Guide to Acceptable Management Practices for Bat Eviction and Structural Remediation
Information in this Guide is both original and has been updated from the bat species article in the Prevention and Control of Wildlife Damage (1994; edited by Scott Hygnstrom, Robert Timm, and Gary Larson, University of Nebraska-Lincoln).
Additional content is from A Guide to the Bats of Virginia. Reynolds, R.J. and R.T. Fernald. 2015. Bureau of Wildlife Resources Special Publication Number 5, Virginia Department of Game and Inland Fisheries.
Cover photos by R. J. Reynolds, Bat Conservation International and A Guide to the Bats of Virginia.
Many thanks to Jan Hygnstrom and Kirsten Smith for their editorial work, as well as the many other unnamed people that have reviewed this document to make it better for everyone.
The National Wildlife Control Training Program is a cooperative work of several people, agencies, and universities. Our material is IPM friendly and available for use in outreach programs with land grant universities and state websites.
Executive Editors of the
National Wildlife Control Training Program
Raj Smith, Cornell University, Department of Natural Resources, NWCTP LLC
Paul Curtis, Cornell University, Department of Natural Resources
Scott Hygnstrom, University of Wisconsin-Stevens Point
This booklet is published by the National Wildlife Control Training Program LLC, October 2018.
Managing bat problems effectively requires that you have good information on the bat species involved and the type of damage the bats are causing. Understanding the biology and habitat of bats allows a technician to use methods effectively to control or eliminate the unwanted behavior, conflict, or the animal itself. After studying this manual, you will be well on your way to understanding bats and how to deal with bat problems effectively. In general, bats do not harm structures except for deposits of guano and urine. Excluding bats from attics and roosting sites eliminates most unwanted behavior.
Little brown bat (Myotis lucifugus). Image in Public Domain by Marvin Moriarty, USFWS.
This research-based manual highlights common problems caused by bats along with recommended management techniques. Bats are important, useful, and a protected species. Lethal means of resolving bat conflicts are a last resort and only an option in unusual circumstances. In most situations, you can resolve bat related issues through exclusion of the bat colony. Once the bats are excluded, problems such as bats accessing human occupied areas, guano and urine buildup, and potential disease issues can be effectively mitigated.
In addition to biology and habitat, technicians need to know the signs and damage caused by various species of bats. Animal-handling and control techniques must be learned, practiced, and mastered. If a bat must be killed or euthanized, it should be done as humanely as possible. Do not hesitate to contact other professional WCOs or state wildlife agencies if the damage situation is complex, or if safety issues exist. If you have concerns about your ability to handle a bat problem, do not hesitate to work with other qualified professionals.
Repairing structures to exclude bats and performing clean-up of guano and urine requires special safety precautions because of heights, diseases, and attic spaces.
After studying this manual, you should be able to complete the following learning objectives. If not, review the appropriate sections until you are comfortable with the objective.
- Know the legal status of bats in your area.
- Identify the bats common in your area.
- List signs that indicate bats are present.
- Understand why it is important to know bats’ breeding season.
- Be able to explain to clients the process involved to exclude bats from their homes.
- Know what types of sealants, screening, flashing, and other exclusion methods can be used to prevent bats from entering structures.
- Know the diseases and health problems associated with bats.
- List 3 scenarios that lead to the assumption that a client has been bitten and the bat must be tested for rabies.
- Know your state’s rabies testing policies.
- Explain how to prevent contamination by white-nose syndrome.
- Explain the key elements of attic remediation and decontamination.
The lethal control of bats, even when there is a proven potential danger to humans, is often subjected to careful scrutiny and interagency coordination. Some states have laws that specifically mention bats, either providing or denying protection. Others have legislation that applies to bats only by interpretation because bats may be considered nongame wildlife, or indigenous state mammals. Some species are protected as either federally or state-listed endangered species. Enforcement and public education must accompany legislation to accomplish the intended goal of protecting the public and endangered bats. Familiarity with the appropriate federal and state laws should precede any nuisance management activities. In addition, special precautions may be required to prevent the spread of white-nose syndrome (WNS), a fungal disease that is decimating bat populations.
As a WCO, you can’t just pack your truck and hit the road when a customer calls with a complaint about bats. Besides knowing if a particular species is federally protected or managed by a state agency, you must know your state’s Health Department guidelines for rabies exposure, as well.
Why the concern about bats? They have important roles in the environment as pollinators and spreading seeds for some plants, especially in the tropics. Insects are an important source of food for many bats, making insectivorous bats important pest managers. Their preferred foods include mosquitoes and agricultural pests. According the US Geological Survey National Wildlife Health Center, consumption of insects by bats saves farmers billions of dollars in pest control services annually. Most people applaud their consumption of mosquitoes, as well! In addition, bat guano still is considered a valuable fertilizer resource in some parts of the world (e.g., Thailand and Mexico). Finally, studies of bats have helped in developing sonar, medications for blood clots, low-temperature surgery, and navigational aids for blind people (White‐nose Syndrome Conservation and Recovery Working Group, 2015).
Silver haired bat. Photo by Rich Reynolds.
Despite the ecological value of bats, many people have phobias about these animals. While customers have the right to live in a home that is free of bats, it is important not to reinforce fears of bats. Many people are afraid of contracting rabies from bats, although in reality only a very small percentage of bats actually have the disease (less than 4%).
A number of bat species are declining throughout the world for various reasons. White-nose syndrome is threatening populations of bats. Some fruit bats in the tropics are harvested for food and fetch a high price in restaurants. The bigger fruit bats are cooked in a mixture of coconut milk, water, spices, and onions and eaten whole! In the US, a number of species are listed as threatened or endangered due to disturbance at hibernacula (overwintering sites such as caves) and maternity sites, loss of habitat, and pesticides.
WCOs regularly work with the public and have an opportunity to contribute to the conservation of bats by following simple guidelines for dealing with bats encountered in structures. WCOs also are an important resource for information on the size and geographical distribution of bat colonies. You are encouraged to share this information with state agencies, regardless of whether a colony is being evicted from a structure or not. State biologists, in turn, can provide additional resources and information on bats to homeowners and cooperating WCOs.
Natural resource conservation is a right and responsibility of all citizens and begins at home. By educating ourselves and our clients, each of us can contribute greatly to protection of our natural resources.
Bats have many common features that are shared among different species. Species specific information is covered in a later section along with information on identification, habitat, reproduction, and other characteristics of bats.
In summer, bats roost beneath loose tree bark, in tree crevices, rocky outcrops, bridges, wood piles, and buildings. For many species of bats, females form maternity roosts where they bear young in early to mid-summer after two months of gestation. These roosts can contain hundreds to thousands of bats. The colonies will return to the same roosts year after year, unless it becomes unsuitable or is not accessible.
Bats mate in the fall and females store the sperm until they ovulate in the spring, a process called delayed fertilization. This delays pregnancy until insects, their primary food source, are more available. Females give birth to one young, called a pup, per year, with some producing twins. Pups nurse until they are able to begin flying, about 4 weeks of age. They may not be able to feed on their own until about mid to late summer. Males are not involved with raising pups and live separately from females. Although usually solitary, they may form small groups.
Near dusk, bats hunt for insects, including moths, mosquitoes, beetles, and midges. Using echolocation, described later, they search for food over bodies of water, in clearings, among trees, and even in areas with houses. They may use night roosts, including under bridges, to rest between feeding forays.
In late summer colonies break apart. In fall, bats mate and build up fat reserves to sustain them through the winter. They migrate to warmer climates or to their winter roosts, called hibernacula, traveling short distances to several hundred miles. They may use buildings or other structures as temporary roosts while traveling. Check out how to build a bat house at the end of the manual.
According to Acceptable Management Practices for Bat Control Activities in Structures – AMPs (2015), “Long‐distance migratory species may use bouts of torpor, a short state of decreased physiological activity, to save energy, while hibernating species engage in repeated, longer bouts of torpor to save energy in cold climates.” They find locations that are 35 to 50oF for hibernation, returning to the same places year after year. These sites include abandoned mines, caves, and sometimes buildings.
To conserve energy during hibernation and periods of torpor, the temperature, heart rate, and respiratory rate of a bat decreases. Whenever a bat is roused during this time, it uses valuable stores of energy.
In spring, bats emerge and return to their summer ranges. Young are born following the 2 months of gestation, and the cycle continues.
Bats are unique in that they are the only mammals capable of sustained flight. When you look closely at a bat wing you can see that it is half arm and half hand.
Gray bat, wing attachment at the ankle.
The upper arm and forearm are relatively proportional to the bat’s body, but the fingers are greatly elongated. Because a bat can move each finger independently, its flying skills rival those of birds. A bat can hover in place, fly backwards, stoop, dive, and roll as needed. All of these maneuvers may be necessary when pursuing prey, such as a highly evasive moth.
Bats use echolocation to navigate and find food. A bat emits a high frequency pulse that travels out, bounces off an object, and then returns to the bat. Through this, the bat can determine the distance, size, and direction of the object. Most sounds are emitted through its mouth, but a bat can emit sounds through its nose. Their ears are sensitive enough to detect slight differences in the time required for an acoustic pulse to return to each ear, and this difference tells the bat the direction of the object. Similarly, the time required for an echolocation pulse to return tells the bat how far away the object is. Bats vary the frequency of echolocation pulses they emit depending on their activity.
Echolocation. Drawing by Guide to Bats in Virginia.
If a bat is chasing a moth for dinner, it will increase the number of pulses emitted as it gets closer to its prey. This gives the bat detailed information on direction and distance to the prey. This is very important when a bat is chasing a moth for a meal!
Bats belong to the Class Mammalia, meaning they bear their young live, feed their young milk, and grow hair. Because they can fly, they have been placed in their own Order, Chiroptera, which means “hand wing.”
The ability to fly, secretiveness, and nocturnal habits have contributed to bat folklore, superstition, and fear. And vampire bats – bats that consume blood – really exist, with species ranging from Mexico to South America, but not in the US.
About 1,100 species are distributed worldwide, second in number only to Rodentia (rodents) among mammals. Bats make up almost 25% of all species of mammals. Forty-five species of bats are found in North America, of which 19 occur in the eastern US.
Identification of Bat Anatomy
Anatomy of a bat. Image by PCWD.
Species of bats can be identified by morphological differences. These include features such as:
- ear shape and size,
- overall size and wingspan,
- the tragus,
- attachment of the foot to wing,
- and the calcar.
The tragus is a prominent lobe that extends from the base of the ear. Its function is not known, but believed to be related to echolocation. The calcar is a spur or spur-like projection found on the on the wing or leg of a bat. Other ways to identify a bat species are given in the following section. Fortunately, while the physical species characteristics can differ greatly, control methods, like those covered later in this manual, do not.
Among the 40 species of bats found north of Mexico, only a few cause problems for humans. Bats that congregate in colonies are called colonial bats; those that do not are solitary bats.
The species most often encountered in and around buildings in the US are colonial and include big brown bats (Eptesicus fuscus), little brown bats (Myotis lucifugus), tri-colored bats (Perimyotis subflavus), Brazilian or Mexican free-tailed bats (Tadarida brasiliensis), pallid bats (Antrozous pallidus), Yuma myotis (Myotis yumanensis), and evening bats (Nycticeius humeralis). Colonial bats that are not inclined to live in human structures often use caves or terrain that supports a maternity hibernacula.
By contrast, Solitary bats typically roost in tree foliage or under bark, but occasionally are found in buildings, usually during migration. These include Keen’s bats (Myotis keenii), red bats (Lasiurus borealis), silver-haired bats (Lasionycteris noctivagans), and hoary bats (Lasiurus cinereus). Excellent illustrations of the bats discussed here can be found at the following web sites.
Bat Conservation International (http://www.batcon.org/index.php/all-about-bats/species-profiles.html)
University of Michigan’s Animal Diversity Website (http://animaldiversity.ummz.umich.edu/site/accounts/information/Chiroptera.html).
Little brown bat. Photo by BCI.
- Wingspan: 0 to 10.6 inches.
- Forearm: 3 to 1.6 inches.
- Ears: 55 to 0.63 inches.
- Foot: About 0.4 inches; long hairs on toes extend beyond claws.
- Color: Pale tan, red-brown, or dark brown, depending on geographic location.
It is a rich dark brown in the eastern US and most of the west coast. Fur is glossy and sleek. Confusion may occur with other “house” bat species. In the east, it may be confused with Keen’s bat, which has longer ears (0.7 to 0.8 inches) and a longer, more pointed tragus (the appendage at the base of the ear). In the West, it resembles the Yuma myotis, which has dull fur and usually is smaller. Yuma myotis and little brown bats may be indistinguishable in some parts of the northwestern US where they hybridize.
Little brown bats are one of the most common bats found in and near buildings, often located near a body of water where they forage for insect prey. Summer colonies are gregarious, commonly roosting in dark, hot attics and roof spaces where maternity colonies may include hundreds to thousands of individuals. Colonies may form beneath shingles and siding, in tree hollows, beneath bridges, and in caves.
Top to bottom. Foot with calcar (cartilage spur), foot with hairs on toes, and ear and tragus of the little brown bat. Photos by BCI.
Litter size is 1 in the Northeast, and twins occasionally occur in other areas. Roosts often are shared with big brown bats, though the latter is less tolerant of high temperatures, often relocating when temperatures exceed 95°F. Keen’s bats may share the same site. Separate groups of males and non-reproducing females tend to be smaller and roost in cool attics, behind shutters, under bark, in crevices, and within caves.
Generally in August or September, little brown bats in the eastern part of their range abandon buildings to hibernate in caves and mines. Hibernacula may be near summer roosts, or up to a few hundred miles away. Little is known of winter habits of little brown bats in the western US. The life span of little brown bats is up to 31 years, though the average life expectancy probably is only a few years.
Distribution of little brown bats. Image by PCWD.
- Forearm: 7 to 2.0 inches.
- Wingspan: 8 to 13.8 inches.
- Ears: Rounded tragus.
- Color: Reddish-brown, copper-colored, or dark brown, depending on geographic location.
Big brown bat. Photo by BCI.
It is a large bat without distinctive markings. Confusion may occur with evening bats, though the latter is much smaller. Big brown bats are hardy and favor buildings for roosting. Summer maternity colonies may include a dozen to a few hundred individuals roosting behind chimneys, in enclosed eaves, in hollow walls, attics, barns, and behind shutters and unused sliding doors. They form colonies in rock crevices, beneath bridges, in hollow trees, and under loose bark. Litter size is 2 in the East to the Great Plains, and 1 from the Rockies westward. Big brown bats frequently share roosts with little brown bats in the East, and with Yuma myotis, Mexican free-tailed bats, and pallid bats in the West. Males roost in small groups or alone in the summer.
Big brown bats are a widely distributed species of bats in the US, and probably are familiar to more people than any other species. People recognize their large, easy-to-observe size, but also to their habit of overwintering in buildings. Their proximity to humans coupled with the tendency to move about during temperature shifts, often brings big brown bats into human living quarters and basements. Big brown bats hibernate in caves, mines, sewers, burial vaults, and underground harborage. They travel up to 150 miles to hibernacula, though winter quarters are largely unknown. Big brown bats may live to 18 years.
Distribution of big brown bats. Image by PCWD.
Mexican free-tailed bat (Tadarida brasilensis).
- Forearm: 4 to 1.8 inches.
- Wingspan: 4 to 12.8 inches, long narrow wings.
- Tail: (Interfemoral) membrane does not enclose the lower third to half of the tail (hence the name free-tailed).
- Foot: Stiff hairs, as long as the foot, protrude from the toes.
- Color: Dark brown or dark gray.
Fur of some individuals may be bleached to pale brown due to ammonia fumes from urine and decomposing guano. Confusion is not likely to occur with other species that commonly inhabit structures.
Mexican free-tailed bats form large colonies in buildings, particularly on the West Coast, and in the Gulf states from Texas east. Hundreds to thousands of individuals may be found in buildings or under bridges. It primarily is a cave species in Arizona, New Mexico, Oklahoma, and Texas. Buildings are used as temporary roosts during migrations. Litter size is normally only 1.
Mexican free-tailed bats often share roosts with other species. In the West, they may be found in buildings with pallid bats, Yuma myotis, and big brown bats. Some males are always present in the large maternity colonies, but they tend to segregate in separate caves. A few individuals may overwinter in buildings as far north as South Carolina in the east, and Oregon in the west. Most migrate hundreds of miles to warmer climates (Mexico) for the winter.
Distribution of Mexican free-tailed bats. Image by PCWD.
Pallid bat. Photo by BCI.
- Forearm: 9 to 2.4 inches.
- Wingspan: 2 to 15.4 inches.
- Ears: Large, widely separated and more than half as broad as long. The ears are nearly 1.5 times as long as the combined length of the bat’s head and body.
- Color: Pale, upper parts are light yellow, the hairs tipped with brown or gray. Under parts are pale creamy, almost white.
It is relatively easy to recognize. Confusion with other species is not likely to occur.
Segregation between bachelor and nursery colonies is common, though both sexes often mix in the same groups. Most colonies are small, ranging from 10 to 100 individuals. Roost sites include buildings, bridges, and rock crevices, and less frequently in tree cavities, caves, and mines. Litter size is most commonly 2. The roost frequently is shared with Mexican free-tailed and big brown bats in the West. While some males segregate during the nursery period (sometimes in the same building), others are found within the maternity colony.
Pallid bats fly close to the ground, hover, and take most prey on the ground rather than in flight. Prey includes crickets, grasshoppers, beetles, and scorpions. They will forage among tree foliage. Pallid bats do not make long migrations, but little is known of their winter habits.
Pallid bat and its distribution. Map by PCWD.
Yuma myotis. Photo by BCI.
- Forearm: 3 to 1.5 inches.
- Wingspan: about 9.3 inches.
- Ears: 55 to 0.59 inches.
- Foot: 4 inches.
- Color: Light tan to dark brown, underside is whitish to buffy.
Confusion may occur in the West with little brown bats, though the latter is larger and tends to have longer, glossier fur. In the Northwest, hybridization occurs with little brown bats, often making the 2 species indistinguishable.
Distribution of Yuma myotis. Map by PCWD.
Summer maternity colonies of up to several thousand individuals form in attics, belfries, under bridges, and in caves and mines. Litter size is 1. Males typically segregate during the nursery period and roost as solitary individuals in buildings and other suitable harborage.
Yuma myotis are more closely associated with water than any other North American species of bat. Nearly all roosts have open water nearby. Yuma myotis are less tolerant of high roost temperatures than little brown bats and will move to cooler areas within a building when temperatures rise above 100o F. Yuma myotis bats abandon maternity colonies in the fall, and its winter habitat is not known.
Evening bat. Photo by BCI.
- Forearm: 3 to 1.5 inches.
- Wingspan: 3 to 11.0 inches.
- Ears: Short, curved, and rounded tragus.
- Color: Medium-brown with some variation to yellow-brown in subtropical Florida. No distinctive markings.
Confusion may occur with big brown bats, which are readily distinguished by their larger size. Evening bats bear some resemblance to the smaller little brown bats, but can be identified by their characteristic blunt tragus. Summer maternity colonies in buildings may consist of hundreds of individuals. Litter size usually is 2. Colonies form in tree cavities, in Spanish moss, beneath dead palm fronds, and under loose tree bark. In the Southeast, Mexican free-tailed bats commonly inhabit the same building with evening bats. Evening bats are common in towns throughout southern coastal states. Little is known of their feeding behavior or seasonal movements except that they almost never enter caves.
Distribution of evening bat. Map by PCWD.
Keen’s bat. Photo by BCI.
- Forearm:3 to 1.5 inches.
- Wingspan:0 to 10.2 inches.
- Ears:7 to 0.8 inches, with a long, narrow, pointed tragus.
- Color: Brown, but not glossy, and somewhat paler in the East.
Confusion may occur with little brown bats that have glossy fur, shorter ears, and no long, pointed tragus.
Keen’s bat distribution. Map by PCWD.
Keen’s bats generally are found singly in the East, except for small maternity colonies, where up to 30 individuals may congregate. They roost behind shutters, under wooden shingles, sheltered entryways of buildings, in roof spaces, in barns, and beneath tree bark. In the West, these bats are known as a solitary species, roosting in tree cavities and cliff crevices. Litter size is probably 1. Roosts sometimes are shared with little brown bats. The sexes likely segregate during the nursery period. In winter, these bats hibernate in caves and mines.
Eastern red bat. Photo by BCI.
- Forearm: 4 to 1.8 inches.
- Wingspan: 4 to 13.1 inches; long, pointed wings.
- Ears: Very short and rounded.
- Tail membrane: Heavily furred on upper surface, with a distinctive long tail.
- Color: Bright orange to yellow-brown, usually with a distinctive white mark on the shoulders.
Red bats are most likely to be confused with Seminole bats, a species of nearly identical size and appearance except for the latter’s deep mahogany color. Confusion also may occur with hoary bats, which are frosted-gray in appearance and larger.
Eastern red bat distribution. Map by PCWD.
Red bats are solitary, coming together only to mate and migrate. Few people are familiar with this species. Red bats typically spend summer days hidden in the foliage of deciduous trees. Litter size ranges from 1 to 4. Red bats often chase insects that are attracted to lights, such as street lamps, which may bring them close to people. Red bats are well-adapted for drastic temperature fluctuations. They do not hibernate in caves, but apparently in trees. Some migrate long distances. During migration red bats have been known to land on high-rise buildings, and on ships at sea.
Silver-haired bat. Photo by BCI
- Forearm: 5 to 1.7 inches.
- Wingspan: 3 to 12.2 inches.
- Ears: Short, rounded, hairless.
- Tail membrane: Upper surface is sparsely furred on the anterior half.
Silver-haired bat distribution. Map by PCWD.
Color: Black with silver-tipped fur, some with dark brown, yellowish-tipped fur. Confusion may occur with larger hoary bats, which have patches of hair on the ears and wings, heavy fur on the upper surface of the tail membrane, and a distinctive throat “collar.”
Silver-haired bats roost in a variety of harborages. A typical roost is behind loose tree bark. Other sites include tree hollows, woodpecker holes, and bird nests. They are a solitary species except when with young. They may be colonial and may roost in and on buildings. Litter size is 2. The sexes segregate through much of the summer range.
Silver-haired bats hibernate in tree crevices, under loose bark, in buildings (including churches, sky scrapers, and wharf houses), hulls of ships, rock crevices, silica mines, and non-limestone caves. They may migrate, during which time they are encountered in buildings (they favor open sheds, garages, and outbuildings rather than enclosed attics), in lumber piles, and on ships at sea.
Hoary bat. Photo by BCI.
- Forearm: 8 to 2.3 inches.
- Wingspan: 0 to 16.1 inches.
- Ears: Relatively short, rounded, edged with black, and with fur.
- Tail membrane: Completely furred on upper surface.
- Color: Dark, but many hairs are tipped in white, giving it a frosted appearance. These bats have a yellowish or orangish throat “collar.”
Confusion may occur with the much smaller silver-haired bats, which lack fur patches and markings on the ears, markings on the throat, and have tail membranes that are only lightly furred on the upper surface. Hoary bats generally spend summer days concealed in tree foliage (often evergreens), rarely enter houses, and are not commonly encountered by people. At day roosts, they usually are solitary except when with young. Litter size is 2. The sexes segregate through most of the summer range. Hoary bats are one of the largest bat species in North America, powerful fliers, and accomplished migrants. Some individuals may hibernate in northern parts of the range.
Hoary bat distribution. Map by PCWD.
Most bat problems fall into 5 main categories:
- bats living in the structure,
- lone bat encounters inside the living space,
- problems related to guano and urine,
- disease-related problems, and
- bats loafing or flying around the exterior of a structure.
Additional problems and concerns are related to the repair of the structure, and the clean-up of guano and urine.
No figures are available to determine the extent of damage caused by nuisance bats or the cost for their control. The problem is widespread in the US and other countries. Bats commonly enter buildings through openings associated with the roof edges and valleys, eaves, apex of the gable, chimney, attic or roof vent, dormers, and siding.
The cost of bat eviction and remediation services can vary greatly depending on the number of bats, the length of time that they have inhabited the roost, and the general condition of the building structure.
According to the NWCOA Bat Standards:
A bat Eviction/Venting Inspection is a non-invasive, visual examination of the interior and exterior of a residential or commercial dwelling, that may be performed for a fee, which is designed to identify current and potential bat entry points, exterior roosts, structural defects, rabies risk, contaminations, and ectoparasites within specific components of said dwelling prior to exclusion and eviction/venting. Components may include any combination of mechanical, structural, or other essential systems or portions of a structure, as identified and agreed to by the client and WCO.
Bats often roost on the outside of buildings, behind shutters, and where the siding and edge boards are not joined, lapped, or sealed properly. They occasionally roost on porches
and patios, in garages, and behind shingles and roof gutters. Other openings may be found under loose-fitting doors, around windows, and gaps around conduits (wiring, plumbing, air conditioning) that pass through walls and utility vents.
Common points of entry for bats. Image by PCWD.
Bats squeeze through narrow slits and cracks. For the management of bats, focus on any gap measuring approximately ¼ x 1½ inches, or hole measuring 5/8 x 7/8 inch. Such openings must be considered potential entries for the smaller species, such as little brown bats. Smaller species require an opening no wider than 3/8 inch, or the diameter of a US dime coin.
Openings of these dimensions are not uncommon in older wood-frame structures where boards have shrunk, warped, or loosened. Big brown bats may hibernate in the cooler recesses of heated buildings, and they may suddenly appear (flying indoors or outdoors) in midwinter during a warm spell or a cold snap as they move about, adjusting to the temperature shift.
We recommend 2 ways to confirm the presence of bats in a structure. The first is to perform a detailed building inspection (interior and exterior). Discovery of rub marks, bat guano, and sightings of bats, coupled with client information usually are sufficient to confirm the presence of bats.
The second way to confirm the presence of bats is a bat watch. It can be conducted by 2 people posted at opposite corners of a structure (more may be necessary to observe large or complex sites). An evening watch begins about 30 minutes before dark, and a morning watch begins about an hour before dawn.
Observations should continue for approximately 1 hour. Bat watches can indicate exit and entry points and the number of bats. It may be necessary to watch for more than 1 night to compensate for weather conditions, bat sensitivity to observers, noisy or inexperienced observers, and improper use of light. Observations can be enhanced with a standard flashlight but be certain to project the beam as far away as possible from the exit hole that is being watched. Bright light will make bats reluctant to exit and may give an incomplete count of the colony. A powerful, rechargeable flashlight equipped with a red filter is helpful during a bat watch. Also, an electric headlamp supplied with rechargeable batteries and fitted to a climbing or caving helmet allows hands-off illumination when exploring roost locations.
Bats are sensitive to light intensity and can visually discriminate shapes and patterns in extremely low-light situations. They see in black and white, so low-contrast illumination and soft shadows produced by red light has little effect on bats.
Guano and urine may be visible, especially near large colonies. Fecal pellets indicate the presence of bats and are found on attic floors, in wall recesses, and outside the house. Fecal pellets along and inside walls may indicate the presence of mice, rats, or even roaches. Most house bats north of Mexico are insectivorous and their droppings are easily distinguished from those of small rodents. Droppings from bats tend to be segmented, elongated, and friable or crumbly. When crushed, they become powdery and reveal shiny bits of undigested insect remains. In contrast, droppings from mice and rats tend to have a tapered end, are not segmented, are harder and more fibrous, and do not become powdery when crushed (unless they are extremely old).
Bat guano (whole, left; crushed, right) looks similar to mouse droppings except for the shiny speckles and susceptibility to crumble. Photo by UNL.
The droppings of some birds and lizards occasionally may be found along with those of bats. Bat droppings never contain the white chalky material characteristic of the feces of these other animals.
Bat excrement produces an unpleasant odor as it decomposes in attics, wall spaces, and other voids. The pungent, musty, acrid odor often can be detected from outside a building containing a large or long-term colony. Similar odor problems occur when animals die in inaccessible locations. The odor also attracts arthropods that may later invade other areas of a building. Bats urinate and defecate when exiting a roost, causing spotting and staining on sides of buildings, windows, patio furniture, automobiles, and objects near entry and exit holes, or beneath roosts.
Rub marks may occur on surfaces along walls, under loose woodwork, between bricks, and around other bat entryways. Rubs often have a smooth, polished appearance. The stained area is slightly sticky, may contain a few bat hairs, and is yellow-brown to blackish brown in color. The smooth gloss of the rub marks is due to oils from fur and other bodily secretions mixed with dust, deposited there as many animals pass repeatedly for a long period over the same surface. Openings marked in this way have been used repeatedly by bats.
Bats that are infected with rabies can transmit the disease to pets and livestock during encounters. Though less than 0.1% of all bats have rabies, the percentage increases to almost 4% for sick bats that interact with people and animals and have been tested.
Bats do not damage gardens or landscapes.
Diseases Associated with Bats
White-nose Syndrome (WNS)
WNS is a fungal infection that affects the exposed skin of a bat’s nose, ears, wings, and tail. It may appear as a white growth on an infected bat’s nose, or may not be visible. No evidence is available that suggests the white-nose fungus can infect humans, but WNS is a health and safety concern for bats as it can kill and decimate whole populations of bats. Every measure should be taken to prevent the spread of the disease by utilizing proper decontamination methods when working with colonies of bats.
Rabies in a bat colony can pass through generations. Estimates vary, but most experts agree that less than 0.1% of all bats in a colony carry rabies. Bats that are found on the ground, and bats that have interacted with humans or display unusual behaviors, are more likely to carry rabies (up to 4%).
Do not assume a client can identify a bite from a bat. Photo by NE Health and Human Services.
A rabid bat usually transmits the disease by biting another mammal. Do not assume that a bite mark from a bat can be identified. As the disease progresses, the bat becomes increasingly paralyzed and dies as a result of the infection. The virus in the carcass remains infectious until decomposition is well advanced. Therefore, every bite from, or contact with, a bat must be considered a potential exposure to rabies.
While aerial transmission of the rabies virus from bats in caves to humans (and some other mammals) has been reported, this is not a likely route of infection for humans entering bat roosts in buildings in temperate North America.
The potential for rabies exposure has occurred if a client or others in the building:
- awoke to find a bat flying in the room,
- found a bat in a room with an unattended child, whether sleeping or not, or
- found a bat in a room with someone who was mentally unable to assess whether they were bitten by a bat.
If possible, capture the bat in a manner that does not damage the head, and deliver it to your local health department for testing in accordance with your state’s guidelines. In situations where the bat’s whereabouts are unknown, the affected individuals should consult appropriate health officials immediately. The following discussion is general information on treating potential rabies exposures. Always consult health officials to ensure that the latest protocols are being used.
Treat bite wounds immediately and thoroughly with soap and water and irrigate the wound with a virucidal agent, such as povidone-iodine solution, when available. If the bat is captured, you can quickly transport it to a testing laboratory, and arrange for immediate testing, post-exposure treatment may be delayed for 72 hours until the test results are known.
Post-exposure treatment must be administered immediately if:
- the bat cannot be captured,
- prompt delivery to the testing laboratory is not possible,
- the specimen is not suitable for reliable diagnosis,
- the victim’s immune system is compromised or suppressed, or
- the test results prove positive for rabies.
The prophylaxis (treatment) has little resemblance to that of many years ago. Today, it consists of 1 dose of rabies immune globulin (human origin) and 1 dose of rabies vaccine (human diploid cell) administered preferably on the day of the incident. Additional shots may be necessary.
Newspapers, television, and other mass media sometimes misrepresent the risk of rabid bats to humans. However, the unfortunate deaths of individuals throughout the country, and around the world, show the need to pay prompt attention to bat bites and other exposures.
Exposure to rabies can be prevented if people don’t handle bats. Adults and children should be strongly cautioned never to touch bats with their bare hands. All necessary measures should be taken to ensure that bats cannot enter living quarters in houses and apartments.
Cats and dogs should be regularly vaccinated for rabies. Valuable livestock also should be vaccinated if kept in buildings that harbor bats or are in a rabies outbreak area. While transmission of rabies from bats to terrestrial mammals apparently is not common, such incidents have been reported. Dogs, cats, and livestock that have been exposed to a rabid or suspected-rabid animal, but are not currently vaccinated, must either be quarantined or destroyed as determined by local health officials.
Histoplasmosis is a common lung disease found worldwide, caused by a microscopic fungus, Histoplasma capsulatum. It is a saprophytic mold (gets its energy from dead and decaying organic matter). It grows in soils high in nitrogen, generally associated with the guano and debris of birds (particularly starlings and chickens), and bats. Wind probably is the main agent of dispersal, but the fungus can survive and be transmitted from one site to another in the intestinal contents of bats and on the appendages of bats and birds. The disease can be acquired by the casual inhalation of windblown spores, but infection is more likely to result from a visit to a point-source of the fungus. Where bats are involved, such sources include roosts in barns, attics, belfries, and soil enriched with guano.
Wild and domestic animals are susceptible to histoplasmosis, but bats are the most important animal vectors. Unlike bats, birds do not appear to become infected with the fungus. Both the presence of guano and particular environmental conditions are necessary for H. capsulatum to multiply. In avian habitats, the fungus apparently grows best where the guano is in large deposits, rotting, and mixed with soil, rather than in nests or in fresh deposits. Specific requirements regarding bats have not been described, although bat roosts with long-term infestations are often mentioned in the literature.
While histoplasmosis in the US is particularly endemic to the Ohio-Mississippi Valley region (which is also an area with the greatest starling concentration) and areas along the Appalachian Mountains, it is found in other states. In addition to areas with the right environmental conditions, scattered sites with high infection rates can be found, usually associated with caves inhabited by bats or birds.
When soil or guano containing H. capsulatum is disturbed, the spores become airborne. People at particular risk of histoplasmosis include cavers, bat biologists, pest control technicians, WCOs, those who clean or work in areas where bats have habitually roosted, and those in contact with guano-enriched soil. An example of this is the foundation of a building where guano has sifted down through the walls.
Infection occurs when spores are in inhaled, and can result in a variety of clinical problems. Severity partially depends on the number of spores inhaled. The infection may remain localized in the lungs where it may disappear on its own. Such infections are identified only by the presence of a positive Histoplasma skin test or calcified lesions on routine X-rays. Other individuals may have chronic or progressive lung disease that requires treatment. Less severe forms of these infections may be accompanied by fever, cough, and generalized symptoms similar to a prolonged influenza. Resolution of the disease confers a degree of immunity to reinfection and varying degrees of hypersensitivity to H. capsulatum. Massive reinfection in highly-sensitized lungs may result in a fatal, acute allergic reaction.
In a small percentage of chronic histoplasmosis cases, the fungus spreads to involve multiple organ systems and may be fatal. This condition usually is found in young children (1 year or older), and in adults with compromised immune systems. In recent years, systemic infections have increased in frequency globally as an opportunistic infection of AIDS patients.
Bat roosts known or suspected to be contaminated with H. capsulatum should only be entered wearing a National Institute for Occupational Safety and Health (NIOSH)-approved protective mask capable of filtering out particles as small as 2 microns in diameter, or a self-contained breathing apparatus. NIOSH states that “only persons trained in the proper selection and use of personal protective equipment should undertake work where this equipment is needed.” Along with training, anyone planning to wear a respirator should have a medical evaluation and fit test. In known contaminated areas, wear PPE that can be removed at the site and placed in a plastic bag for later decontamination via formalin and washing. According to NIOSH, “because of the potentially serious health hazards associated with formaldehyde exposures, this chemical should be handled only by persons who know how to apply it safely.” Clean all footwear before leaving the site to prevent dissemination of spores in cars, the office, at home, and elsewhere. Actual control measures may not be necessary unless bat droppings become a problem or the risk of human contact is significant.
Guano may provide a growth medium for microorganisms, some of which are pathogenic (e.g., histoplasmosis) to humans. Accumulations of guano may fill spaces between walls, floors, and ceilings. Guano may create a safety hazard by causing slips on floors, steps, and ladders. Accumulations can stain ceilings, soffits, and siding, producing unsightly and unsanitary conditions. The weight of droppings and urine can potentially compromise structures, even causing ceilings to collapse. Excrement may contaminate stored food, commercial products, and work surfaces. Guano is still used as a fertilizer in many parts of the world.
Urine readily crystallizes at room temperature. In warm conditions, such as under roofs exposed to sun and on chimney walls, urine evaporates so quickly that it crystallizes in great accumulations. Boards and beams saturated with urine acquire a whitish powder-like coating. With large numbers of bats, thick and hard stalactites and stalagmites of crystallized bat urine occasionally are formed.
Although fresh urine from a single bat is relatively odorless, that of any moderate-sized colony is obvious. The odor increases during damp weather. Over a long period of time, urine may cause mild deterioration of wood. As urine saturates the surfaces of dry wood beams and crystallizes, the wood fibers expand and separate. The fibers then are torn loose by the bats crawling over such surfaces, resulting in wood fibers being mixed with guano accumulations underneath.
The proximity of bat roosts to human living quarters can result in excreta, animal dander, fragments of arthropods, and various microorganisms entering air ducts as well as falling onto the unfortunate residents below. Such contaminants can result in airborne particles of public health significance.
Deposits of guano and guano-enriched soils should not be disturbed unnecessarily. Dampening with water or scheduling outdoor work at a time when the ground is relatively wet will minimize airborne dust. To protect the environment, decontamination must be conducted in accordance with state and local regulations. Decontamination of an active bat roost should be conducted only after the bats have been excluded or after bats have departed for hibernation.
Arthropods (fungivores, detritivores, predators, and bat ectoparasites) often are associated with large or long-term bat colonies in buildings. The diversity depends on the number of bats, age and quantity of excreta deposits, and season. Insects such as dermestid beetles (Attagenus megatoma) contribute to the decomposition of guano and insect remnants, but also may become a pest of stored goods and a nuisance within the living quarters. Cockroaches (e.g., Blatta orientalis) attracted to guano may invade other parts of a building. Bat bugs (Cimex spp.) are sometimes found crawling on the surface of beams, or around holes leading to secluded recesses used by bats.
Ectoparasites (ticks, mites, fleas, and bugs) rarely attack humans or pets and quickly die in the absence of bats. Ectoparasites may become a nuisance, however, following the exclusion of large numbers of bats from a well-established roost site.
Area fumigation with a total release fogger using pyrethrum may seem to be an appropriate solution for knockdown of arthropods within an enclosed space. However, its effectiveness is questionable and total release foggers have caused severe health problems for some people.
Bat bug (left) with bed bug (right). Photo by Jim Kalisch.
Treatment can occur only after bats have been evicted. For long-term control of arthropods, lightly dust appropriate surfaces (e.g., affected attic beams, soffits) with boric acid powder or diatomaceous earth. Carefully read all product labels before using any pesticide. Neither rabies nor Lyme disease are transmitted by any arthropods associated with bats.
Some people are frightened by the presence of bats flying around the outside of their house. Bats often fly around swimming pools, from which they drink or catch insects. White light with an ultraviolet component, commonly used for porch lights, building illumination, street and parking-lot lights, may attract flying insects, which in turn attract bats. The mere presence of a bat outdoors sometimes is beyond the tolerance of some homeowners. Education is a good remedy for such situations.
Disturbing sounds may be heard from vocalizations, grooming, scratching, crawling, or climbing in attics, under eaves, behind walls, and between floors. Bats become particularly noisy on hot days in attics, before leaving the roost at dusk, and upon returning at dawn. However, rustling sounds in chimneys also may be caused by birds or raccoons. Scratching and thumping sounds in attics and behind walls may indicate rats, mice, or squirrels.
The following section on Damage Prevention and Control Methods is consistent with Acceptable Management Practices (AMPs) for wildlife control operators (WCOs) to reduce impacts on bats during bat control or removal activities in structures. These guidelines were developed by WCOs, state and federal agencies, private conservation organizations, and the Centers for Disease Control in response to recent catastrophic population declines and changes to the protection status of many bat species due to white-nose syndrome (WNS). These AMPs are recommended for use with all structure-dwelling bat species, regardless of their conservation status.
Methods that include poisoning, trapping (e.g., cages, sticky traps), exterminating, translocating, or in any other way harming, harassing, or killing bats do not meet the AMPs.
Lethal control methods are illegal in many states and can result in increased cost to the homeowner. In addition, some of these methods may actually be dangerous to people and pets. In some states, bats are protected under state law, and should never be intentionally exterminated, harassed, or harmed.
The AMP document is meant to provide practices for safely addressing human-bat conflicts, while minimizing disturbance to bats and preventing the further spread of WNS. The following overview, followed by more detailed information on damage prevention and control methods align with the AMPs. https://www.whitenosesyndrome.org/sites/default/files/resource/wns_nwco_amp_1_april_2015.pdf.
- Use exterior light bulbs that are less attractive to insects. Bulbs that emit a yellow light attract fewer insects.
- Use lights and fans to make roosting sites unattractive to bats.
- Use one-way doors or check-valves, caulk, flashing, screening, soffit repair, chimney and vent caps, ridge vent screening, roof and siding exclusion.
- Exclusion is the recommended method for bat eviction and long-term control.
- Not effective.
- Naphthalene flakes registered in some states.
- None registered.
- Not practical, illegal in some states.
- Use bat traps as a last resort. Prevent unnecessary stress or death to the bats.
Use glue boards only in emergency situations.
Bat problems in houses or buildings usually occur when either an individual bat or a colony of bats is found in a building such as a house, barn, church, or warehouse, and they usually roost in the attic or rafters. Each situation calls for specific tactics to resolve the problem.
See the discussion on handling bats later in this section for dealing with a lone bat encounter.
You may discover a colony of bats residing in an attic. Unless they frequent the attic, the client may not have discovered them until they had been there for weeks, or much longer. Generally, these are maternity colonies made up of females and their young. Females become pregnant in the spring and give birth in early to mid-summer. The young will be able to fly by mid- to late-summer. Most colonies found in attics will leave in the fall for their preferred winter hibernacula. Temperatures in attics are usually too variable in the winter to provide good hibernating roosts, thus bats will generally not stay through the winter.
While bats in the attic may be unwanted guests, these maternity sites are important to the species. Allowing a maternity colony to stay until the fall (when they will leave), can be beneficial to the species. If the client chooses this alternative, then you should close off any access points into the house from the attic until the bats have departed. Late summer and fall are the best times to apply exclusion techniques to prevent their return.
Costs for remediation services are highly variable and depend on the nature of the problem and who will do the work. Remediation services include the costs to execute a bat eviction plan, such as exclusion and structural modifications to the building, as well as the cleanup of guano and urine and damaged attic insulation. For example, to fabricate a few bat check-valves on the average 2-story house would probably require 2 workers about a half day, mostly on ladders, and less than $100 in materials. Much more time would be required to seal all the other active and potential bat exit and entry holes. In addition, if a roof, eaves, or other woodwork has deteriorated and needs replacement, costs can increase rapidly. Bat removal, exclusion, and home remediation services can be expensive. For example, extensive ladder work or the use of a mechanical lift increases costs.
It often is difficult or expensive for the public to obtain the services of reliable, licensed pest control operators (PCOs). Many PCOs have limited knowledge of the basic biology of bats and are apprehensive to work with bats. Some PCOs will try to sell their services using the client’s fear of bats and diseases. Others may want to avoid liabilities (especially around the existence of rabies) should bat-human contact occur.
In outdoor settings, swap white-lights for bulbs less attractive to insects. Illumination has been reported to be effective at moving bats out of structures. Floodlights strung through an attic to illuminate all roosting sites may cause bats to leave. Large attics may require many 100-watt bulbs or 150-watt spotlights to be effective. Fluorescent bulbs may be used. In some situations, such lighting is difficult, costly, and may pose an electrical hazard. Where possible, the addition of windows to brighten an attic will help to reduce the desirability of the roost site and is not likely to introduce additional problems. All wiring should be done by a qualified electrician. Note that bright light may drive bats into wall voids where control can be more difficult.
Air drafts have been successful in repelling bats in areas where building owners can open doors and windows, or create strong breezes with electric fans. The addition of wall and roof vents will enhance this effort, as this will lower roost temperatures. The above-described measures will increase the thermoregulatory burden on the bats, making the roost less desirable. Colonies in soffits, behind cornices, and other closed-in areas can be discouraged by opening these areas to eliminate dark recesses. Discourage bats from roosting behind shutters by removing the shutters completely or by adding small blocks at the corners to space them a few inches away from the wall.
Exclusion is the best way for eliminating and preventing bats from residing in structures. The challenge is to avoid trapping young pups, and to prevent bats from relocating in the structure through other openings. Repairs or modifications to the attic, soffit, and roof may be necessary. Although tedious, it is necessary to locate all active and potential openings available to bats. This may require a lot of ladder work, and a machine lift may be necessary. Active holes can be identified by rub marks, guano, and sometimes odor.
Look for gaps or openings around chimneys, fireplaces, plumbing, piping, attic doors or hatches, windowsills, air conditioners, ducts, louver fans, and pet doors. Broken window and door screens, and even open windows, can provide entry points. Because bats use some of the same holes in buildings where heated or cooled air is lost, bat-proofing often reduces energy costs for the client.
Except for the actively used holes, seal all gaps of ¼ x 1½ inches and openings 5/8 x 7/8 inch or greater. However, be aware that sealing gaps can have disastrous effects on bat pups if done at the wrong time.
Timing of Exclusion
Timing is important to reduce the risk of separating adults from young that cannot fly during the maternity season. This could result in death of the young. Depending on the bat species, region, and weather, the maternity season may be from early April to late August in North America. You should not install 1-way doors or any other exclusion devices from early May through August 31. You do not want to trap young bats that cannot fly inside a building and exclude their mothers. One option is to seal unused holes but leave active holes open until September. Consult with your local biologist for exclusion dates. The exclusion process may cause bats to find their way into the living quarters of a home, a behavior most often associated with young bats.
In addition, do not exclude or evict bats during winter months if there is a history or evidence of winter bat activity in the structure, unless there is a public health threat. According to the White‐nose Syndrome Conservation and Recovery Working Group (2015), “Some species (e.g., the big brown bat) hibernate in buildings and do not regularly exit the structure because there is usually no food source available. Therefore, eviction/venting activities are likely to be ineffective, or may result in bats exiting the structure in inclement weather. Improper exclusion work may result in trapped bats dying inside the roost, or ending up in the living space while searching for another exit.”
Timing of bat-proofing
|Jan. – April||Seal entrances before bats return|
|May – Aug.||Do not seal. Watch for bat activity to identify entrances.|
|Sept – Oct||Install 1-way doors|
|Nov-Dec||Seal entrances after bats have left the building.|
Exclusion materials and methods
Exclusion materials and methods for bats are less rigid than for rodents, as bats do not chew into structures. Caulk, flashing, screening, and insulation often are needed to complete an exclusion job. The combination of materials used will depend on the location, size, and number of openings and the need for ventilation. Weather stripping and knitted-wire mesh (e.g., Guard-All®, Stuf-fit®) are best applied during dry periods when wood cracks are widest. Caulk can be applied with a caulking gun (in gaps up to 0.4-inch-wide) and include latex, butyl, and acrylic compounds, which last about 5 years. Elastomeric caulks, such as silicone rubber, will last indefinitely, expand and contract, do not dry or crack, and can tolerate temperature extremes.
To prevent bats from entering chimney flues, completely enclose the flue discharge area with rust-resistant spark arresters or pest screens secured to the top of the chimney. They should not be permanently attached (e.g., with screws) in case they must be rapidly removed in the event of a chimney fire. Review fire codes before installing flue covers. Dampers should be kept closed except during the heating season.
Oakum packs easily and firmly into small cracks. Other fillers include sponge rubber, glass fiber, knitted-wire mesh, and quick-setting putty. Self-expanding polyurethane foam applied from pressurized containers can be used for openings larger than 3 inches. It must be applied carefully so clapboards, shingles, and other surfaces are not lifted. Surfaces that are exposed should be sealed with epoxy paint to prevent insect infestation and ultraviolet degradation. Conventional draft sweeps (metal, rubber) and other weather stripping supplies (felt, vinyl, metal) will seal the space around windows.
Treat attic and basement doors whenever the gap exceeds ¼ inch. Flashing may be used to close gaps at joints (e.g., where the roof meets a chimney). Materials include galvanized metal, copper, aluminum, stainless steel, and self-adhesive stainless-steel “tape.” A potentially useful intervention for the wall-ceiling interface is the application of a wide 45° molding strip to eliminate the 90° angle corner and force the bats to roost in a more exposed area.
Insulation provides some barrier to bat movements. It is available in several forms and types including fiberglass, rock wool, urethane, vermiculite, polystyrene, and extruded polystyrene foam. Inorganic materials are fire and moisture resistant. The safest appear to be fiberglass and rock wool.
Exclusion techniques for doors. Image by PCWD.
The mesh size of screen must be small enough to prevent access of bats and other species. Hardware cloth with ¼-inch mesh will exclude bats and mice. Screen with 16 squares per inch will exclude most insects. Soffits (the underside of overhanging eaves) usually have vents of various shapes and sizes. The slots should not exceed ¼- x 1-inch and should be covered on the inside or outside with insect mesh.
Exclusion can be difficult on tile roofs. Bats, particularly Mexican free-tailed bats, often roost under Spanish or concrete tile roofing by entering the open ends at the lowermost row or where the tiles overlap. Tight-fitting plugs are difficult to make due to the variation in opening sizes and thermal expansion and contraction. A layer of coarse fiberglass batting laid under the tiles so that bats entering holes contact the fiberglass can be an effective barrier. A layer of knitted wire mesh also will work well for this purpose and will not hold moisture. Bats also may be excluded from the tiles if rain gutters are installed directly under the open ends. Gaps under corrugated and galvanized roofing may be closed with knitted-wire mesh, self-expanding foam (avoid causing roofing to lift), or with fiberglass batting (may retain moisture).
One-way doors are excellent for moving bats out of structures, when used outside of maternity season. A variety of 1-way doors and check-valves are available, and no single device is suitable for every situation. Install 1-way doors on holes that are actively used by bats to enter or exit the structure. One-way doors should be left in place for at least 3 to 5 nights before final exclusion. When weather is not conducive to bat flight (heavy and sustained rains, winds above 10 mph, temperatures below 50oF), leave 1-way doors in place longer. As with any exclusion intervention, the excluded animals will go elsewhere. The shift may be to an alternative roost already in use, such as a night roost, or to a roost used in previous years or to another opening in the house.
Netting with ¼-inch mesh will exclude bats from entrance openings.
Netting with ¼-inch mesh can be used to exclude bats. Photo by PCWD.
The basic design attaches netting (fiberglass mosquito netting works well) around an exit hole except at the bottom where the bats will escape. Designs must be open enough so they do not impede exiting bats.
Install netting as a check valve for excluding bats. Image by PCWD.
Do not lay netting flush against the wall, as this will prevent bats from exiting. Fold the netting to provide the exiting bats a little gap to move into and then down and out. The width and shape of netting check-valves is highly variable so they can cover the necessary exit points such as a single hole, a series of holes, or a long slit-like opening. The top can be much larger than the bottom. Restrict the bottom opening to no larger than 1.6 x 1.6 feet. The length of the netting (the distance from the lowest enclosed point of egress to the bottom of the netting) should be about >3 feet. The above specifications usually are sufficient to keep bats from re-entering the space. Once all bats have left the roost, the netting must be removed and final exclusion devices installed. When their routine exit points are blocked, bats may seek alternative exits, often causing some bats to find their way into living quarters of homes.
Tubes, such as the Batcone®, provide another tool to exclude bats. Center the tube hole over the exit used by the bats to provide an easy exodus.
Batcone® by Westchester Wildlife. Photo by Westchester Wildlife.
Frightening devices are not appropriate for the control of bats. Ultrasonic devices have been tested under natural conditions, both indoors and outdoors, to repel little brown and big brown bats both in the roost, and as they fly toward an entrance hole. Numerous ultrasonic devices have been removed from clients’ homes because the bats remained in the roost after the devices were activated. Little brown bats exposed to ultrasound in semi-natural roosts have shown little response. This lack of known scientific efficacy for ultrasonic devices has caused at least 1 state to caution against the use of such devices. Part of the concern is that such devices will provide consumers with a false sense of security and may prevent them from taking effective preventive actions.
Distress cries of bats that are recorded and rebroadcast can be used to attract other bats to nets or traps, but they do not serve as an effective repellent. Little brown and big brown bats respond to their own distress cries, but not to the cries of other species.
Many chemical aromatics and irritants have been proposed and tested for bat repellency, though efficacy has proven very limited. Naphthalene crystals and flakes are the only repellents registered by the Environmental Protection Agency (EPA) for the control of bats in attics or between walls. This repellent is not registered in all states. The chemical may be placed in loose-mesh cloth bags and suspended from the rafters. About 2.5 pounds per 1,000 cubic feet is recommended to repel bats chronically as the chemical vaporizes. Dosages of 5 pounds per 1,000 cubic feet may dislodge bats in broad daylight. Bats will return when the odor dissipates. Naphthalene vapors are flammable and the prolonged inhalation may be hazardous to human health. The area must be sealed so odors do not enter the living space, as naphthalene vapors may cause health problems for some individuals.
Contact repellents, such as sticky-type bird repellents and rodent glues, have been used successfully in situations where roost surfaces are coated. First apply masking tape to the surface if you plan to remove the repellent after treatment. Replenish contact repellents occasionally, because dust accumulation causes a decline in tackiness. Apply coatings that will be sticky but will not entrap bats. As with the use of naphthalene, this technique may not be legal in some states.
No toxicants are registered for the control of bats.
Shooting bats is not practical and it is not legal in many states.
Trapping of bats is controversial and does not follow AMPs. Bat Conservation International (BCI) does not promote WCOs who use bat traps. For purposes of WDM, exclusion is less complicated, less time-consuming, more effective, and requires no handling of bats. Only trap bats in emergency situations when required for public health reasons.
Occasionally, an individual bat will enter a home during the summer months when a door or window is left open. These situations are easy to deal with by allowing the bat time to find its way back out. Trying to chase a bat out of a room will only complicate the issue by distracting the bat from finding its way out and prolong the incident. The bat will usually circle a room several times looking for an exit. In a situation like this, it is best to close off the room and open all windows and doors leading outside. Do not leave the room, or if you must, leave the lights on and watch from a side door or window to see that the bat leaves.
If the bat tires and lands on the wall or curtain, you can capture the bat and release it outside without directly touching it. If you try to capture a bat in a house, you should wear leather gloves and use a container to trap the bat. A large plastic bowl and a stiff piece of cardboard work well. Place the bowl over the resting bat and slide the rigid cardboard between the bowl and the wall. If the bat is to be tested for rabies, the head must not be damaged.
Interview the client to determine if humans have been exposed to the bat. This is critical information. Review “Rabies Exposure” covered earlier in this manual. If you are certain there has been no exposure, while keeping a tight seal on the bowl with the cardboard top, carry the container outside. It is best to release the bat on a raised surface (tree trunk or wooden fence post) to avoid potential predators and help the bat take flight. Most bats prefer to drop from a perch to fly as opposed to taking off from the ground. It may take several moments for the bat to orient itself before flying off.
Find out the procedure to submit specimens for rabies testing prior to providing bat control services. You may need this information outside of normal business hours.
In emergency situations and when bats have landed in locations too difficult to place a ladder, attach a glue board onto your painter’s extension pole. After capture and removal of the bat to the outside, use vegetable oil to free the bat from the glue. Wearing leather gloves, handle the bat according to health department guidelines.
On-site Release (Relocation)
Bats can be released outside provided they have not contacted people or pets. Bats should be released on a tree at least 5 feet above the ground.
It is not legal to translocate bats in most states and we do not recommend moving bats. If it is legal in your state, drive at least 20 miles and release bats in an area with abundant insects (e.g., near lakes and ponds). Realize that this is not in line with AMPs, and the effectiveness of this method is questionable. Many bats often migrate hundreds of miles between summer and winter roosts.
Check federal, state, and local regulations regarding euthanasia of bats. Some bats are protected, and all are beneficial to the environment. In some states, WCOs may humanely dispatch bats. Bats should only be euthanized if they are ill or have bitten a human or domestic animal. Where euthanasia is allowed, use carbon dioxide or cervical dislocation.
Check state regulations regarding disposal of carcasses. All bats that may have been in contact with people or pets should be tested for rabies. Be careful to avoid damage to the head or brain of the animal.
After bats are excluded, repelled, or have departed at the end of the summer, measures must be completed to make re-infestation less likely, and to eliminate odor, bioaerosols, and risks from diseases and insects.
Safe handling and removal of bat guano has been discussed previously in this chapter. In addition to the bulky accumulations of excreta, deposits of guano may be found under and among insulation materials, caked urine and guano on roof beams, and splattered urine on windows. Such clean-up work during hot summer weather may be the least desirable activity of a management program, but it is necessary. Contaminated insulation needs to be removed and replaced.
All caked or crystallized bat urine and droppings should be scraped and wire-brushed from all roof and attic beams. For this procedure, workers should take the same precautions as outlined for histoplasmosis-related work. Accumulated excreta and contaminated insulation should be sealed in plastic bags and removed for disposal in accordance with state and local laws. Remove all remaining droppings and debris with a vacuum cleaner, preferably one that has a water filter to reduce the amount of dust that escapes in the exhaust or uses a vent to exhaust directly to the outdoors.
Wash all contaminated surfaces with soap and water. Allow the surfaces to dry and disinfect them by misting or swabbing on a solution of 1 part household bleach and 20 parts tap water. Ventilate the roost site to allow odors and moisture to escape. Installation of tight-fitting window screens, roof and/or wall ventilators in attics will enhance this process. Sanitation and cleanup accompanies bat-proofing and exclusion measures. It does not replace them.
Now that you understand methods to prevent and control conflicts with bats, think about how to use that information when you work with a customer. First, study the bats in the problem area. View them early in the morning, and again in the evening. This will tell you how many bats are present. Are they adults or juveniles? Are they resident bats, or are they migrating? Where do they eat and drink? What is attracting them to the area?
Next, make some decisions. Answers to the following questions will help you develop a responsible and effective control plan.
Are the bats a nuisance? Are they causing physical damage? Do they pose a health risk? Is exclusion or habitat modification possible? Are these actions practical? What are the costs of exclusion and clean up? If the bats disperse, where will they go? Should you build bat houses? What are the legal and public relations considerations? Are there disease or health problems? Do not frighten clients by exaggerating health and safety issues.
The NWCOA Bat standards recommend that:
- Inspection reports and/or service agreement/contract/proposals may also contain recommendations for post inspections/monitoring/bat watches or further evaluation by other industry professionals.
- Inspection reports and/or service agreement/contract/proposal may also contain photo documentation. This documentation shall be of all of the inspected areas of the structure with current and/or potential bat entry points, exterior roosts, structural defects, and contaminations as evidence/proof to the client of the issues associated with the bat and to protect the inspector from any potential liabilities.
- Inspection reports and/or service agreement/contract/proposal shall contain a description of the warranty, its limitations, and its time period. Warranties shall not start until the final eviction/venting devices are installed on the structure, or 100% eviction is confirmed by the WCO and the client. Warranties may be limited to colonies of bats entering the interior of the structure. Warranty time periods shall not be less than one year and shall not exceed the manufacturer’s minimum service life of any exclusion material used on the structure. Warranties must not exceed the ability of the installing company or WCO to service said warranty.
- Inspection reports and/or service agreement/contract/proposal shall contain a price to exclude and evict/vent the bats from the structure. Price shall reflect the company’s or WCO’s ability or experience to perform the bat removal services based upon these standards, material costs, labor, and profit.
As a WCO, you have an important role, not only in preventing and controlling damage caused by bats. You also are an important source of science-based information to the public. The following section provides more information concerning why bats are facing problems, and what can be done to reduce those threats.
Two relatively new impacts to bat populations are wind energy development and white-nose syndrome (WNS). Loss of habitat or disturbance at important sites, is a long-standing threat to bats, as for many other wildlife species.
With increasing demand for renewable energy, development of wind energy facilities has grown considerably in the US. Unfortunately, a high number of bat fatalities have been documented at several major wind facilities. These fatality rates have raised concern among scientists regarding the cumulative effect of operating large numbers of turbines over their 20- to 30-year life spans.
On a positive note, research has revealed that the vast majority of fatalities occur at low wind speeds during the bats’ migration period. Curtailing turbine operations during night time hours under certain seasonal and weather conditions may be an effective way to reduce fatalities due to wind turbines.
Little brown bat with white-nose syndrome. Public domain photo by Marvin Moriarty, USFWS.
The fungus Pseudogymnoascus destructans has been associated with the disease known as white-nose syndrome (WNS). It has killed an estimated 5 to 6 million bats in eastern North America since 2007. The disease is named after the white fungus that is often detected on the muzzles (noses) of infected bats. This psychrophilic (cold-loving) fungus grows best at temperatures below 70°F, with ideal growing conditions found in hibernacula, especially the caves in which many colonial bats live.
It’s uncertain how the disease kills the bats, but bats with WNS exhibit wing damage that affects body fluid levels leading to dehydration. The loss of fluids and irritation due to fungal growth cause bats to arouse more often during hibernation, using up more of the precious body fat needed to survive the winter.
Wing damage from white-nose syndrome.
Photo by Rick Reynolds
WNS is extremely harmful to colonial bats. At many hibernacula in the Northeast, fatality rates exceed 90 percent. At hibernacula where WNS has been present for 3 years or more, little brown bat numbers have declined by over 98 percent, and tri-colored bats have declined by over 90 percent. As of October 2017, WNS has been found as far south as Mississippi; north in Ontario and Quebec, Canada; and west in Nebraska, Texas, and Oklahoma, with some found in Washington State.
With the expanded movement of WNS, we are seeing great reductions in bat numbers, raising the likelihood of extirpation of local and regional populations. Some biologists suggest possible species extinctions. Research continues to better understand and monitor this disease with the goal of controlling or eliminating it.
USGS Map showing the occurrence of WNS as of fall 2017.
USGS Map showing the occurrence of WNS as of July 2018. This map is constantly being updated. https://caves.org/WNS/
Many caves in the eastern US are closed due to the occurrence of WNS or the potential for contamination. Strict equipment decontamination protocols are strongly recommended.
Decontamination is extremely important to protect bats and their habitats. It is the primary management option available at this time to reduce the risk of spreading WNS. Current recommendations focus heavily on treating equipment in cave and mine habitats. In addition, WCOs should realize the risks associated with bat-related work. WCOs should follow the National Decontamination protocol to minimize their risk of transmitting the fungus when working in close contact with bats, their environments, and/or associated materials. For the most recent WNS decontamination protocol, see https://www.whitenosesyndrome.org
For pesticides registered by the EPA, “the label is the law.” This includes anti-microbials used for disinfection. Label directions indicate the types of materials (e.g., porous vs. impervious surfaces) that can be treated. Safety Data Sheets provide important information on the pesticide products as well. Some WCO equipment may need to be cleaned according to the manufacturer’s specifications and then, where permissible by the manufacturer’s guidance, be decontaminated following the WNS protocol.
In addition to items mentioned in the National Decontamination Protocol, consider disposal or decontamination of items used by WCOs that frequently come in contact with bats or their habitat. These include cones, tubes, chutes, and mesh used to construct 1-way doors.
Associated with the loss of habitat such as caves, is the increase in disturbance at those sites. Bats enter hibernation with enough stored fat reserves to carry them through the winter months. During hibernation, bats reduce their body temperature and basically shut down their body functions to conserve fat supplies. When bats are disturbed during hibernation, they must raise their body temperature to wake and flee the disturbance. This uses fat that is essential for their winter survival.
During the maternity period, females need large energy reserves to raise their young. Disturbance at maternity sites may cause them to move their young to other sites, or to abandon their young altogether.
Several agencies and organizations in the US are very active in the research, management, and conservation of bats. Reducing disturbance, and increasing habitat are ways to enhance bats populations.
Collecting a wing punch from a Virginia big-eared bat for genetic studies.
One of the primary management activities has been the protection of bat hibernacula and maternity sites through gating of cave entrances. The gating of caves keeps people from disturbing bats during hibernation and maternity periods by not allowing access to the site. Thick angle-iron bars are constructed across the entrance with enough space to allow bats to fly through, but not large enough to allow human access to the cave.
Cave gate at an Indiana bat hibernacula.
In some states, most of the endangered Indiana and gray bat sites have been gated to reduce disturbance. This was accomplished through the cooperative efforts of state and federal agencies, non-government organizations, local conservation groups, and interested citizens.
Bat houses are a great way to enhance local populations and to mitigate the removal of a large bat colony from a structure. Bat houses can be placed in yards or around the home where they will not impinge on daily human activities. Because bats are major night time predators of insects, they will provide a natural form of insect control.
Some of the early work with bat houses resulted in mixed success, but years of research have documented what makes a good bat house. The primary elements to consider in building and installing a bat house are design and placement.
This small bat box could house up to 100 bats.
Design considerations include size, color, and building materials. One of the first “bat houses” was simply heavy tar paper or corrugated metal wrapped around the trunk of a mature tree. This is an effective way to provide roosting or nesting habitat for bats but may not look as nice as a manufactured house. If there is a large tract of land and you want to enhance or maintain the bat habitat, however, this is a cost-effective approach.
A small maternity colony box (12″ x 24″ x 8″) can attract up to 150 bats. If this seems too large for the setting, halving the box width can downsize the box. As the box gets smaller, the chances of occupancy diminish. A large maternity colony box (18″ x 24″ x 8″) can handle from 150 to 300 bats.
The “rocket box” is a newer design that appears to be well-liked by bats. This design is more compact than the traditional bat houses and can be placed in most settings. Rocket boxes are typically made of a 4″ x 4″ post and 6″ x 1″ boards with spacers to provide openings. The boxes are sealed at the top to prevent rain from entering the box.
Bat houses should be made of exterior grade plywood, cedar, or redwood. Do not use pressure-treated wood. Construct bat houses with multiple baffles to provide ample roosting space. Baffles should be spaced 3/4 to 1 inch apart with horizontal grooves to provide footholds. Hardware should be galvanized, and using screws as opposed to nails will increase box longevity. Finally, all seams should be caulked. A variety of good bat house designs can be found on the internet (see Additional Resources).
The first consideration is the correct orientation of sunlight (provides heat for the bats), and to minimize disturbance or traffic by humans or pets. To further reduce disturbance, fence or plant low shrubbery around the box.
You need to consider the box’s exposure to sunlight. The bat house should be oriented to maximize sun exposure (preferably 8 hours or more). Generally, this means a southern exposure is best. In addition, boxes painted a dark color (use water-based paint) will absorb more solar radiation than lighter-colored boxes or natural wood. An alternative to paint is to put black roofing paper on the upper section of the box. Because there will be variation in the amount of solar radiation a box receives, bat boxes should be constructed with vents to facilitate a temperature gradient in the box. This allows the bats to move within the box to locate the temperature that suits them best.
Placement height of the box is another factor. Boxes should be placed at least 12 feet above the ground, and preferably around second floor height (approximately 20 feet). Boxes can be attached to the side of a house or chimney, or on a pole. Boxes should usually not be attached to trees, as they will likely not receive sufficient sunlight.
Plan when to erect your bat box. Bats generally leave their winter roosts for summer sites in April. Having your box out before bats leave their hibernacula will provide greater exposure to wandering bats and increase your chances of success. If you’re late getting your box built, go ahead and put it where you expect to have the greatest success. It may take a couple of years for bats to accept the box.
Finally, research has established that maternity sites are usually located within 1/4 mile of a natural water resource. Providing a water resource often is not practical, so if you are unaware of a nearby water resource, build the bat box anyway. The bats may know of a water source you weren’t aware of.
Like any other wooden structure, bat boxes will need limited maintenance. The small 3/4-inch openings should help to exclude wasps. However, if wasps occupy the box, they should be cleaned out during the winter months when they are inactive. Repairs, if necessary, should be made when bats have left for winter hibernation.
Along with bats come bat droppings, or “guano.” This should not be a problem if you place the box where droppings can accumulate on the ground until the bats leave in the fall. At that time the droppings, if substantial, can be gathered up with a shovel and placed in a compost pile. The bat droppings make excellent fertilizer.
calcar: A spur or spur-like projection, such as one found on the base of a petal or on the wing or leg of a bird or bat.
coniferous: Any of various mostly needle-leaved or scale-leaved, chiefly evergreen, cone-bearing gymnosperm trees or shrubs such as pines, spruces, and firs.
dimorphic: Existing or occurring in 2 distinct forms; exhibiting dimorphism (a dimorphic crystal; dimorphic organisms ).
echolocation: Process of emitting high-frequency sounds and interpreting the reflected sound waves. Typically used by an animal to orient itself or identify objects.
eviction: use of 1-way doors and exits to remove bats from a structure by using their natural tendency to leave the roost at night.
exclusion: closing gaps and sealing holes to prevent bats from entering or re-entering a structure.
extirpate (extirpation): To remove or destroy completely; exterminate. In wildlife biology, the term is used to indicate the extinction of a species in a given area or geographic region, but not from its total range.
gestation: The period of development in the uterus from conception until birth; pregnancy.
glean (gleaner): To harvest grain left over after reaping. In wildlife biology, the term is used to indicate foraging from a surface (leaves, bark, ground, etc.) as opposed to foraging while in the air.
guano: A substance composed chiefly of the dung of sea birds or bats, accumulated along certain coastal areas or in caves and used as fertilizer.
hibernaculum (pl. hibernacula): The shelter of a hibernating animal. In the case of bats, it usually indicates a cave or mine, but could be a rocky crevice or tree cavity.
hibernate (hibernation): A period of inactivity, usually in the winter, in which normal metabolic processes and body temperatures are greatly reduced.
karst: An area of irregular limestone in which erosion has produced fissures, sinkholes, underground streams, and caverns.
mammae: A milk-secreting organ of female mammals.
ovulation: To produce and discharge eggs from the ovary or ovarian follicle.
parturition: The birth of offspring in most mammals.
riparian: Pertaining to the bank or shore of a stream , river, lake, or other water forms.
tragus: Prominent lobe extending from the base of the ear, the function of which is unknown, but believed to relate to echolocation.
venting: See exclusion.
Will bats fly into your hair?
An old misconception people have is that bats will fly into your hair and lay eggs! The origin of this rumor may come from bats foraging low, just over the heads of individuals. While it may look as though these bats are out of control and trying to fly into someone’s hair, the reality is that these bats are in complete control chasing insects that are in evasive mode trying not to be caught. When you consider that a bat can pick a mosquito out of the air while on wing, then they certainly can avoid an object as big as our heads. As to where the egg-laying part of the rumor came from, we’re at a loss!
Are bats blind?
While “blind as a bat” remains a popular phrase, it lacks credence. It would have been more accurate to say “blind as a mole,” but such colloquialisms don’t die easily. Despite their small eyes, bats see perfectly well. However, their sense of echolocation is a more reliable means of finding prey and navigating in the dark, and is very highly developed. When you pursue food as small as a mosquito or no-see-‘um, then you’d better put every sense you have to use.
Are there vampire bats in the US?
There are 3 species of vampire bats that occur in Mexico, and in Central and South America (none occur in the contiguous United States). Vampire bats feed on the blood of other animals, but they do not kill their food source. Vampire bats use their razor-sharp teeth to make a small incision, and then they lap up, rather than suck, the blood of their host. They lap blood from open wounds on animals. Some bat species specialize in feeding from birds, while others prefer mammals.
Do all bats have rabies?
While bats can contract rabies, they are no more likely to test positive than other rabies vector species. In an Ohio study of animals submitted for rabies testing, 3.5% of bats, 11.9% of skunks, 3.1% of deer, and 3.1% of horses tested positive. Regardless of prevalence, any wild mammal showing abnormal behavior should be treated with caution and the proper authorities contacted.
Other phrases contributing to the poor popular image of bats include “dingbat,” “old bat,” “bats in their belfries,” and “just plain batty.” While these phrases are less than complimentary, they do not portray bats correctly. Hopefully, what you have gained from our discussion on bats is that they are an important component of our natural environment. Without bats in this world, a domino effect of problems, ranging from insect outbreaks to loss of certain plant species, could be anticipated. So, next time you’re talking with your client and see bats flying overhead, spread some of your knowledge and help dispel the unflattering myths surrounding this fascinating and important group of mammals.
Material updated and adapted from the book, Prevention and Control of Wildlife Damage, 1994, published by the University of Nebraska–Lincoln Extension.
Bat species and conservation information are from Reynolds, R. J. and R. T. Fernald. 2015. A Guide to the Bats of Virginia. Special Publication Number 5. VDGIF. Bureau of Wildlife Resources.
White-nose Syndrome Conservation and Recovery Working Group. 2015. Acceptable Management Practices for Bat Control Activities in Structures in Georgia – A Guide for Nuisance Wildlife Control Operators. U.S. Fish and Wildlife Service, Hadley, MA at https://www.whitenosesyndrome.org/sites/default/files/resource/wns_nwco_amp_1_april_2015.pdf
- Michael Mitchell of Alabama;
- Reginald Murray of Oklahoma Wildlife Control, LLC;
- Eddie Hope of Virginia Professional Wildlife Removal Services, LLC;
- Mike Mengak of Warnell School of Forest Resources, University of Georgia.