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Bats may enter the living space where they can pose a disease risk. In addition, their droppings and urine can contaminate and deface surfaces.
Familiarity with the appropriate federal and state laws should precede any nuisance management activities. The lethal control of bats, even when a proven potential danger to humans exists, 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.
Bats are the only mammals that can truly fly. The ability to fly, secretiveness, and nocturnal habits have contributed to bat folklore, superstition, and fear. About 1,100 species are distributed worldwide, second in number only to rodents among mammals. Among the 40 species of bats found north of Mexico, only a few cause problems for humans. Vampire bats are not found in the US or Canada.
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 northeast are colonial and include little brown bats (Myotis lucifugus, Figure 1), and big brown bats (Eptesicus fuscus, Figure 2).
Solitary bats typically roost in tree foliage or under bark but occasionally are found in buildings, usually as transients 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 herein can be found at Bat Conservation International and the University of Michigan’s Animal Diversity Website.
While species characteristics can differ greatly, control methods do not.
Little brown bats have glossy fur that can be dark-brown, golden-brown, reddish, or olive-brown. The fur on the belly is lighter than the fur on their back. Wings and membranes between the legs are dark brown or black, and have almost no hair. Little brown bats have small ears and large hind feet. The hind foot has hairs that extend past the toes. These bats are tiny, and weighing between 2/10 and ½ ounce. They are between 2½ and 4 inches long, and have a wingspan between 8½ and 10½ inches. Females are larger than males, especially during the winter.
As for the little brown bat, the big brown bat’s name is highly descriptive. Its fur is uniformly medium to dark brown on the upper parts, with slightly paler under parts. The fur is relatively long and silky in appearance, compared to other bats. The ears and wing membranes are dark brown. The species is larger in size than little brown bats, from about 4 to 5 inches in body length, with an 11-13 inch wingspan, and weighing 1/2 to 5/8 ounce.
Little brown and big brown bats are found throughout the northeast. Numbers of bats have declined dramatically during the past decade due to white-nose syndrome (WNS) caused by a fungal infection at their winter cave roosts. The US Fish and Wildlife Service estimates that more than 5 million bats have died from contracting WNS.
Health and Safety Concerns
Bats are associated with 2 important diseases that can be transmitted to humans, rabies and histoplasmosis. These diseases can be avoided, and should not be used as an excuse to kill bats. Although less than one-tenth of 1% of all bats has rabies, the percentages increase to less than 5% for bats that interact with people and animals, and are tested.
If a bat has possibly contacted a person or pet, it should be captured for rabies testing. This includes finding a bat in a room with a sleeping person, a previously unattended child, or a mentally disabled or intoxicated person. In these situations, it is very important to catch the bat using thick gloves, a towel, or a box that can be placed over the bat (avoid any direct contact). Tell the client to avoid damaging the bat’s head during capture, as brain tissue is needed for the test. Do not release the bat. Contact local health officials for additional information on rabies treatment and testing. If the bat is not available for testing, consult health professionals about needed treatment for people or pets.
Histoplasmosis is a fungal disease that is associated with guano from bats. The fungus grows best in dark and humid areas (e.g., attics) with large accumulations of guano. Inhalation of fungal spores is the primary mode of infection. Spores become airborne when guano is disturbed. Do not remove large guano deposits without appropriate training and protection. To learn more read the online document published by the Department of Health and Human Services, Histoplasmosis-Protecting Workers at Risk (Publication number 2005-109).
Bats generally mate in the fall and winter but females retain sperm in the uterus until spring, when ovulation and fertilization take place. In the northeast, pregnant females usually congregate in maternity colonies in buildings.
Birth typically occurs from early May through July. Young bats grow rapidly and are able to fly within 3 weeks. Weaning occurs in July and August, after which nursery colonies disperse. Young bats in the northeast should be able to fly by mid-August.
Pregnant females usually congregate in building attics or bat boxes. High temperatures are needed for rapid development of their young. Bats do not build nests.
Bats prepare for winter around the first frost. Big and little brown bats migrate relatively short distances, usually less than 150 miles. Bats in the northern US and Canada may hibernate from late September through late April. Big brown bats in the northeastern US may become active in buildings during warm, sunny days in winter.
Bats tend to inhabit structures that have significant exposure to sunlight, are large, and are within ½ mile of a fresh water source.
Bats in the northeast feed on a variety of flying insects. Many of the insects are crop pests. Although there are some limitations such as body size, flight capabilities, and jaw opening, insectivorous bats apparently consume a wide range of prey.
The diet of little brown bats includes mayflies, midges, mosquitoes, caddis flies, moths, and beetles. An individual can consume insects equal to ⅓ its body weight in ½ hour of foraging. A big brown bat may fill its stomach (roughly 0.1 ounce) in about an hour with prey including beetles, moths, flying ants, true bugs, mayflies, caddis flies, and other insects. A colony of bats can eat an extremely large number of insects each night.
Voice, Sounds, Tracks, and Signs
Most bats emit high frequency sounds (ultrasound) inaudible to humans and similar to sonar. This allows them to avoid obstacles, locate and capture insect prey, and communicate. Bats also emit audible sounds that may be used for communication between individuals.
Damage to Landscapes
Bats do not damage landscapes although their droppings may accumulate around extremely large colonies.
Damage to Crops and Livestock
Bats do not damage crops. Bats that are infected with rabies can transmit the disease to pets and livestock during encounters.
Damage to Structures
Bats may live in attics and walls. 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 also indicate the presence of mice, rats, or even roaches. Because house bats in the northeast are insectivorous, their droppings are easily distinguished from droppings of small rodents. Bat droppings tend to be segmented, and elongated (Figure 3). When crushed, they become powdery and reveal shiny bits of undigested insect remains. In contrast, droppings from mice and rats tend to taper, are not segmented, are harder and more fibrous, and do not become powdery when crushed (unless they are extremely aged).
Damage Prevention and Control Methods
Prevention is the best way to avoid having bats in your home. Fortunately, bats cannot create their own entry holes. They do not gnaw like mice and rodents. Therefore, it is important to seal all cracks, plug all holes, and use good screens and tight fitting doors prior to having a problem. Hardware cloth (¼-inch or smaller mesh), caulk, sealant, and weatherproof foam strips are excellent materials for excluding bats. Foam insulation will degrade under UV light and is not recommended. Install a stainless-steel chimney cap with ⅜-inch wire mesh to reduce access by bats. Never secure an opening unless the client is certain it is not being used by wildlife.
In outdoor settings, swap white light bulbs for bulbs less attractive to insects. Increased lighting has been reported to be effective at moving bats out of structures. Where possible, add windows to brighten an attic and reduce the desirability of the roost site. 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 be an electrical hazard. All wiring should be done by a qualified electrician. 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 people 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.
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 option for eliminating and preventing bats from residing in structures. It is tedious, but important, to locate all active and potential openings available to bats. Conduct a bat watch at dusk to determine where bats are exiting and entering a building. You can also identify active holes by rub marks, guano, and sometimes odor. Except for the actively used holes, seal all gaps of ¼ x 1½ inches and openings ⅝- x ⅞-inch or greater. Bats use some of the same holes in buildings through which heat (or cooled air) is lost.
Install one-way doors on holes that are actively used by bats to enter or exit the structure. Timing is important to reduce the risk of separating adults from flightless young. In the northeast, do not install one-way doors from May 1 through August 15. One option is to seal unused holes, but leave active holes open until the exclusion date is past. After installation, leave one-way doors in place for at least 5 days. During periods of inclement weather (e.g., rain), leave the doors in place longer.
Screening and netting with ¼-inch mesh will create a check-valve and exclude bats. Tubes, such as the Batcone® (Figure 4), provide another tool to create a one-way door. Center the tube hole over the exit used by the bats to provide an easy exit.
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. Some bats may shift to an alternative roost already in use, such as a night roost, or to a roost used in previous years.
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 (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 tolerate temperature extremes.
Conventional draft sweeps (metal, rubber) and other weather stripping supplies (felt, vinyl, metal) will seal the space between a door bottom and the threshold or around windows.
Treat attic and basement doors whenever the gap exceeds ¼ inch.
Use flashing 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.”
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.
Gaps under corrugated and galvanized roofing may be closed with knitted-wire mesh, self-expanding foam (but avoid causing roofing to lift), or with fiberglass batting (may retain moisture).
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. Do not attach these permanently; they may need to be removed rapidly in the event of a chimney fire. Review fire codes before installing flue covers. Keep dampers closed except during the heating season.
Frightening devices are not appropriate for the control of bats.
Numerous ultrasonic devices have been removed from 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.
Recorded distress cries of bats can attract bats to nets or traps, but 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.
No repellents are registered for use on bats.
No toxicants are registered for use on bats.
Shooting bats is not practical and is illegal in many states.
Trapping of bats is controversial and should only be performed by experienced individuals with rabies pre-exposure immunization.
Release bats outside provided they have not been in contact with people or pets. Place the bat at least 5 feet above the ground on a tree or ledge. Bats need the height to obtain the lift needed to fly away.
If legal in your state, drive at least 20 miles and release bats in an area with abundant insects, such as near lakes and ponds. Effectiveness of this method is questionable and many bats often migrate hundreds of miles between summer and winter roosts.
A carbon dioxide chamber is an excellent way to euthanize bats. It will not invalidate rabies testing if such testing is required.
Resources and Acknowledgments
Government or private agencies, universities, extension service
Bat Conservation International at http://www.batcon.org/index.php/all-about-bats/species-profiles.html)
University of Michigan Animal Diversity at http://animaldiversity.ummz.umich.edu/site/accounts/information/Chiroptera.html
- Bat Management in AlabamaPDF – Bat Management in Alabama
- Bats In and Around Your HomePDF – Arkansas: Bats In and Around Your Home
- UC IPM: BatsPDF – California – UC IPM: Bats
- Bats in BuildingsPDF – Florida: Bats in Buildings
- Indiana BatPDF – Indiana Bat
- Big Brown BatPDF – Kansas – Big Brown Bat
- Townsend’s Big Eared BatPDF – Kansas – Townsend’s Big Eared Bat
- Bats Information for Kentucky HomeownersPDF – Kentucky – Bat Information for KY Homeowners
- Got Bugs Get BatsPDF – Maryland – Got Bugs Get Bats
- Coping With Bats in HomesPDF – Montana – Coping with bats in MT homes
- Eastern Red BatPDF – New Hampshire – Eastern red bat
- Indiana Bat in New HampshirePDF – New Hampshire – Indiana Bat
- Indiana Bat in New JerseyPDF – New Jersey – Indiana bat
- Life History Notes: Big Brown BatPDF – OH – Life History Notes: Big brown bat
- Life History Notes: Indiana BatPDF – Ohio – Life History Notes: Indiana bat
- Living With BatsPDF – Oregon – Living with bats
- Bat in Rhode IslandPDF – Rhode Island – Bats
- Colonial BatsPDF – South Carolina – Colonial Bats
- Gray BatPDF – USFW – Gray bat
- BatsPDF – USDA – Bats
- Living With BatsPDF – Washington – Living with bats
- Bats Information for Wisconsin HomeownersPDF – Wisconsin – Bats information for homeowner
- Bats in West VirginiaPDF – West Virginia – Bats
- Bats Information and Management Techniques for Wyoming HomeownersPDF – Wyoming – Bats information and management techniques for homeowners
- Bats in Pennsylvania PDF – Pennsylvania – Bats
Bat, Bats, NWCO, wildlife control, wildlife damage management