Ministry of Environment
Integrated Pest Management
For some people, the sound of bats squeaking in the attic sends shivers up the spine. If you are frightened by these furry, nocturnal mammals, remember that they don't bite and are actually beneficial animals. Far from being pests, all species of bats found in British Columbia are voracious insect predators (and none drink blood!). Using their highly successful radar to catch night- flying insects, bats consume up to half their weight every night in moths, mosquitoes, beetles, crickets, grasshoppers and flies. For example, a single little brown myotis (weighing 6 grams) may catch up to 600 insects an hour in its preferred habitat, near water.
There are 16 species of bats in the province and all are protected from being killed and harassed under the provincial Wildlife Act. Most species are dark brown with short ears and small bodies about the size of mice. Wingspans range from 20 to 42 centimetres (about 9-16 inches). The most common species found in buildings are the little brown bat (Myotis lucifugus) and big brown bat (Eptesicus fuscus), which thrive throughout the province, and the Yuma myotis (Myotis yumanensis), which is found only in the southern part of the province.
Bats enter a building for a variety of reasons, including simply flying in by accident. They may use buildings as a temporary, daytime roost, as a nursery to rear their young or, occasionally, as a hibernation site. Attics are a favourite bat refuge. If bats are living in your attic during the spring or summer, chances are it is a nursery colony of a common species. Female bats bear one pup a year in spring; they choose dark, private attics as maternity roosts because the warm, protected conditions are ideal for rearing their pups. People in the house often hear the pups making distress calls to their mothers, who can identify their squeaking pup from among hundreds of others in a maternity roost. The pups depend on their mothers to feed them until they learn to fly and hunt on their own. By late summer, they are able to fend for themselves. In September, bats travel to a winter hibernation site, usually in a cave, old mine shaft, attic or wall void, where they spend the next 6-8 months until spring.
Like other mammals, bats can carry fleas, mites and ticks; in rare cases, they contract rabies. Unlike other mammals with rabies, however, they tend to get sick and die before becoming aggressive. According to Agriculture Canada, which tests about 100 injured, sick or dead bats annually, no one has contracted rabies from a bat in B.C. (People are more likely to contract rabies from family pets, which should be vaccinated against rabies.) Bats found on the ground may be injured or sick or they may be healthy, but stranded, since bats have a hard time taking off from a level surface. They should be gently scooped up with a shovel, placed in a ventilated box and taken to your local WLAP office or Agriculture Canada veterinarian.
Note: Always wear leather gloves when handling bats.
Removing Bats from Buildings
Most bat-human encounters occur when bats get into houses, usually through open windows, doors, chimneys or narrow cracks. Flying bats will leave on their own if you close the doors to other rooms, turn off the lights and open outside doors and windows. A sleeping bat can be safely picked up by using a pillowcase as a glove. Reach through the open end of the pillow case and grasp the bat gently in the sewn end. Invert the bag, with the bat still inside, then turn the bag inside out to free the bat outdoors. Try to place the bat in a safe spot, out of reach of cats.
The best way to prevent bats from entering the house in the first place is to keep up with maintenance. Inspect the house regularly and fill cracks under eaves and around pipes, vents, doors and windows as soon as they begin. If the holes are big enough for bats to enter (they can get through a dime-sized hole), make sure no bats are roosting inside before you plug the gaps. Not only will a decomposing bat in a wall smell, but it may also breed fly maggots and other pests.
If bats are roosting in a remote part of the house, isolated from human activity you may want to consider leaving them undisturbed and providing them a home (see below). They do not gnaw on wood, wires or insulation and will not damage the structure. If the bats are a nuisance, however, there are effective, non-toxic ways to evict them. It is illegal, as well as ineffective, to use pesticides to remove bats. Don't bother trying to chase bats away by playing loud music, installing ultrasonic devices, scattering mothballs or setting up bright lights in an attic. The only sure way to keep bats out of a building is to close up any entrance and exit points while the bats are out hunting. These openings are often small and inconspicuous since bats can squeeze through cracks only 5 mm wide.
Evicting Resident Bats
The first step in evicting bats is to find the entry and exit points to their roost:
- listen for squeaks
- inspect walls for brownish oil stains that rub off their coats as bats squeeze through small cracks
- look for bat droppings (guano) on the ground where they may exit the roost
- watch the suspected entrance at dusk, which is when bats leave the roost to find food
Check areas a metre or more off the ground, near wall and roof joints and under loose chimney flashing. Check for unscreened louvres and air intakes, gaps in the fascia boards, along the eaves and near dormer windows. In the evening, watch for bats flying out of these openings. Between foraging flights, bats may rest and digest food under the eaves, behind shutters, under loose siding or chimney flashing. If you see bats roosting near your house at night, don't assume they live nearby; they may be kilometres away from their daytime roost.
Close off bat entrance and exit points after dusk during the late summer, after pups have started flying on their own, so that orphaned pups are not left to die while their frantic mothers are stranded outside. You can make a one-way escape valve from screening to install over the main exits, which ensures that bats do not re-enter. Use polypropylene bird netting or fly screen to make a large flap over the escape hole. Use duct tape or staples to attach the netting above the exit. The netting should hang loosely and extend at least 20 cm (about a foot) on each side and below the exit point. The bats can slip out from under the mesh, but they can't climb back in. Leave the exclusion device in place several nights to ensure that no bats remain inside, especially in rainy or cool weather, when some bats may have entered torpor (similar to a self- induced coma) to conserve their energy.Evicting bats is useless unless it is followed by repairs to the building to permanently exclude them. Since bats don't chew their way in, you can block most entrance and exit points using light building materials such as window screen, caulking, duct tape or moulding. In some cases, it is not possible to evict bats. For example, bats frequently roost under wood shakes on roofs or under cedar siding. In this case, the best option is to learn to live with the bats, especially since they do not damage the building.
Living with Bats in Buildings
As a biological pest control, bats are vital links in the balance of nature as well as invaluable allies for farmers. If you've discovered a bat roost nearby, consider hosting them permanently and reaping the benefits. Bats and humans can peacefully coexist when bat roosts are isolated in one part of a well- maintained building or housed in a separate building. Keeping them out of human living quarters maintains the comfort and safety of both people and bats. If the idea of maintaining or providing a bat roost in the attic appeals to you, certain modifications are recommended. First, ensure that there is no way for bats to find their way into human living quarters. Next, cover ceiling joists with flooring to support the weight of bat guano. Cover the flooring with plastic, particularly underneath rafters where bats hang out. This contains the guano, allows for easy cleaning and protects the wood from staining and rotting.
You may want to limit the number of entry points to the attic by blocking undesirable access points. In future, do any home repair work and a seasonal clean- up (guano makes excellent fertilizer) in late November or December when bats have left their summer roosts. On rare occasions, bats choose attics as their winter hibernation sites. Make sure no bats are hibernating in the attic when you do your seasonal clean- up. Hibernating bats have accumulated as much as 40 per cent of their summer weight in fat to make it through winter. If disturbed by human activity, hibernating bats burn off valuable stored energy and may die of starvation.
While bats in this region pose no significant health risks, bat guano can smell. In extremely humid environments, guano dust can carry a disease organism that causes a lung infection called histoplasmosis. This is rare in the Pacific Northwest, although it is common in other regions. Therefore, always wear a respirator mask when investigating or cleaning a bat roost.
Providing a Bat House
If you want to exclude bats from your house, or just want to attract bats to your yard for mosquito control, you can provide them with a safe and suitable alternate home in a bat house. Used for more than 60 years, bat houses look like bottomless bird houses. Small houses may consist of a single narrow chamber, while large ones have partitions inside to divide the space in narrow, bat-sized hangouts. Build the houses from untreated, rough- sided wood, such as cedar or pine boards. Space any inside partitions 2-4 centimetres apart, and cover one side of each partition with fibreglass window screening (do not use metal mesh) to provide a secure foothold for the bats. In cool areas, paint the box a dark color or cover the outside with tar paper to increase the solar heat absorbed by the box and position it so that it receives at least five hours of sunlight a day.
Bat houses should be installed at least 3-4 metres (12-15 feet) above the ground, where the entry is unobstructed and out of the reach of predators. Bat houses attract more occupants when they are located near a permanent water source - especially a marsh, lake or river. A bat house can be hung in a tree, but those attached to the side of a building or mounted on a pole have been most successful. Bat houses should face south or southeast to receive morning sun exposure. In regions with hot summers, they may require shade by mid-day.
Using bat houses is still experimental, therefore, if the bat house remains empty after one year, try moving it to improve the sun exposure or put up other bat houses in different locations. For more detailed information on bat and bat houses, contact Bat Conservation International*. Beyond providing bat houses, people can enhance the urban environment for bats by preserving barns, sheds, caves and snags as potential roost sites.
Footnote: Bat Conservation International, PO Box 162603, Austin, Texas 78716. Write for quarterly publication "Bats".
Nagorsen, D.W., and R.M. Brigham. 1993. Bats of British Columbia. Royal British Columbia Museum & UBC Press, Vancouver, B.C. 164.