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Like most birds and other wildlife, all 12 species of bats inhabiting Illinois are protected by law. Species most commonly found in structures include the little brown bat ( Myotis lucifugus ) and big brown bat ( Eptesicus fuscus ). These species have a wingspan of less than 12 inches and weigh ½ ounce or less. Four other species are classified as endangered species. It is unlawful to harm or kill a bat. Only under special circumstances are permits to kill bats granted by the Illinois Department of Natural Resources.

Popular “home remedies” for eliminating bats are temporary, ineffective, and/or illegal. No pesticides are registered for bat control. Moth crystals (naphthalene) can be temporarily effective, but the typical attic requires three to five pounds to be used and changed every few weeks. Other types of repellents may not be registered for use as bat repellents and therefore cannot legally be used to harm or repel bats. Bright lights, as well as fans and air-conditioners (used to cool down the roosting area), may be effective but temporary controls. In addition, ultrasonic and electromagnetic devices do not effectively repel bats, rodents or insect pests, despite advertising claims.

Exclusion remains the best way to prevent and control bats in a structure. Bats can be excluded by sealing exterior openings larger than ½-inch, using caulk, expandable foam, plywood, mortar, metal flashing, steel wool or ¼-inch mesh screen or netting. Make sure doors, windows and vents have screens and are securely framed; chimneys are capped; and gaps around utility lines are plugged.

In May and June, one or two “pups” are born to pregnant bats in Illinois. By the end of July, the young bats have taken wing, though they will continue to nurse until able to feed themselves exclusively on insects. Most bats, especially those in northern Illinois, leave their roosting places in September and early October to migrate south where they will overwinter in caves, rocky ledges and cliffs, and occasionally accessible walls and attics. Therefore, bat entry points in structures are best sealed during the months of September through April, when no bats are present. Proper exclusion at this time will prevent bats from entering the structure in spring. Only at certain times can exclusion be performed while bats are roosting within the structure. This involves sealing openings after the young bats are old enough to fly (August or later in Illinois).

Some skill is required to identify all entry points and to apply exclusion materials to openings. Openings through which bats are entering and exiting a structure may be identified from inside the structure by entering the roosting area, an attic for example, on a sunny day when light can be seen through the openings. Another method is to turn on a bright light in the attic at night and look for light escaping through the openings on the building’s exterior. Dark stains may be seen around and beneath openings used by bats. These result from bat guano and from “rub marks” where oils and dirt accumulate as bats pass through the openings. Yet another method of finding bat entry points is to watch for bats leaving the structure at dusk to make their evening feeding flights.

When all openings are identified, a “one-way valve” can be applied to each opening. One-way devices are those that allow bats to leave the structure, and prevent them from reentering. These can be as simple as a sheet of plastic or plastic bag attached above the opening and allowed to hang flush against the building’s exterior. The plastic should be wider than the opening and long enough to hang at least one foot below it. The sides (but not the bottom) of the plastic can be attached to the building by staples or duct tape, to prevent wind from lifting the flap. At dusk, the bats will find their way out beneath the plastic flap, but will not be able to lift the flap to reenter the structure.

Similar devices can be constructed from screening or polypropylene netting of ¼-inch mesh, or a short length of PVC pipe can be positioned in the opening. A tube sock should be fitted around the outside end of the pipe and allowed to hang down with the toe cut out. Bats will exit the pipe and crawl through the sock to get out but will not be able to reenter through the collapsed opening in the toe of the sock. Once all resident bats have exited the structure, the one-way devices can be removed and the openings immediately sealed as described above. Again, this type of exclusion should NOT be performed when young bats, incapable of flying, are present (typically May-July).

Although exclusion is the best way to rid structures of bats, knowledge and timing are critical for effective “bat proofing.” Especially in older construction, there may be several bat entry points that can be difficult to discover. If all openings are not found and sealed, bat problems will continue. Installing sealing materials and one-way devices can also be difficult because bat entry points are often several feet off the ground, requiring the use of ladders (note that falling is a much more common accident than being bitten by a rabid bat). For these reasons, bat exclusion may be best left to professionals. A list of wildlife control specialists, who may be familiar with bat exclusion procedures, can be obtained from the Illinois Department of Natural Resources (217-782-6384).

Illustrations courtesy of Penn State University and the University of Missouri.

For more information, contact the Illinois Department of Public Health, Division of Environmental Health, 525 W. Jefferson St., Springfield, IL 62761, 217-782-5830, TTY (hearing-impaired use only) 800-547-0466.