Overview of Prevention and Control Methods
- Change to exterior light bulbs less attractive to insects
- Use lights and fans to make roosting sites unattractive to bats
- One-way doors or check-valves, caulk, flashing, screening
- Not effective
- Naphthalene flakes registered as a repellent in some states.
- No toxicants registered
- Not practical; illegal in some states
- Not reccommended
- Bat traps; prevent unnecessary stress or death to bats
Damage Prevention and Control Methods
Costs for remedial services are highly variable, depending on the nature of the problem and who will do the work. For example, to fabricate a few bat check-valves on the “average” 2-story house would probably require 2 workers about a half day, mostly on stepladders, and less than $50 in materials. Much more time would be required to seal all the other active and potential exit and entry holes. In addition, if a deteriorated roof, eaves, or other woodwork needs replacement, costs can increase rapidly.
In outdoor settings, swap white-light bulbs for bulbs less attractive to insects. Illumination has been reported to have limited effectiveness at moving bats out of structures. Floodlights strung through an attic to illuminate all roosting sites may cause bats to leave. For this method to be effective, the light level must be bright by human standards. Fluorescent bulbs may be used. In some situations, such lighting is difficult, costly, and is an electrical hazard. Note that bright light may drive bats into wall voids where control can be more difficult.
Air drafts have been successful in repelling bats in areas where building owners can open doors and windows, or create strong breezes with electric fans. The addition of wall and roof vents will enhance this effort, as this will lower roost temperatures. The above-described measures will increase the thermoregulatory burden on the bats, making the roost less desirable.
Colonies in soffits, behind cornices, and other closed-in areas can be discouraged by opening these areas to eliminate dark recesses. Discourage bats from roosting behind shutters by removing the shutters completely or by adding small blocks at the corners to space them a few inches away from the wall.
Eviction/venting and exclusion is the gold standard for eliminating and preventing bats from residing in structures. It is tedious to locate all active and potential openings available to bats. Active holes can be identified by rub marks, guano, and sometimes odor. Except for the actively used holes, seal all gaps of ¼ x 1½ inches and openings 5/8 x 7/8 inches or greater. Bats use some of the same holes in buildings through which heat (or cooled air) is lost. Bat-proofing can reduce energy costs for the client. As with any exclusion intervention, the excluded animals will be forced to go elsewhere. The shift may be to another building nearby.
There is no guarantee that bats will use a bat house, but research has shown that bat houses can be successfully occupied during and after an eviction. Ideally, bat houses should be erected a few months to a year before a scheduled exclusion to give bats time to find and explore the new roosting option. Bat houses should be installed near the original roost to increase the likelihood of bats finding the new habitat. Choosing the proper location, placement, design, color, and materials are all important factors for increased success.
Timing is important to reduce the risk of separating adults from flightless young. Although regulation does not prohibit removal of bats during the pup season, eviction and subsequent exclusion generally should not be attempted between May 15 and August 15. One-way eviction devices installed during the maternity season will allow adult bats to leave the structure but flightless young bats will be trapped inside. Mothers of trapped pups may attempt to reach their young by flying back into the living spaces of homes. Trapping bats inside an attic will increase the chance of them finding their way into the living quarters of a home as they search for a way out. An alternative to exclusion is to seal unused holes, but leave active holes open from May 15 until August 15. During the maternity season, your primary strategy should be to concentrate efforts on locating and sealing off entries into the client’s living spaces. In structures used by bats for hibernation, eviction and venting during the winter months should be avoided as well. Clients should be made aware of the increased chances of exposure that are the result of eviction/venting at the wrong time of year.
Exclusion materials and methods for bats are less rigid than for rodents, as bats do not chew into structures. Caulk, flashing, screening, and insulation often are needed to complete an exclusion job. The combination of materials used will depend on the location, size, and number of openings and the need for ventilation. Weather stripping and knitted-wire mesh (Guard-All®, Stuf-fit®) are best applied during dry periods when wood cracks are widest. Caulk can be applied with a caulking gun (in gaps up to 0.4 inch wide) and include latex, butyl, and acrylic compounds, which last about 5 years. Elastomeric caulks, such as silicone rubber, will last indefinitely, expand and contract, will not dry or crack, and can tolerate temperature extremes.
To prevent bats from entering chimney flues, completely enclose the flue discharge area with rust-resistant spark arresters or pest screens secured to the top of the chimney. They should not be permanently attached (e.g., with screws) in case they must be rapidly removed in the event of a chimney fire. Review fire codes before installing flue covers. Dampers should be kept closed except during the heating season.
Oakum packs easily and firmly into small cracks. Other fillers include sponge rubber, glass fiber, knitted-wire mesh, and quick-setting putty. Self-expanding polyurethane foam applied from pressurized containers can be used for openings larger than 3 inches. It must be applied with caution so clapboards, shingles, and other surfaces are not lifted. Surfaces that are exposed should be sealed with epoxy paint to prevent insect infestation and ultraviolet degradation. Conventional draft sweeps (metal, rubber) and other weather stripping supplies (felt, vinyl, metal) will seal the space between a door bottom and the threshold or around windows.
Treat attic and basement doors whenever the gap exceeds ¼ inch. Flashing may be used to close gaps at joints (e.g., where the roof meets a chimney). Materials include galvanized metal, copper, aluminum, stainless steel, and self-adhesive stainless steel “tape.” A potentially useful intervention for the wall-ceiling interface is the application of a wide 45° molding strip to eliminate the 90° angle corner and force the bats to roost in a more exposed area. Insulation provides some barrier to bat movements. It is available in several forms and types including fiberglass, rock wool, urethane, vermiculite, polystyrene, and extruded polystyrene foam. Inorganic materials are fire and moisture resistant. The safest appear to be fiberglass and rock wool.
The mesh size of screen must be small enough to prevent access of bats and other species. Hardware cloth with ¼-inch mesh will exclude bats and mice. Screen with 16 meshes per inch will exclude most insects. Soffits (underside of overhanging eaves) usually have vents of various shapes and sizes. The slots should not exceed ¼- x 1-inch and should be covered on the inside with insect mesh.
Exclusion on tile roofs can be difficult. Bats often roost under Spanish or concrete tile roofing by entering the open ends at the lowermost row or where the tiles overlap. Tight-fitting plugs are difficult to make due to the variation in opening sizes and thermal expansion and contraction. A layer of coarse fiberglass batting laid under the tiles so that bats entering holes contact the fiberglass can be an effective barrier. A layer of knitted wire mesh also will work well for this purpose and will not hold moisture. Bats also may be excluded from the tiles if rain gutters are installed directly under the open ends. Gaps under corrugated and galvanized roofing may be closed with knitted-wire mesh, self-expanding foam (avoid causing roofing to lift), or with fiberglass batting (may retain moisture).
One-way doors are excellent for moving bats out of structures. A variety of 1-way doors and check-valves are available, and no single device is suitable for every situation. Install 1-way doors on holes that are actively used by bats to enter or exit the structure. One-way doors should be left in place for at least 3 to 5 days. During periods of inclement weather (e.g., rain), 1-way doors should be left in place longer.
The basic design attaches netting (fiberglass mosquito netting works well) around an exit hole except at the bottom where the bats will escape. Designs must be open enough so they do not interfere with bats exiting the hole.
When their routine exit points are blocked, bats may seek alternative entry points, often causing some bats to find their way into living quarters of homes. Do not lay netting flush against the wall, as this will prevent bats from exiting. Fold the netting to provide the exiting bats a little gap to move into and then down and out. The width and shape of these “check valves” (allowing bats to exit but not enter) is highly variable so they can cover the necessary exit points such as a single hole, a series of holes, or a long slit-like opening. The top can be much larger than the bottom. The length of the netting (the distance from the lowest enclosed point of egress to the bottom of the netting) should be about 3 feet, and stick out about 3 to 5 inches from the wall so the bats can crawl beneath the screen to leave. The above specifications usually are sufficient to keep bats from reentering the space.
Tubes, such as the Batcone®, provide another tool to exclude bats. Center the tube hole over the exit used by the bats to provide an easy way out.
Frightening devices are not appropriate for the control of bats. Ultrasonic devices have been developed for certain specialized commercial applications, but generally are too expensive for domestic use.
Many chemical aromatics and irritants have been proposed and tested for repelling bats, though efficacy has proven very limited.
Naphthalene crystals and flakes are the only repellents registered by the US Environmental Protection Agency (EPA) for indoor bat control and are to be applied in attics or between walls. Confirm that it is legal to use in your state. Sometimes the chemical may be placed in loose-mesh cloth bags and suspended from the rafters. About 2.5 pounds per 1,000 cubic feet (1.2 kg/30 m3) is recommended to chronically repel bats as the chemical vaporizes. Dosages of 5 pounds per 1,000 cubic feet (2.4 kg/30 m3) may dislodge bats in broad daylight. Bats will return, however, when the odor dissipates. The prolonged inhalation of naphthalene vapors may be hazardous to human health. The area must be sealed so odors do not enter the living space, as naphthalene vapors may cause health problems for some individuals.
Contact repellents, such as sticky-type bird repellents and rodent glues, have been used successfully in situations where roost surfaces may be coated. Apply masking tape to the surface first if you desire to remove the repellent after treatment is finished. Replenish contact repellents occasionally, since dust accumulation causes them to lose their tackiness. Apply coatings that will be sticky, but will not entrap the bats. This technique may not be legal in some states.
No toxicants are registered for controlling bats.
Shooting bats is not practical and it is not legal in many states.
Trapping of bats not recommended. Bat Conservation International (BCI) does not promote NWCOs who use bat traps. Eviction/venting and exclusion is less complicated, less time-consuming, more effective, and requires no handling of bats.