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Flying Mammals Driving You Batty?

By Dr. Jim Pease, Emeritus and Dr. Rebecca Christoffel
Extension Wildlife Specialists (ISU - Retired and ISU-Current)

August, already? If the calendar didn't tell me so, I can tell by the number of bat calls coming in on my phone. About every third call is related to bats: bats in churches, in homes, in barns.

Big Brown Bat ( Eptesicus fuscus) eating a beetle - © Merlin D. Tuttle, Bat Conservation International,

While everyone is concerned about them, the tenor of the calls has definitely changed. Ten years ago, every caller asked, "How do I kill them?" Now, only rarely do I hear that question. People know bats are extremely valuable in insect control. We want them flying around outside gobbling mosquitoes, crop damaging insects, and others. We've even learned to construct and put up bat houses for them. We just don't want to share their living space with them. Fair enough. I can live with that -- and so can the bats! 

Eight species of bats are found in the state of Iowa. One of them, the Indiana bat, is a federally endangered species, exceedingly rare. Five species -- the northern myotis, the silver-haired, the eastern pipistrelle, the red, and the hoary bat  -- are either solitary or are found primarily in dense woodland areas near water. As a result, people rarely encounter them. Only two bats, the big brown and little brown, commonly use buildings in the summer for colonies. Only the big brown bat uses buildings in winter. It is mainly these two species that bring the deluge of phone calls to this and other offices.

There are no state or federally registered baticides available for use against bats. Even if there were, one would still have to locate and close up entrance holes to prevent other bats from recolonizing the same area in a building. Like almost all other animal species, Iowa law protects bats. While homeowners are allowed to protect their property, repellents are largely ineffective, including those expensive "sonic" or "ultrasonic" devices and the relatively inexpensive mothballs. Thus, excluding bats from buildings is the only way to deal with them. Understanding some bat habits is the key to successful exclusion.

First, bats are nocturnal. They emerge from their roosts at dusk each evening, searching for food and water. Thus, exclusion activities must be done after they've emerged, NOT during the daytime. Blocking entrances during the day only guarantees more severe problems, increasing the likelihood that they will come down through the walls in search of a way out. It also increases the chance that they will die a slow and painful death, something few people would wish on any animal.

Second, bats follow air currents. Any spaces--say, your attic and a bedroom--that have different temperatures and are connected by a crack or hole, automatically have airflow between them. Bats simply follow those air currents. Blocking those air currents is the key to successful bat exclusion.

Third, September is an excellent time to do the exclusion. Bats are mammals and often form maternal colonies, a mother and her one or two young hanging together with dozens of other mothers and young. Since young bats are naked and blind, mothers leave them behind each night to seek food and water, returning later in the night to nurse them. If exclusion is done prior to September, you may simply exclude the mothers and condemn the young to starvation and death. By waiting until September, you can be assured that the young are going on the evening flights with their mothers.

Finding the entrance can be a family affair. Take lawn chairs, your favorite drinks, and a flashlight and sit outside in your yard about sundown. Watch for emerging bats. Check the obvious places first: around the chimney, gable vents, or roof vents. Don't forget the not-so-obvious places also: under the eaves, behind the rain guttering, under torn shingles. All these are common entrance sites and indicate that some repair is in order. Some buildings have more than one entrance site.

The next day, check the entrance points more closely. Sometimes, simply replacing a rusted screen, a broken window, or caulking some flashing is all that is required. Other times, more extensive repairs may be necessary. You can determine whether or not your skills are up to the challenge or if you need to call in a carpenter, mason or roofer. Remember, however, that no repair or blocking should be done in the daytime since you will only trap the sleeping bats inside.

The problem, of course, is finding a home repair person willing to climb up on your roof or elsewhere after dark. While some may do this, an alternative is to create a oneway door so the bats can get out and not back in. A simple oneway door can be created with one-half-inch mesh bird netting (available at garden stores) or with screen wire. Let's say, for example, the entrance is a crack one half inch wide and 6 inches long. Cut a piece of netting or screen. Place it over the entrance crack so that the entrance is in the upper half of the net/screen. Use duct tape to tape the top and two sides of the screen to the building, leaving the bottom edge open and just loose enough for the bats to squeeze out.

Bats will emerge that evening, hit the screen, crawl around until they find the bottom loose, and then fly out. When they return, they return to where the air current is--the crack--not to the bottom of the screen. If you leave this up for 2-4 days, you can be assured that all bats are out and the repairs can be made, this time during the daylight hours.

Bats are our only flying mammals. All Iowa bats are insectivorous, finding insects using sophisticated sonar. Each bat can devour hundreds of flying insects each night. They are an interesting and valuable part of Iowa's biodiversity, one that we can live with, even if not in the same dwelling.

Photo Credit: Big Brown Bat - © Merlin D. Tuttle, Bat Conservation International,