University of Maryland Extension


bats in house

****Bats are very beneficial. However, they can become a problem when roosting in structures. To attract bats, see (PDF) UME Publication #791 Got Bugs? Get Bats! *****


They are the only mammals that can fly. Contrary to popular opinion, they are not related to mice but are in their own separate order of mammal. There are ten species of bats that live in Maryland. The two most common bats are the little brown bat and the big brown bat. The natural habitat for bats is in caves and hollow trees. When these places are not available, they will roost during the day in attics, behind window shutters, in church steeples and even storm drains. They roost in groups ranging from just a few individuals to as many as several hundred bats. Pregnant bats will congregate to form nursery broods where they deliver one, or sometimes two, young in April through July. The young bats are fed by their mothers, and the young learn to fly by the time they are three weeks old.

Unfortunately, bats evoke fear in many people. This is unwarranted because bats are actually gentle, peaceful animals that are very beneficial to people and gardens. Since bats are so beneficial they should not be deliberately killed. Bats can consume as much as one-third of their weight in flying insects every ½ hour. It is a familiar sight to see groups of bats swooping down to catch mosquitoes and other flying insects on a warm summer evening. They also dip to find drinking water from swimming pools and fish ponds.

If bats become a nuisance, they can be discouraged or removed from an area but should not be killed. When roosting in a building, bats may not be welcome, because their droppings, known as guano, could build up and cause odors and attract insects. Bats or their guano may also be associated with certain diseases, such as rabies or histoplasmosis. Bats most often will roost in attics, between roofs and ceilings, fascias or other crevices around chimneys, or down spouts. Fecal droppings and stains may be seen near eaves, beneath entrance holes, and below roosts. Bats often can enter homes through open windows or uncovered fireplaces, and through the overhang of the roof or eaves.

Exclusion with 1/4-inch hardware cloth is the best way to exclude bats from a roost. Bats should be out of the house before the openings are sealed or else they will be trapped inside, which could result in bats entering the living quarters of a house looking for a way out. Batproofing should be done after September 1 and before hibernation begins, which is normally in November. Bats may not be excluded during the summer months because nursing pups (young bats) may be trapped inside. It is advised not to exclude during winter to avoid trapping hibernating bats inside. Close the main entrances after the bats have left for the evening to feed. Open the main entrances the following evening to permit any bats trapped inside to escape. Close the opening permanently after you are certain there are no more bats trapped inside. Repellents may be effective in small spaces. Lighting the attic with a bright floodlight or the draft from a large floor fan can also encourage them to leave. An individual bat that is inside a house can be easily removed by confining the bat to a single room and opening all windows and doors to the outside and allowing it to escape. This should be done in the evening so the bat feels comfortable to leave. If this does not work, the bat can be captured with a hand net or a small container placed over the bat. Slide a piece of cardboard under the container to trap the bat inside. These bats can be set free away from the house.

Photo Gallery:

bat guano
When roosting in a building, bats may not be welcome, because their droppings, known as guano, could build up and cause odors and attract insects.

The little brown bat is a common species in Maryland.

Additional Resource 

Penn State Extension A Homeowner's Guide to Northeastern Bats and Bat Problems

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