Big brown bat
Photo by: J. Darren Harris, USDA Forest Service
Bats are among Indiana's most interesting and unique
mammals and probably one of the most misunderstood.
There are many myths concerning bats that cause some
people to unnecessarily fear these mammals. Bats are,
for the most part, harmless. They rarely, if ever attack
people or "get tangled in your hair." In fact, most bats in
the U.S. are biologically useful mammals. They feed
primarily upon insects, many of which are pests to agriculture. However, bats can be pests if they take up residence
within buildings. There are good reasons for not tolerating
their presence. The scratching and squeaking noises they
create are annoying. Their droppings and urine not only
stain walls and ceilings, but also cause objectionable,
persistent odors that may attract insects and other bat
colonies, even after the original colony is eliminated.
Long-term accumulation of these droppings in attic spaces
has been associated with the respiratory disease Histoplasmosis. This disease is caused by a fungal spore
called Histoplasma capsulatum. Bat and/or bird droppings that have decomposed from 2-5 years or longer
provide an ideal habitat for spore growth and reproduction. And finally, there is a slight chance of someone
contacting a rabid bat, although the great majority of
house-infesting bats in Indiana are NOT rabid.
Bats belong to the order of mammals called the
Chiroptera, which means "winged hand." Because they
possess wings, bats are the only mammals capable of
true flight. Although they have functional eyes, they rely
on their "radar system" to navigate their swift and unerring
flights through dense tree growth and other obstructions.
There are 12 resident species of bats in Indiana. Most
roost in caves, hollow trees and other natural shelters. A
few species, however, commonly roost and breed within
occupied buildings during the warm-weather months. The
two species most frequently encountered by homeowners
are the big brown bat (Eptesicus fuscus) and the little
brown myotis bat (Myotis spp.).
The big brown bat is relatively large, measuring about
4 inches in length and having a wing span of 12-14 inches,
but weighs only 1/2 ounce. The body and wings are
usually dark brown; the ears are blackish. The little brown
bat is about 3 inches long, chocolate brown and weighs
1/4 ounce. Although these two species vary in size, they
are easily confused and usually only an expert can tell
them apart. Brown bats live an average of 4-10 years.
Big brown (1/2 ounce), Little brown (1/4 ounce).
Big brown (4 inches), Little brown (3 inches).
Big brown: 60 days, Little brown: 50-60 days.
Big brown: 2, Little brown: 1.
During the first warm days of spring, the brown bats
leave their overwintering sites and enter structures. The
majority of these bats are females preparing to give birth.
These groups are called nursery or maternity colonies.
The number of individuals in a colony may range from a
few to several hundred. Generally, if the colony contains
more than 200, it is likely they are little brown bats.
The babies are born during June and July. The big
brown bat female produces two young, while the little
brown bat female produces only one. Juvenile bats are
breast-fed until they are mature enough to fly and obtain
their own food (from 3 to 7 weeks). Bats usually remain in
their roosts all summer.
During September and October, males increase in
numbers at the roost and mating occurs (the sperm is
"stored" by the female during the winter, and fertilization
occurs in the spring when she departs from hibernation).
As temperatures begin to drop in mid-October, the bats
leave their summer roosts and travel to their overwintering quarters.
Hibernation sites vary among different colonies of
bats. Little brown bats hibernate as colonies, selecting
caves instead of buildings to pass the winter. Big brown
bats on the other hand, disperse and hibernate individually or in small groups utilizing various sites including
hollow trees, rock crevices, drainage pipes, and caves.
Usually, they do not hibernate as colonies within buildings. But it is common for small numbers of bats (typically
1-4) to hibernate in homes and other buildings if the attic
or walls provide a suitable environment for surviving the
winter. These bats sometimes become active during
either unseasonably cold or warm days of the winter,
causing concern among homeowners that a colony of
bats might be inhabiting the home all year long. But the
same bats which overwinter in one structure may use a
completely different building for their summer roost. In
cases of winter sightings, the attic should be inspected for
structural deficiencies, and all openings should be sealed
as described under bat proofing.
While all types of buildings are susceptible to bat
infestation, certain factors seem to make some structures
especially inviting as "maternity roosts." For example,
bats usually select buildings located near water (streams,
lakes, ponds, etc.) and food (i.e., where insects are found
in adequate numbers). Also, there is some support for the
notion that bats are attracted to buildings by odors from
previous infestations, coupled with high roost temperatures which are conducive for rearing young. And, of
course, the structure must be accessible (i.e., the bats
must be able to get in and out). Actual roost sites within a
building are either (1) in concealed areas like roof and
ceiling voids, brick voids, etc. or (2) in exposed areas such
as ceiling joists and roof rafters (Figure 2).
Fig. 2. Among the roosting sites of bats within
buildings are dark, draft-free, upper exposed areas like roof rafters or ceiling fans.
During the day, bats remain in a restful or semi-restful
state. They emerge from the structure beginning at dusk.
Most of the bats will be out of the roost within 60 minutes
from the first bat's exit. Then they fly to feeding and
watering areas, which often will be a field and/or stream
near the roost structure. By dawn, the bats are back in the
There is no need to panic if a bat is seen flying around inside the home. It may mean nothing more than a bat flew in through an open window or chimney. However, it's presence could signify that a colony is living in the attic. Bat colonies within homes and buildings may range between a dozen to several hundred bats.
To determine if a colony exists, a simple outdoor inspection of the home's roof area should be conducted at dusk, when house-infesting bats emerge for feeding. This inspection should reveal the exit and entry points of the bats or structural deficiencies of the building and size of the infesting colony. It is important to remember that if bats are present, they will emerge each evening, except in cases of inclement weather.
To make the inspection, situate two people at opposite
corners of the structure so that each can view two roof
areas at the same time (if a building has several wings,
more than two people may be needed). Begin the inspection approximately 1/2 hour before dusk and continue for
about an hour after the first bat emerges.
Common exit points include attic vents, rooflines (e.g.,
between the roof sheathing and the fascia boards), and
any openings resulting from building deterioration (Figure
3). Openings of only 3/8 inch are sufficient for bat entry. In
some cases, such as barns, exit and entry points may be
the actual doorways. Exit/entry locations can also be
determined by preemergence noise, droppings below the
exit points, odor, or smudge marks in the areas of entry.
Fig. 3. A) Bats only need an opening of 3/8 inch to invade a space; B) Cloth funnel exit;
C) Plastic netting exclusion.
An inspection inside the home, particularly in attic
areas may also reveal evidence of bat activity. You may
see the bats themselves hanging from rafters in dark,
secluded spots in the attic, you may find the remains of a
dead bat, or you may find accumulations of bat droppings
on the floor. Old, dried bat droppings sometimes have a
"coffee grounds" appearance. To minimize any risk of
exposure to Histoplasma, precautions should be taken to
safely clean up and remove those droppings if significant
accumulations of old bat droppings are found. Lightly mist
the droppings with water in order to avoid stirring up dust.
Scoop up the droppings and put them in plastic bags. If
desired, cleaned surfaces may be sprayed with a 2%
bleach/water sanitizing solution or a hospital quality fungicide. At minimum, a respirator and disposable gloves
should be worn during any droppings clean up.
Making a structure "bat proof" is the best long-term, cost-effective, and biologically acceptable way to control bats. Remember, the objective in controlling bats is to rid a building of the colony permanently. Exclusion is the only method that can guarantee this goal.
The best time of the year to bat-proof a building is
either in late fall after the bats depart for hibernation, or in
late winter/early spring before the bats arrive. If, for some
reason, it can only be done during the summer, the
preferred time is mid-August or later and in the following
- seal all but one or two of the principal
openings, and wait 3-4 days for the bats to adjust to using
the remaining openings;
- install a one-way exit so that
the bats can leave but not re-enter the building (one-way
exits can be made by using a mesh screen funnel over the
exit hole, installing a "bat checkvalve", or by securely
attaching the end of an old pant leg around the remaining
exit holes, thereby forming a chute for the bats to crawl out
through (Fig. 3B)); and then
- permanently seal those
openings after there is no longer any evidence of bat
External exclusion programs should never be
attempted mid-May through mid-August (nursery phase),
because the young will be trapped within the structure and
die, creating odor problems. Preventing bats from entering occupied rooms is usually a primary concern for those
people experiencing a bat infestation. During this nursery
phase, exclusion programs can be focused on the interior
areas of the building using the same exterior guidelines
All openings 3/8 inch and larger must be sealed (Fig.
3A). For permanent sealing, use materials such as 1/4-
inch hardware cloth, sheet metal, plywood, mortar, or
aluminum flashing. For temporary exclusion, rags, steel
wool, weather stripping, or foam sealants can be used.
In some cases, such as very large, older buildings,
excluding the bats will require a substantial amount of
time to repair all the holes the bats are using to gain entry.
There are, however, various products that provide practical, economical, permanent exclusion even for buildings
One product is a plastic bird netting (Fig. 3C). Although
developed primarily to protect high-value agricultural crops
from birds, it is also used to exclude birds from structures
and can be used in bat control. The netting is very light, supple, yet relatively tough and resilient; it is easy to work with and relatively inexpensive. Depending on the situation, it can be draped over entire roof areas, or cut and applied as needed to cover only certain sections.
It may be practical in some situations to net the susceptible parts of a building during April and May (when the bats begin arriving). The net then can be removed and stored until the following spring. Information concerning manufacturers of plastic netting for bat control can be obtained by contacting the USDA APHIS Wildlife Services Program located at Purdue.
Various caulks or foam sealants can be used to seal cracks and crevices in walls.
If, for some reason, bat proofing is not possible or bats need to be forced out of a building before an exclusion program is begun, repelling methods can be attempted. In most cases, they are not very successful.
Sudbury's Bat Repellent and Dr. T's Bat Scat are
currently the only chemical bat repellents registered in
Indiana that can be used in structures. In most cases,
chemical repellents are not very effective. When sprinkled
in areas where only a couple of bats are roosting in small
confined air spaces, such as tight attic spaces, double
walls, between roof and attic ceilings or other boxed-in
areas, it may encourage the bats to leave the building.
Keep in mind that the repelling effect is only temporary,
and treatments must be repeated every few weeks.
Moreover, the bats may simply move to another part of the
building where there is no repellent odor. Also, repellents
should not be used if humans are exposed to constant
inhalation of the odors. Some individuals are sensitive to
these products and, thus, should avoid contact with the
Fiberglass Insulation. Some bats may roost temporarily during the night in open structures or on outside
areas of homes, such as garages, carports, patios, behind shutters and building overhangs. Bats sometimes
can be discouraged from using these areas by tacking
coarse fiberglass batting to the specific areas where they
Bright Lights. In cases of small attic areas, bright lights
may repel bats. Attics can be illuminated using four or
more 150-watt bulbs; direct the lights at the entry points.
These lights should be turned on after the bats have left
for their evening feeding. Thus, they will have to return into
the bright light, something they will be adverse to doing.
Although illumination has been successful in a few cases,
it is not very practical for most situations, such as bat
infestations in larger buildings. Bats have sometimes
been observed feeding on the insects attracted by outside
Drafts and Air Conditioning. Drafts of cool air within
roosts will sometimes repel bats from a structure. Thus,
carefully directed breezes produced by electric fans may
Air conditioners installed in bat roosts and turned on
during April and May discourage bats from roosting since
they prefer high temperatures for rearing young. This
approach might be worth a try where exclusion cannot be
Ultrasonic Noise. Despite some claims, none of the ultrasonic sound devices have been found effective for repelling bats from buildings.
People occasionally inquire among pest control professionals as to whether or not the nuisance bat colony
can be "exterminated" by applying some kind of poisonous material. This is not an acceptable control alternative
because there are no poisons that can be legally used for
bat control. Besides, poisoning bats might only worsen
the situation. The bats may die in an inaccessible area
within a building, thereby creating an odor problem. Also,
bats affected by the poison could disperse over an entire
community before dying. This may increase the chance of
contact with a child, dog, cat, etc., thereby increasing the
potential for a "rabies scare" and/or a possible rabies
Recently, bat houses have become commercially available. Most of these houses are designed to be attached
to trees or the sides of buildings. But the factors involved
in bats selecting their roosts are relatively complex. Location, temperature, humidity, orientation of the house,
roost height, and other biological factors determine whether
or not bats will utilize a particular area or structure.
Although there have been a few cases of success, research thus far has shown that most of the commercially
available "homeowner style" bat houses are not very
effective in attracting bats.
Fig. 4. A) Bat house; B) Little Brown Myotis
Nevertheless, bat houses are fun to try, provide a great
educational vehicle to teach young people about bats,
and may even be successful if they are carefully installed.
The following tips may increase the success of the bat
- Bat houses should be erected from 10 to 15 feet
above the ground.
- The house should be protected from the prevailing (north
and west) winds.
- Do not situate the house where the entrance is
obstructed by tree limbs or vegetation.
- Bat houses placed within 1/4 mile of a permanent
water source are more likely to attract bats.
- Do not paint the house, or use any chemicals on
the house as bats are very sensitive to chemicals.
- Bats typically will not occupy a house right away. Most bat houses are not used the first year, and some may never be used.
Consumers should be careful about purchasing bat houses. There are specific construction factors for an effective bat house which must be followed, or the house may not have any chance of becoming inhabited. For more detailed information on bat houses and suppliers, contact: Bat Conservation International, 800-538-2287, P.O. Box 162603, Austin, TX 78716, or your local Department of Natural Resources.
On occasion, one or two bats may get into a home and fly around. Most times, it is a big brown bat that entered accidentally through an open window, door or ungrated fireplace. When this happens, there is no need to panic. These bats will not attack people.
Usually, the bat will find its way back outdoors by following fresh air movements. Therefore, leave windows and/or doors open to help it escape. Also, turn off all the lights; if any are left on, the bat may seek refuge behind wall hangings or drapes.
If a bat refuses to leave, it can be caught with an insect net, coffee can or gloved hand and released outside.
Many species of bats are found in Indiana and
some are protected by state law. Before conducting any type of bat control or relocating any bats
that have been captured, contact local representatives.
The information given herein is for educational purposes only. Reference to commercial products or trade names is made with the understanding that no discrimination is intended and no endorsement by State or Federal Agencies is implied. This program serves people of all ages regardless of socioeconomic level, race, color, sex, religion, disability, or national origin. The Wildlife Conflicts Information Hotline is a cooperative program of the United States Department of Agriculture - Animal and Plant Health Inspection Service - Wildlife Services, Indiana Department of Natural Resources-Division of Fish & Wildlife, and Purdue Cooperative Extension Service.