Bringing knowledge to life in Greater Flint

Bats

General Description

Bats are our only true flying mammal. They are distributed worldwide and include as many as 1,000 species. They benefit us by consuming large numbers of insect pests and pollinating crops and other plants.

Several bat species in the United States have declined in number during the past few decades for several reasons: people kill them unnecessarily, insects have decreased due to chemical pesticides, contaminated insects are ingested by bats, roosts and hibernation sites are disturbed or by cave explorers and vandals, and bat habitat is reduced due to mine and cave closures.

The primary species in southern Michigan that roost in houses are the big brown and the little brown bat. They both roost in buildings, and the big brown bat will occasionally fly during warm winter spells and make sudden appearances in a house.

Because of their beneficial value bats should be appreciated and not needlessly killed. In some circumstances, when their accumulated droppings, noise, intrusion, odor, mites, or lice become a nuisance, control is necessary. Also, if there is the potential threat of rabies, there is no alternative but to capture the animal.

Habitat - Bats roost in attics, behind chimneys, under eaves, between walls, behind shutters, in ceilings, under shingles, under bridges, in tree hollows, under loose bark and in caves.

Diet - Bats in North America primarily feed on insects. Common food includes mayflies, moths, midges, mosquitoes, caddis flies, beetles, and flying ants. They can consume large quantities of insects, with some eating up to one-third their body weight in one hour. They are often attracted to insects around swimming pools, ponds or white lights.

Reproduction - Mating generally occurs in the fall and winter, but the egg is not fertilized until spring. Pregnant females leave hibernation and congregate in maternity colonies to give birth to usually one offspring between May and July. Young bats are able to fly within 3 weeks and are weaned in July and August when the nursery colonies disperse.

Behavior - Bats use ultrasound, or high frequency sounds like a dog whistle, to locate prey and avoid obstacles. They also emit audible sounds to communicate with each other. They usually leave their roosts to feed between 1/2 hour before and 1/2 hour after sunset, usually on calm days. They return during the night to rest, leave again before dawn and return to sleep during the day. Around the time of the first frost, bats prepare for the winter hibernation. Many bats in Michigan migrate to Indiana to hibernate from October until May.

Bats enter homes for four main reasons:
1) Through open windows, doors, repairs or chimneys. Once inside they hide in closets, behind curtains, bookcases, etc.
2) A few males (usually less than 4) roost during the day in shaded windows, behind shutters, under an awning, etc.
3) Bats are repeatedly (2 or more times) seen in the home, especially in May and August. These bats are usually females and their young in a summer maternity roost.
4) Bats occasionally blunder into a house from November - March when warm weather causes hibernating bats to wake up and move around.

Bats are able to squeeze through narrow slits and cracks as small as 3/8 inch in diameter.

Rabies - Rabies is the most important public health hazard associated with bats because it can be fatal if not treated within the incubation period (2 weeks to several months). Humans may be infected if they are bitten or handle the infected bats. If a bite or exposure occurs, the bat should be captured, without damaging the head, and transported to a lab. If a bat has rabies, it will show symptoms for as long as 10 days after an incubation period. Symptoms include activity during the day, inability to fly, or, less frequently, unprovoked attacks on people or pets. Although rabies can be transmitted by bats, only 1% appear to be infected.

Histoplasmosis - Histoplasmosis is a common lung diseased caused by a microscopic fungus which grows on accumulated bat guano and bird droppings. The fungus enters the lungs usually after the guano has been disturbed. The infection usually causes only minor flu-like symptoms and is resolved uneventfully. In a small percentage of cases it can be fatal, especially to young children. Histoplasmosis can be prevented by avoiding areas with high concentrations of guano or wearing a protective mask and clothing.

Damage Identification

Rub marks - A smooth, polished, yellow-brown to blackish brown mark made by the passage of bats through entrances.

Noise - Scratching, crawling, climbing, and vocalization may be heard in attics, under eaves, and behind walls, especially on hot days, before leaving at dusk, and returning at dawn.

Guano and Urine - Bat dropping contain bits of undigested insects, are powdery when crushed, and are not white and chalky like bird droppings. Guano has an unpleasant odor and can be an airborne health hazard. Bat urine crystallizes at room temperature, appearing white and powdery where it has accumulated. Bats can urinate and defecate in flight, causing spotting and staining of objects near their entrances.

Damage Prevention Methods

Exclusion and Habitat Modification

Sealing openings - The best method to exclude bats permanently from a structure is to prevent their gaining entrance. Locate any potential or current openings the bats are using and temporarily seal them while the bats are out flying at night. The seal can then be made permanent at the homeowners' leisure. Sealing the openings should be done before June 30 or between August 15 and September 15 to avoid sealing in baby bats or hibernating adults.

To locate the openings, watch each side of the house from sunset to 1 hour after sunset or until the bats are seen leaving. As an alternative, watch 1 hour before sunrise. Watch only on warm, still evenings or mornings because the bats will not leave the roost in inclement weather. Other alternatives include illuminating the inside of the roosting area at night and observe where the light can be seen from the outside, observing the darkened roosting area from the inside during a bright day, using smoke bombs, or homemade air leak indicators.

Openings are usually found behind chimneys, under eaves, soffits, ventilators, and door and window frames. Seal all openings of 1/4 X 1 1/2 inches or openings of 5/8 X 7/8 inch or greater. Seal with masking tape, duct tape, fine steel wool, fiberglass insulation, caulking, or hardware cloth.

If it is dangerous or not convenient to seal the openings after the bats leave, the use of a bat excluder (see next section) will temporarily prohibit the bats from entering the opening until it can be sealed in daylight hours.

Use the procedures outlined in the section below to exclude the bats before making repairs.

Removal of Bats from large roosts

Bird Netting - Hang polypropylene bird netting over the entrance hole 1 inch from the outside wall, 1 foot on each side and 1 foot below. The bats will be able to fly out, but they can't get back in.

Excluder - Attach a 5 inch long, 2 inch diameter hollow pipe to an old pillowcase or T-shirt and place over the bats’ major entrances after the other entrances are sealed. The T-shirt allows bats to leave but the narrow pipe prevents their re-entry. After 1 week, check for remaining bats, remove the excluder and permanently seal the opening. A commercial excluder, called the EX-100 Hanks Bat Excluder can be purchased and used in the same fashion.

Removal of bats from small roosts

Eliminate temporary roosts behind shutters and eaves by blocking the access points, using a sticky substance used to prevent bird roosting, and trimming the plants that shade the window. Coarse fiberglass batting tacked to where the bats like to roost sometimes discourages them.

Repellents used for temporary control

The following methods of control may provide a temporary solution to the problem, although they tend to be costly, slow acting or needlessly destructive of a beneficial animal. The best method of control is to exclude them, since the same problem may occur again the next roosting season. Methods not mentioned below include bat traps, poisonous tracking powders and fumigation.

Sound - Bats are may be repelled by high-frequency sound devices, although this is the least effective method and not highly recommended. An inexpensive device can be made by attaching a silent dog whistle to an aquarium pump. Place the device in the roosting area and run continually until the bats leave. Since sound does not penetrate solid objects, many sound devices may be required.

Distress cries have been used to lure bats away from roosts, but they are an ineffective repellent.

Light - Bats may be repelled by brightly illuminating their roosting area, although bright lights can be a fire hazard, are inconvenient and expensive. The attic of an average size house may require four or more 100 watt bulbs to drive out the bats. Illuminate all potential roosting sites and check the roost after several days. The addition of windows in an attic may help reduce the likelihood of bat roosts.

Odor - Bats may be repelled by the odor of naphthalene (moth balls) in confined spaces such as attics or walls. Use 2 1/2 lb per 1,000 cubic feet of space, or hang moth balls from a net bag for easier removal. Bats will return, however, when the odor dissipates. The gases are also harmful to humans if they are inhaled for too long.

Air Drafts - Strong breezes and cooler temperatures created by open windows or fans can repel bats.

Sticky Substances - Sticky substances used to prevent birds from roosting is also effective for bats. The substance does not trap the bat, but makes it unpleasant for the bat to stay.

Removal of Occasional Bat Intruders

Let it find it’s way out - A bat that has blundered into a house will usually find its way out by detecting air movement. Confine the bat to one room, open windows and doors and let it escape. At night, lights should be dimmed so it can find the openings and won't hide behind drapes or in closets.

Catching - If the bat can't escape, catch it with a hand net or fishing net. If it rests, catch it with a coffee can or similar container. Once caught, release it outside at night. Use work gloves at all times. Chasing a bat with a broom, racquet, or newspaper is often futile and will often cause a bat to seek safety, making escape more difficult.

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Trapping - Trapping is not recommended, since it is more effective and safe to exclude bats.

Sanitation - Since bats may be attracted by the odor of bat urine and droppings, all droppings should be cleaned up and the roost area washed after the bats are removed. To remove guano, lightly spray the droppings with water to control dust, then remove with a wet/dry shop vaccuum. Dispose of the droppings in a sealed bag in the trash or buried in your yard. If there is too much to vaccuum, wear protective clothing and a respirator while shoveling the droppings. Scrape off all crystallized urine, then wash the area with soap and water. Disinfect the area with one part bleach and 20 parts tap water.

Bat houses

Artificial roosts have been used in Europe for over 60 years, but have been slow to gain popularity in the U.S. It appears that they may attract bats that are displaced from a structure. Bat houses should be located away from human high use areas. Bat house plans are available in libraries and MSU Extension offices.

Resources