Removing a bat colony

Bats that roost in Texas buildings can be evicted using special exclusion techniques either before or after maternity season. Young bats cannot leave a building until they are old enough to fly. Evicting the adults during maternity season will orphan and kill the babies inside the building. Maternity season varies by species and location, but it is usually from spring to late summer. Generally, the best time for eviction efforts is between September and April, depending on your location in the state.

Removing a bat colony can require significant amounts of time and money. In recent years, Texas school districts have paid from $5,000 to $60,000 to remove and exclude bats from school buildings.

If bats are seen in or near a school building:

  1. Identify the species.
  2. Inspect the building for entry points and roosting areas.
  3. Consider the risks and benefits of providing alternate housing for the bats.
  4. Seal potential but unused entry points.
  5. Make and install bat-eviction devices.
  6. Remove the one-way eviction devices and permanently seal the entry points.
  7. Clean up the area as needed once the bats are gone.

1. Identify the species.

Before taking action, know the species involved. A few very rare bats are federally protected. It is important to comply with the laws that protect these animals. There may be more than one bat species sharing the building.

Species identification can also help if the district is considering building alternative housing for the bats. Each species of bat has different housing requirements. For example, the structures for Mexican free-tailed bats need to have an open bottom design. Many bat houses available commercially are unsuitable for North American species.

Although some bats are easy to identify, proper identification requires training, including the use of identification keys. The Texas Parks and Wildlife Department or Bat Conservation International can help school districts develop a list of the bats most likely to be encountered. The Texas Department of State Health Services Laboratory also routinely identifies the species of bats submitted for rabies testing.

2. Inspect the buildings for entry points and roosting areas.

A bat colony will often leave stains at entry/exit points. Photo courtesy Fly By Night Inc.

Identify areas in and around buildings where bats can enter. This step is vital for effective placement of bat-eviction tubes and nets.

To witness bats entering or exiting the building, monitor it during early evening (dusk) and just before dawn. Note all the locations where the bats leave the building. During cooler months, you may need to inspect several nights in a row to establish exit/entry points, because bats do not leave the roost at night in cold weather.

When inspecting the exterior of the building, look along roof lines and behind gutter placement for rub marks, which are stains left by the oils and dirt rubbing off the bats’ hair. Like rodents, bats will leave some evidence of staining; however, bat stains are harder to see. Also look at ground level for guano—in most cases, the bat-entry points will have some guano buildup if the colony is large enough.

Inside, identify all parts of the building where they may have established roosts. These areas can include chimneys, attic spaces, wall spaces, ceiling spaces, expansion joints, and roof overhangs. Bats also roost behind gutters, in sports stadiums, and beneath or behind signs and fixtures.

3. Consider the risks and benefits of providing alternative housing for the bats.

Some school districts have erected bat houses to provide alternative housing (Fig. 10) for the bats removed from school buildings. Such bat houses can make it less likely that the bats will infest nearby homes or buildings.

After a Texas school district removed a bat colony but did not build alternative housing, the bats moved into neighboring structures. This caused problems for the neighbors and created adverse publicity for the district.

Alternative bat houses should be big enough to house several hundred bats. (Copyright photo by Merlin D. Tuttle, Bat Conservation International, www.batcon.org)

However, erecting bat houses on school property can raise concerns about odors, bat waste, and increased bat/human contact.

If you decide to provide alternative housing, erect it before beginning exclusion efforts. Each bat house should be appropriate for the species and large enough to hold several hundred bats. Place them at the edge of school property, away from students.

For information on how to build, buy, and install a bat house, visit Bat Conservation International at http://www.batcon.org/. This organization keeps a list of certified bat houses and vendors that sell them. Plans for bat houses are available at http://free.woodworking-plans.org/bat-house-plans.html.

Building alternative housing for the bats does not guarantee that they will move into the housing. If they do, however, the colony may be integrated into classroom teaching, including lessons on conservation and public health issues related to wildlife.

4. Seal all other potential entry points.

Without disturbing active access areas, seal all potential but inactive entry points using caulk, weatherstripping, flashing, or hardware cloth (heavy-duty, ¼-inch, polyethylene mesh). Use the techniques described in the section on preventing human/bat contact to seal these entry points.

5. Install bat eviction devices.

  1. Buy one-way chutes or make them from 2-inch-diameter PVC pipe, clear sheets of plastic, and empty, clean caulking tubes with the ends cut off. Netting also may be used. Specific directions on how to make or where to buy bat eviction devices, see http://www.batcon.org/index.php/bats-a-people/bat-exclusion-instructions.html.
  2. If using tubes, place the tubes over the holes in the roof or soffit used by the bats. These tubes will allow them to leave but not reenter the building. If bats are roosting in a long horizontal crevice, place a tube roughly every 4 to 6 feet along the entire distance to make sure all the bats can get out.

    Eviction tubes (Photo courtesy Fly By Night Inc.)

  3. If using netting, tack down the top and sides, leaving an opening on the bottom for bats to escape. Be sure the netting is secure, or bats can get stuck or reenter the building because of faulty design.
  4. To ensure that all the bats exit the building, leave these one-way devices in place for at least 1 week during warm weather and 2 weeks in cool weather (less than 50 degrees F).

6. Remove the one-way devices and permanently seal the entry points.

Make sure that there are no new signs of bats leaving the building. If you remove the one-way devices too early, you could permanently seal bats inside, killing them and causing odor and sanitation problems for the school.

7. Clean up.

Once the bats have left the building, begin remediation procedures. Remove the guano from interior structures to avoid attracting other pests such as cockroaches or flies.

Guano can pose two risks: excessive weight on structures, and disease transmission from contaminated materials. A naturally occurring soil fungus, Histoplasma capsulatum, is sometimes found in bat droppings. A person inhaling the fungus spores can develop histoplasmosis, a flu-like respiratory disease (http://www.dshs.state.tx.us/idcu/disease/histoplasmosis/faqs/).

To prevent illness, employees should take precautions when cleaning up guano in a confined area:

  1. Wear personal protective equipment, including leather gloves, long-sleeved shirt, long pants, approved eye protection, and a respirator that can filter particles smaller than 2 microns in diameter.
  2. Before removing the guano, lightly dampen it with a disinfectant to minimize the amount of dust and spores dispersing into the air. The Centers for Disease Control (CDC) and the National Institute for Occupational Safety and Health recommend using a 10 percent bleach solution (1 part water to 9 parts of bleach) as a wetting agent.
  3. If the guano buildup is more than 2 inches deep, follow CDC procedures (http://www.cdc.gov/niosh/docs/2005-109/) to remove it from the building.
  4. Bag the affected material or use a professional vacuum (high-efficiency particulate-absorbing, or HEPA) that exhausts to the outside.
  5. Check with the local landfill or the Texas Commission on Environmental Quality for the appropriate place to send this material, and send it there.

Like other mammals, bats can attract pests such as mites, ticks, fleas, and flies. Depending on the roosting location, a licensed pesticide applicator may need to apply a desiccant or insecticide dust after eviction to kill the pests and keep them from entering areas occupied by students and staff.