Bats love old houses: All those crumbling chimneys, cracks and holes, and vents with missing screens are open doors for little mammals that can squeeze through a 3/8″ x 1″ crack, or into a hole smaller than a quarter. Forget mothballs, aerosols, and ultrasonic deterrents. They don’t work, particularly if you have an established colony. (A few bats during migration season may be a temporary situation and nothing to worry about. If there’s bat poop—guano—all over the place, you’ve got a problem.)
Bats are a critical part of the ecosystem, controlling the insect population. It’s impractical, inhumane, and probably illegal to kill them, so you have to go through a live exclusion. If the infestation is large or has been recurrent over years, call in a pro for both exclusion and cleanup. Every state has a wildlife or conservation department that can help you find a licensed wildlife removal specialist. (Not an exterminator!)
Inspection
Where are the bats getting in? Do a sunny-day inspection to look for missing roof shingles, deteriorating eaves, holes in soffits, etc. Then watch the house (all sides) on a warm, clear summer evening, beginning just before dusk, noting any bat activity. Also, entry points may have “bat tracks,” or greasy brown marks, around them.
Exclusion
In parts of the country, bats migrate in fall to hibernate for the winter; if yours have left for the season, and you know where they’re coming in, late fall is the time to plug up all holes and cracks around windows, fascia and soffits, cornices, chimney flashing, etc. If the bats were living in the chimney, cover the top with a “box” made of fine-mesh screening.
If the bats have taken up residence, plan to evict them in late summer or early spring, not after birthing season, when pups (who can’t fly) would be orphaned and die. Don’t evict them in winter, when they’re hibernating and can’t fly out. The process of exclusion involves using netting or tubes at entry points, which allow the bats to drop down and take flight but which confound re-entry. The excluders are left in place for a week, so that the bats give up. After they’re gone, the plugging and sealing and caulking can take place.
Cleanup
The mess must be thoroughly removed, deodorized, and disinfected. Bats easily sniff out a prior roost. The guano could contain a fungus that may cause a serious respiratory infection called histoplasmosis in humans. Even if it’s a small cleanup, wear eye protection, gloves, long sleeves, and a mask that filters particulates over two microns. Clean affected surfaces after removal with a bleach solution.
Most people prefer to call in the pros. They’ll mist the guano to prevent dust, then remove it with a professional HEPA vacuum, containing and disposing of the waste. They may have to remove finish materials like drywall to get rid of urine. Then, with doors and windows sealed, they’ll use an odor eliminator and an antibacterial.
You or a crew can then get to work sealing all the holes and cracks. Pros may find entry points more easily and they’ll use an array of materials—sealants, foam, mesh—to prevent re-entry. Finally, consider putting up a bat house at the edge of the property. That way the bats have somewhere to go, and your mosquito population remains under control.