Have you encountered a stray bat flying around in your house? Bats that fly into human living quarters are usually lost youngsters whose primary goal is a safe escape. They often will leave on their own if a window or door to the outside is opened while others leading to the rest of the house are closed. Bats are not aggressive, even if chased, but may bite if grabbed. As with any wild animal, bats should not be handled with bare hands. An exit can be hastened by catching the bat in flight with a butterfly net (swung from behind), or when the bat lands, covering it with a coffee can and slipping a piece of cardboard over the opening, and then simply releasing it outside. Or you may also catch it by hand, using leather work gloves to avoid being bitten. Any sick bat found in the home should be tested for rabies.
Exclusion is the only effective method of ridding your home of bats. There is no magic potion or spray on the market to kill or repel bats, and there isn’t any pesticide labeled for use on bats. Bats must be excluded only in late summer or early fall from living quarters/attics because these large colonies are usually made up of mothers and their young. If you attempt to exclude when the young have not learned the skill of flying, starved young could create a serious odor and insect problem, not to mention needless cruelty. Begin exclusion in early fall after bats have left the building, by covering chimneys and vents with half-inch hardware cloth screens, by installing draft guards beneath doors, and by sealing any other possible access routes, especially around screen doors, windows, and plumbing. Bats potentially can enter holes as small as 3/4” in diameter or 3/8” by 7/8”. They do not chew insulation or otherwise make new holes. Their entries can be plugged with silicone caulking, steel wool, or temporarily even with tape. If a large bat colony must be evicted from a wall or attic, careful observations should be made at dusk to find entry holes (also sometimes recognizable by stains around used holes or crevices or by droppings beneath). The bats must emerge each summer evening to feed. Once roost entrances have been located, the bats can be excluded. Again, this should not be attempted when flightless young may be present (usually June through August in South Carolina). Most bat species leave in winter, permitting exclusion in their absence. However, some bats hibernate in buildings, especially in warm climates, and when attics are heated. When this is the case, or when one does not wish to wait for winter, there is a relatively simple exclusion technique that can be used after young are flying but prior to the winter months.
Inexpensive lightweight polypropylene netting (which can be found at large hardware and DIY home centers) with a mesh size of 1/4” or smaller (1/6” is preferred) can be obtained in quantity to cover areas of nearly any size. It can be hung during daylight hours above areas where bats emerge, using duct tape or staples. A strip of netting at least two feet wide, hung one to two inches in front of bat exit holes, and extending at least two feet below and to the side of exit points will allow the bats to emerge, but later they will be unable to find their way back. Thus the netting acts as a simple one-way excluder until repairs can make the exclusion permanent. A sheet of clear, heavyweight plastic (also available at any hardware store) will have the same affect. The netting or plastic should be left in place for 2 to 3 days to assure that all bats have left the roost.
It is important to note that once bats are excluded from a building they will attempt to return to the same building the following spring. Therefore, supplying the bats with an alternative roost in the form of a bat box is beneficial to the bats and to the homeowner, as bats are voracious insect consumers. Be sure the bat box is of sufficient size to house the size of the colony displaced.
Harmless repellent devices would seem ideal, but none are known to be effective. The U.S. Environmental Protection Agency once fined a Chicago manufacturer $45,000 for misleading claims involving an ultrasonic device. All ultrasonic sound generators thus far tested by reliable bat experts have proven ineffective and some may endanger people or even attract bats.
Naphthalene flakes (moth balls) are no more effective. To be at all effective, they must evaporate rapidly, requiring frequent replacement.
Aerosol dog and cat repellents may discourage bat use of a particular roosting spot for periods of up to several months. They have been used effectively to prevent bats from night-roosting above porches. The spray is applied by day when bats are not present. Aerosol repellents are not an adequate substitute for exclusion in the case of day roosts and never should be applied when bats are in a roost. In many cases, suspending 2” wide by 7-10” long strips of aluminum foil or helium-filled Mylar balloons at a roost will deter bats. In addition, using bright, artificial lighting in the attic during the evening will also discourage bats from roosting.
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This article is a publication of Clemson University Cooperative Extension's Forestry & Natural Resources team.
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