Late fall or early spring, after bats enter caves to hibernate, is a good time to bat-proof your attic.

By sealing holes in your attic from October to April, you can prevent bats from re-entering your house in the spring.

Within that time frame, you shouldn’t have to worry about sealing bats inside.

The bats that live in houses, the little brown bat and the big brown bat, once roosted in hollow trees.

But after early settlers wiped out large tracts of forest, these “house bats” moved their roosts into hot attics, which act as incubators for their growing pups.

Although people often aren’t thrilled about sharing their living quarters with bats, bats make good neighbors.

One little brown bat can eat 600 mosquitoes per hour, and big brown bats eat many agricultural pests.

If you see bats flying around your neighborhood at night, they’re doing you a great service.

They are eliminating a lot more insects than that bug zapper you have out back, and they also help to reduce our use of insecticides.

If you’re not sure if bats are sharing your home, look for bat droppings in your attic.

Bats make dry, black droppings the size of rice grains, filled with shiny insect wings.

If you find large accumulations of bat droppings, you probably have a summer maternity colony, a roost where female bats gather to raise their pups.

Because house bats have only one or two pups each year, protection of maternity colonies is important for their survival.

Destroying just one maternity colony can have a long-term impact on the populations of both bats and insects in a local area.

So, what should you do if you find yourself with these guests?

First, look for areas in your attic where bats can get through.

Bats enter through spaces where joined materials have been pulled away.

They often get through louvered vents with loose screening, roof peaks, dormer windows or areas where flashing has pulled away from the roof or siding.

Bats can travel through holes the size of a quarter.

To cover louvered vents or large gaps or cracks, use window screening or hardware cloth.

Fill smaller cracks with expanding foam insulation or caulking compound.

When bat-proofing, timing is crucial.

Never seal holes May through September because you can trap the females and their pups inside.

After sealing your attic, provide a bat box near your house as an alternative roost.

Bats are very site faithful. They tend to come back to the same place year after year.

With a bat box, the bats still have a safe place to raise their pups, and you get the bats out of your house while still benefiting from their insect control.

The small bat boxes available at garden centers serve mostly as bat motels.

During the summer, while females are gathered in maternity colonies, males basically are single, flying around.

When you put up a small bat box, often you’ll get a male bat using it for a night or two, then moving on.

To provide housing for maternity colonies, you can build your own bat boxes.

These boxes are larger, holding from 100 to 300 bats.

The interior should be divided into multiple roosting crevices, and the design should allow for proper incubation temperatures.

Siting is important. The most successful bat boxes get at least seven hours of sunlight each day.

Although you may find them creepy, bats are our friends.

“Growing Green” is contributed by Diane Dorn, Lehigh County Extension Office Staff, and Master Gardeners. Information: Lehigh County Extension Office, 610-391-9840; Northampton County Extension Office, 610-813-6613