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Extension > Garden > Yard and Garden > Wildlife > Bats in houses

Bats in houses

Jennifer Menken, Bell Museum of Natural History

Importance

Minnesota has seven species of bats. Two of the most common, the big brown bat and the little brown bat, will commonly roost in people's houses. Bats do much more good than harm eating tons of insects, including mosquitoes, each year. But when they move in with people they can cause problems. Of greatest concern is the potential for diseases such as histoplasmosis found in bat droppings and the rare occurrence of rabies in a bat that might fall sick and be found by a pet or child.

Identification

Bats usually keep a low profile inside a house staying up in the eaves and attic. Some signs that you might have bats include high-pitched squeaks and scratching at night. These sounds come from high inside the walls or ceiling. Wear a dust mask, and look for droppings in attics and under eaves. The droppings will be dark in color, and greasy and may have insect body parts inside. Bats will often stain the sides of a building with these droppings as they are flying in and out at night. Rounded or cylindrical droppings are usually signs of rodents. A strong, pungent odor is often associated with droppings. Of course a bat flying around inside the house or bats leaving at dusk through attic windows are also good signs that bats are around.

Biology

Bats are part of the order Chiroptera, that means "hand wing." They are the only mammals with true flight. Minnesota's bats are all insectivores and eat large numbers of night flying insects, including beetles and moths. Some of our bats migrate in the winter and others, such as the big brown and little brown bats, hibernate over the winter in caves, hollow trees and buildings. Bats are usually active as long as the evening temperatures stay above 50 degrees. The females give birth in spring and 3-4 weeks later the babies can fly.

Surprising to most people is that bats can live a long time, with some species living 30 years or more. This is important to note since bats will come back to the same roosting spot year after year.

Of concern with bats is the potential for disease. The most likely disease is Histoplasmosis, which is caused by a microscopic fungus that grows in the bat's droppings. When the droppings are disturbed the fungus gets into the air and can affect the lungs. If droppings need to be cleaned, a dust mask should be worn and the droppings should be lightly misted with a bleach and water solution before being swept or vacuumed. People with lung problems should have someone else clean up the droppings.

Rabies is also a possibility, although the percentage of bats with rabies is small. In general bats should never be held. If any bat is found, it should be left alone or moved with a shovel to someplace far away from people and pets. Dead bats should be disposed of in a plastic garbage bag. If a person or pet is bitten by a bat they should be taken to a doctor immediately. If the bat can be caught, place it in a hard sealed container, such as a coffee can, and take it to be tested for rabies. Very thick leather gloves will protect hands from a bat's tiny teeth. If rabies is not treated it is always fatal.

Prevention

Bats can enter a house through any opening 1/4 inch wide or bigger. So to keep bats out make sure all cracks and openings are sealed with weather stripping or caulk. Chimneys and vents should be capped. Doors and windows should have weather stripping.

Management strategies

If you should discover bats in your house the only effective long-term solution is exclusion. Exclusion is a time consuming process but is worth the effort. Check the outside of the house to figure out how the bats are getting in and out. Search around the edges of the house at dusk. Look at chimneys, along eaves, loose siding etc. Once you have discovered the bat's exits you need to fill every possible entrance but one, with caulk or weather stripping or hail screen. This will leave one entrance to focus on in the next steps.

You will need to seal the last hole but you don't want to seal bats inside the house. To prevent this, install a one-way bat door. The simplest method is to take a tube of heavy fabric, such as a pillowcase with both ends cut open, and tack it over the last exit hole so that the bats can crawl into the tube and out the other side. When the bat returns it won't be able to crawl back through the pillowcase. After a couple of weeks, seal the last hole.

In Minnesota the best time to exclude bats is very early spring (as soon as it gets above 50 degrees at night) and September, which gives the bats enough time to find a new hibernation site.

Other additional tactics to encourage bats to leave include bright lights and/or fans in the attic. Be careful of the fire hazard posed by lights. The smell of mothballs may be a deterrent but can also be a health hazard for people. Placing a bat house nearby may give evicted bats a place to move into.

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M1281 Reviewed 2009

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