Case Studies Illustrating the Complex Dynamics of WDM

Case Studies Illustrating the Complex Dynamics of WDM



The following three case studies illustrate the issues and challenges involved in resolving human-wildlife conflicts. While these case studies are composites and do not necessarily reflect a specific incident, they represent real occurrences. We have selected these three species based on the frequency and diversity of issues they raise. Resolving these concerns will illustrate the overall process for implementing wildlife damage management (WDM) approaches.

As you review the case studies, keep in mind the material that was presented in the Core modules. Apply the basic principles of WDM and Integrated Pest Management (IPM, Figure 1). Be conscious of safety and protect yourself from zoonotic diseases. Identify the problem wildlife species. Determine what animal control techniques to use and if there will be extensive animal handling. Will euthanasia be required? Will there be any ethical or legal considerations?


Figure 1. Parts of the IPM Strategy

Remember that successful IPM solutions contain the following elements:

  1. identify the pest,
  2. locate and monitor the pest or damage,
  3. determine the legal status of the pest and obtain necessary permits,
  4. choose prevention or control methods,
  5. set an economic or damage threshold, and
  6. evaluate the success of the action.

Case Study #1: Gray Squirrels in an Attic

As a group, tree squirrels (e.g., gray, fox, red, and flying squirrels) frequently cause human-wildlife conflicts. Their climbing ability, high reproductive rate, and gnawing power make them a challenge to control. You have just received a call from a homeowner complaining about squirrels in the attic and she is asking your help on how to deal with them. Follow the steps below to help diagnose the problem and make a recommendation.

Diagnosing the Culprit

Question 1: Determine where the client lives. For example, if the client is in eastern New York State, you know that the only squirrels living there are gray, red, and flying squirrels. In far western New York, fox squirrels may be present in some locations as well.

Question 2: Determine what kind of habitat the client lives in. For example, gray squirrels can be almost anywhere in New York. But red squirrels prefer areas with stands of evergreen trees and flying squirrels tend to live in old forested areas.

Question 3: Ask the homeowner how she knows that squirrels are in the house. If she has seen the squirrels entering a hole in the attic, that is helpful. However, often the client says she hears scratching or scurrying noises in the ceiling, and just assumes it is caused by squirrels because they are too loud to be mice. The client could be correct. However, noise is a difficult sign to evaluate because people “hear” things differently.

Question 4: Ask the client to walk around the house and look at the eaves, roofline, and vents for holes ½ inch or larger (Figure 2). If you are confident that red or flying squirrels are not present, then have the client look for holes 1½ inches wide. Ask if tree branches are within 4 feet of the roof line or if they have noticed squirrels on the roof, or walking power lines to the house.

Question 5: If the client cannot locate a hole large enough for a squirrel to enter, should you recommend that she enter the attic? We suggest that clients enter attics only if they have proper respiratory protection and are physically capable of looking in the attic safely. In our experience, about 90% of squirrel holes can be identified while standing on the ground outside of the structure. Another 5% can be identified from the attic, and the remainder requires inspection with a ladder. If the person is not capable of entering the attic safely for inspection, have her call a wildlife control professional.

Gray Squirrel Hole in Soffit

Figure 2. Gray squirrel hole in a soffit.
Photo by Stephen M. Vantassel.

Potential Solutions

Once the wildlife species causing the problem has been positively identified, you must explain the various control options.

Question 6: Does the client want non-lethal control, or the animal killed and removed?

Many clients will say they want non-lethal control. Various options include:

Frightening devices – Clients may ask about whether radios, strobe lights, or effigies will convince squirrels to leave. You need to explain that the effectiveness of these devices is mixed. For instance radios and strobe lights can cause animals to move, but the squirrel may simply move to another part of the house, damaging more insulation along the way. Research has not demonstrated that ultrasound and similar devices are effective for evicting squirrels from locations.

Repellents – Clients will ask whether mothballs or ammonia are effective on evicting squirrels. Both materials are noxious to mammals. The problem with their use is the legality. These home remedies cannot be recommended by Extension staff or volunteers. Also, application of the repellents at sufficient strength to cause the animal(s) to leave the building may be hazardous to some people.

Translocation – Capture and translocation may be illegal in your state unless it is conducted by a licensed wildlife control operator. Otherwise, you should inform the client that translocation has numerous humane and environmental issues. Translocation may just move a problem to another building, and survival of animals moved out of their home range is often poor.

One-way doors – One-way doors allow squirrels to leave a structure but not return. Although simple in concept, one-way doors require some careful consideration. First, is there a possibility that young squirrels are present that could be abandoned (e.g., March-April or August-September rearing seasons)? Second, is the structure hardened enough to reduce the likelihood that the evicted squirrels would re-enter elsewhere? If not, will the owners be able to screen vents, seal unused holes, and repair weak boards or soffits prior to installing the one-way door? If not, then the one-way door should not be used.

Question 7: If the person chooses lethal control, there are also several options. As for using one-way doors, killing nursing females will result in the young perishing. This is inhumane and should be avoided.

Shooting – Shooting is a very effective method for the lethal control of squirrels. In most situations, laws governing or prohibiting the discharge of firearms in suburban areas will eliminate the use of this technique. Homeowners should note that air rifles and other projectile-shooting devices will also be treated as “firearms.”

Trapping – Trapping is the most common method for removing squirrels from structures. Because in this case the problem is caused by gray squirrels, the owner has several options. We suggest that cage traps are the best tool for non-professionals. We have broken down the process into several steps.  Always be safe and wear gloves when handling traps. Have a plan for dealing with captured squirrels before catching the first animal. Remember, translocation may not be legal in your state. Follow these steps for catching squirrels in cage traps.

Step 1- Determine the travel route used by the squirrels to enter the house. Typically, squirrels will climb a tree, walk on a branch, jump to the roof, and scurry directly to the hole. Select a spot on that trail to set your trap(s). Use multiple traps to remove animals quickly. Location is important because the goal is to capture the problem squirrel(s), not all the squirrels in the neighborhood. Do not approach or trap near power lines unless they have been properly shielded by the power company. Place traps as close to the entry hole as feasible. Make sure traps set at heights are securely anchored to prevent injuries to people below. Trap sizes should be at least 5 x 5 x 18 inches, and ½- x 1-inch mesh is preferred as it reduces likelihood of squirrels getting scraped noses.

Step 2- Remove bird feeders and other sources of food that could interfere with the trapping process.

Step 3- Bait traps with peanut butter and/or sunflower seeds. Spread bait on the treadle. Cover the back half of the cages with cloth, cardboard, or other material to provide a sheltered area. In summer, include sliced apples for moisture.

Step 4- Monitor traps daily. Ideally, check traps in the morning and evening. Euthanize squirrels with methods allowed in your state. Re-bait and return traps to their original location. Plug the entry hole used by the squirrels with newspaper to monitor movement through the hole. Replace paper as needed.

Step 5- Continue trapping until the newspaper has not been moved from the entry hole for at least 5 days of good weather. Clients should not hear squirrel activity in their walls or attic.

Step 6- Secure the entry hole by filling it with expanding foam and covering it with aluminum flashing or solid wood. Paint to match the building or trim.

Preventing Future Problems:

Conflicts with squirrels are preventable. The following list contains tips on preventing squirrel problems on your property. Implement as many of the suggestions as possible for best results.

  1. Trim tree branches near the roof line. Ideally, branches should be 10 feet from the building. Shrubs should be no closer than 4 feet.
  2. Modify bird feeders to prevent squirrels from accessing food. Remove bird feeders during the summer months if possible.
  3. Install professionally-manufactured chimney caps on all flues.
  4. Install protective metal screening on all passive attic vents. Depending on the situation, use ¼-inch weave hardware cloth or professionally manufactured screens.
  5. Never screen a motorized fan vent from the inside. Always protect them from the outside to prevent animals from placing nesting material on the motor.
  6. Check the roof and gutter line of the structure for gaps. Secure gaps with sealant, wood, or metal flashing as soon as possible to prevent squirrels from enlarging them.


Case Study #2: Bats in Structures

Bats in structures present special challenges to property owners. Bats can transmit zoonotic diseases, such as rabies and histoplasmosis, as well as cause structural damage through their feces and urine. Many people have phobias about bats due to their association with Halloween. You have just received a call from a homeowner complaining of bats. Your first task is to determine the type of problem, and to rule out the possibility that the client, a family member, or a pet has been exposed to rabies.

Potential for Rabies Exposure

If there is a possibility that a bat has made contact with a person or pet, capture the bat for rabies testing (Figure 3). Possibility of contact 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. Wear thick gloves. Place a towel or box over the bat to avoid any direct contact. 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 treatment needed for people or pets.

Rabies Treatment Protocol

Figure 3. Rabies treatment protocol. Image by Department of Health and Human Services, Nebraska.

Exterior Bat Issues

Problems occur when bats are found on the outside of the structure either flying or resting in unscreened porches, behind shutters, or flying around lights. You can direct bright light at the resting site to encourage bats to abandon the location. Bats roosting under over-hangs may be moved by shining bright lights at the roost site. Ultrasound devices have not been proven to be effective in deterring bats. Remove or secure shutters away from buildings to prevent bats from roosting behind them.


Turn off exterior lights to prevent flying insects from gathering. If light is needed for safety, change the bulb to a yellow “bug light” to reduce the number of insects attracted to the area.


Consider adding a motion sensor to reduce the duration of illumination.


Bats inside a Building

Bats are sometimes found in the living space of a home. Bats occasionally find their way into houses in the early evening, when seeking shelter, or feeding on insects attracted by doorway lights. If there is a possibility that the bat has made contact with a person or pet, do not release the bat (see Figure 3). Capture the bat following the instructions below and contact your local health department as soon as possible.

Discuss with client how the bat may have entered the living space. Options include an open damper in the fireplace, an open furnace vent, open window, door or screens with a gap, poor sealing to attic areas or pipe chases, etc. Seal all openings that could be potential entryways to prevent further problems.

Individual Bat Capture

Fortunately, these incidents can be dealt with quite easily. A bat flying inside the house usually will circle a room several times in search of an exit, and may find its own way out. Resist the urge to chase or swat at the bat, as doing so will cause it to panic and fly erratically around the room, making capture more difficult. Follow the steps below, and remember to never handle a bat, or any other wild animal, with your bare hands.

  1. Remove pets or children from the room.
  2. Put on a pair of leather gardening or work gloves.
  3. Shut all doors leading into other rooms to confine the bat to as small an area as possible.
  4. Watch the bat and wait for it to land. Then place a container, such as a large plastic bowl or coffee can, over the bat as it rests on the wall. At this point, the bat is probably exhausted and disoriented, and will not fly as you approach it.
  5. Slide a piece of rigid cardboard (if unavailable, use a magazine, or a lid from the coffee can or bowl) between the container and the wall to trap the bat. Hold the cardboard firmly against the container and carry the container outside.
  6. If the bat is located in a hard-to-reach place, such as the corner of a room, and a flat surface is not available for use of a coffee can or bowl, use a fine-meshed insect net with a long pole to capture the bat. Forceps (9 to 12 inches in length, rat-toothed) also may be used to capture the bat and transfer it to a container.
  7. Once the bat is captured, and you are positive no contact with people or pets has occurred, take the container outside, place it on its side (facing away from you) on a secure place above the ground—such as on a ledge, or against a tree—and slide the cardboard away. The bat will not fly right away, so releasing it above the ground keeps it safe from predators until it has its bearings. Unlike birds, most bats must drop from a perch and catch air under their wings before they can fly.

If you have recurring problems with bats entering your home, you should inspect your home, including your attic, to determine if you are housing a bat maternity colony. Wear appropriate personal protection equipment before performing any inspection.

Removing a Bat without Capture

  1. Remove pets or children from the room.
  2. Put on a pair of leather gardening or work gloves.
  3. Open all windows and doors leading outside to give the bat a chance to escape on its own. Leave the lights on, stand quietly against a wall or door, and watch the bat until it leaves. Make sure that someone stays in the room with the bat at all times. Otherwise, the bat may move out of sight and appear to have left when it is actually still inside.
  4. Do not try to herd the bat toward a window. Just allow it to calmly get its bearings, and don’t worry about it swooping at you. When indoors, a bat makes steep, banking turns, so it flies upwards as it approaches a wall and swoops lower near the center of the room.
  5. Within 10 to 15 minutes the bat should settle down, locate the open door or window, and fly out of the room.
  6. If the bat does not leave on its own after 15 minutes, wait for it to rest on a wall or other surface where it may be captured safely. Release the bat outdoors high on a tree branch or ledge (NOT on the ground).

Bat Colonies

Bats may be actively roosting inside the structure (usually the attic), but not in the living space. People frequently wonder if the discovery of a bat means that bats have a colony in their home. Bats likely have a colony if they are discovered during the winter, or when two or more isolated bats are found in the living space between March and October. Homeowners frequently encounter bats in July, when the young move around but are not strong enough to forage with adults. Once they begin flying, young bats also are more likely to make mistakes, like inadvertently flying inside a window or door.

Droppings are the most common sign that bats are present in a building. Individual droppings are small (⅛ to ¼ inch long), black, and dotted with speckles. Bat droppings can be distinguished from mouse droppings by their speckled appearance (fragments of insect wings), rough surface, and the number of droppings in one location.

Droppings often are found on attic and porch floors and under eves and shutters. Droppings and urine accumulate beneath a colony of bats over time, leading to staining and a rather pungent odor in enclosed spaces.


Bats can enter through openings as small as ⅜ inch in diameter. When bats are residing in a structure, they will use the same opening(s) every evening. The presence of smudges, caused by oil and dirt rubbing off of the bats’ fur, indicate that an animal has used the opening. Look for rub marks at entry points near eaves, vents, chimneys, cracks, and other openings as well as bat droppings. Occasionally the squeaking and scratching of individual bats can be heard in a wall void, ceiling, and other spaces.

Another way clients can determine if bats are present in their house is to simply enter the attic and look for roosting bats. During the day, bats will likely be roosting in narrow crevices in the attic walls, between or on the rafters, or tucked into the space between the rafters and roofing material. Before entering attic, we suggest the owner wear a protective respirator (HEPA filter) and gloves. People with lung or heart issues should be examined by medical personnel before wearing a respirator. Upon entering the attic, bats will quickly retreat out of sight rather than taking flight. If they can’t be seen, look for piles of guano (Figure 4) and/or listen for the squeaking or scurrying sounds that will verify their presence.

Bat Dropping Below Entry Hole

Figure 4. Bat droppings below the hole the bats use to enter the structure. Photo by Stephen M. Vantassel.

If you find bats living in your attic during the day, or if you find large accumulations of bat droppings, then you probably have a maternity colony in your house.

Dealing with Bat Colonies

If bats are residing in the attic or a hidden area in the structure, the only definitive way to remove them from a home is by exclusion. One-way doors, check-valves, or netting allow bats to leave, but not re-enter the structure. Exclusion must occur before young bats are born in early May, or after mid-August when the young will be mature enough to leave the roost. Do not exclude bats during summer when young are present. Young bats abandoned or sealed in a structure will move about the attic looking for a way out, and will eventually die. Dead bats in a structure will decay and may cause odor or insect problems. Exclusion can be performed using the following method.

  1. Locate all exit and entry points by standing outside of the structure at dusk on clear summer evenings and look for bats that are exiting the building to forage at night (a bat watch).
  2. Secure all unused openings with ¼-inch screening or sealant.
  3. Cover all exit or entry points by hanging 1-foot strips of flexible ¼-inch netting or exclusion tubes to permit bats to exit but not re-enter the building.
  4. Allow at least one week to pass, then seal and patch all entry points.
  5. Conduct another bat watch to confirm all bats are out of the structure and no entry points were missed.


The best way to remove bats is by using the exclusion and prevention methods already described. Several commercial products are available to repel bats. However, the effectiveness of these products is questionable, and has not been well-studied. Do not use mothballs to repel bats. The two ingredients used to manufacture mothballs, naphthalene and para-dichlorobenzene, are considered possible human carcinogens by the Environmental Protection Agency (EPA).


No toxicants are registered for bat control in the US. Because bats feed exclusively on live insects, it is unlikely that any toxic bait would entice bats to eat it.

In extreme cases, (e.g., where public health is threatened by rabies), the Centers for Disease Control and Prevention (CDC) and the EPA may provide special permits for licensed professionals to use toxic tracking powder to control bats.


Bats are associated with two 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. Because untreated rabies exposure is almost always fatal, it was discussed at the beginning of this case.

Histoplasmosis is a fungal disease that is associated with guano from bats. The fungus grows best in dark and humid areas 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).

Preventing Future Problems with Bats

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 (Figure 5). 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.

Bats hanging from attic screen

Figure 5. Bats hanging on an attic screen (view from inside attic looking out).
Photo by Stephen M. Vantassel.

Case Study #3:
White-tailed Deer Damaging Plants

Deer populations have increased dramatically in suburban areas over the past several decades. As a result, conflicts with landowners have increased in both number and severity.

Diagnosing the Culprit

Typically, most callers will know that the damage they are experiencing is caused by deer. However, to ensure that the culprit has been identified properly, you may need to review the following points with the client.

  • Deer have been seen in the yard during day or night.
  • Branches and leaves have a ragged, torn appearance (not clipped sharply).
  • Damage occurs up to 6 feet above ground on plants reaching that height.
  • Tracks and/or scat have been identified.

Potential Solutions

Homeowners have a variety of tools available to them for managing damage caused by deer. The suitability of the techniques depends on their particular situation and the intensity of damage being experienced.

Situation 1– Deer damage is infrequent and does not last long. In situations where deer are perceived by the owner as more of a nuisance, suggest the following techniques.

  • Use deer-resistant plant varieties (contact your local Extension office for local plant lists).
  • Spray repellents on affected non-food plants. Repeat applications every 4 to 5 weeks as needed. Research has shown that repellents containing rotten eggs or capsaicin are often the most effective.
  • Use fencing, such as rope fencing with repellents, snow fencing, or a peanut-butter polytape fence.

Situation 2- Deer are causing structural damage to plants several months of the year. In situations where the client complains of significant economic losses, and unacceptable levels of damage, more aggressive approaches must be used.

  • Peanut-butter, polytape electric fence (for damage during the growing season or small areas).
  • Individual plant protection with plastic mesh or wire fences (Figure 6).
  • 8-foot-tall, non-electric barrier fencing.
  • Hunting to reduce deer population density (where discharge of firearms and bows are legal and feasible).

Fence protecting sapling

Figure 6. A fence to protect a sapling from browse damage. Photo by Stephen M. Vantassel.

Deer Vehicle Collisions

Deer-vehicle collisions cause more than a billion dollars in vehicle damage and nearly 200 human fatalities annually. Defensive driving is the primary way to reduce the risk of having a deer-vehicle accident. Research in the US has indicated that deer whistles and other ultrasonic devices are ineffective. About two-thirds of deer-vehicle accidents occur during October, November, and December, especially in the early morning and at dusk. If it appears that you will collide with a deer, do not swerve to avoid the animal. Human injuries often occur when drivers swerve to avoid deer and instead collide with roadside obstacles or vehicles in the opposing lane.


Legal Issues

State wildlife agencies regulate the management of deer. Outside of hunting season, deer may be killed only under the authority of a depredation or nuisance deer permit. In addition, the discharge of firearms (including bows) may be regulated by both the state and municipality. Consult with your state wildlife agency to explore deer management options.

Some communities forbid the use of electric fences, while others forbid the installation of any fence that doesn’t meet with community aesthetic standards or height restrictions. Recommend that clients consult relevant local authorities before constructing a fence to determine if there are restrictions.


Deer have been implicated in the distribution and transmission of Lyme disease. According to the CDC, in 2009 there were 29,959 confirmed cases of Lyme disease nationwide. In 2012, 95% of Lyme disease cases were reported from 13 states:

  • Connecticut
  • Delaware
  • Maine
  • Maryland
  • Massachusetts
  • Minnesota
  • New Hampshire
  • New Jersey
  • New York
  • Pennsylvania
  • Vermont
  • Virginia
  • Wisconsin

Deer serve as one of several potential hosts for the black-legged tick (Ixodes scapularis), which transmits the bacterial spirochete (Borrelia burgdorferi) that causes Lyme disease. Larval and nymphal ticks commonly feed on white-footed mice and other mammals and birds. Adult ticks prefer deer as their host, although they also occur on medium-sized mammals such as raccoons and opossums. Tick abundance has been found to be positively correlated with deer density. People who are active outdoors, particularly hunters who handle and transport deer, should examine themselves and their clothing frequently for deer ticks. If you are bitten by a tick, consult your doctor. In order to transmit Lyme disease, ticks must be attached to a person for at least 24 hours, so frequent inspection and removal of ticks is the best prevention.

Ticks may be managed on your property by reducing shrub cover, cutting grass, and spraying acaricides. However, spraying individual properties has done little to reduce overall tick densities and the incidence of Lyme disease. For sensitive areas such as children’s playgrounds or school yards, use deer-proof fencing to exclude deer and ticks from those areas.

A new tool, the Four-Poster Device (Figure 7), is EPA- and state-registered on Long Island, NY and in other states to control tick numbers on deer. These devices are expensive, and should be used on a community-wide scale to be effective. A certified pesticide applicator must apply acaricide to these devices on a weekly basis while ticks are active (March through November in NYS). Also, a deer baiting permit may be needed to supply corn used in these feeding stations.

White tailed deer using four poster

Figure 7. A white-tailed deer using a Four-Poster Device. Photo by Dan Gilrein.

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