The presence of bats inside a structure can be a distressing discovery for any property owner. While these nocturnal animals are incredibly beneficial to the environment, consuming vast numbers of insects, their indoor presence poses health and nuisance concerns. The goal is to encourage the bats to leave the structure safely and permanently, which requires understanding the only truly effective technique: physical exclusion. This approach focuses on preventing re-entry rather than attempting to frighten the animals away.
Exclusion Versus Deterrence
The process of safely clearing bats from a building involves a critical distinction between exclusion and deterrence. Exclusion is the preferred and effective method, involving physical modifications that prevent bats from re-entering once they have exited. This strategy works because bats cannot chew or create new entry points; they must use existing openings to enter and exit a roost.
In contrast, deterrence involves using lights, sounds, or smells to scare the bats away from the roosting site. Deterrent methods are generally ineffective for long-term removal because they do not physically seal the entry points. At best, a deterrent might cause the bats to move to a less accessible area within the same structure, shifting the problem rather than solving it. Successful bat removal requires the physical barrier approach of exclusion.
Implementing Physical Exclusion Methods
Effective exclusion begins with a thorough inspection to locate all entry and exit points, which can be openings as small as one-half inch in diameter. Common entry sites include gaps where pipes or wires enter the building, loose roof shingles, or openings around chimneys and vents. Once all potential and active openings are identified, the majority should be permanently sealed using materials like weather-resistant caulk, hardware cloth, or wood.
The remaining active entry points must then be fitted with specialized one-way exclusion devices, often called bat valves or funnels, to allow the bats to exit but not return. These devices are typically constructed from lightweight plastic netting or sheeting, attached firmly at the top and sides, with the bottom left unattached to hang 18 to 24 inches below the opening. For smaller, round openings, tubes made from PVC pipe or flexible plastic tubing can be used.
The one-way devices must remain in place for a minimum of five to seven nights to ensure every bat has left the roost. This period should include several nights of weather conducive to flight, meaning temperatures above 50 degrees Fahrenheit with minimal wind or heavy rain. Once monitoring confirms that no bats remain, the devices are removed, and the final openings are permanently sealed. Avoid using expanding foam, which can trap and injure bats as it hardens.
Deterrent Methods That Are Ineffective
Many methods are marketed as bat repellents, but wildlife professionals regard them as non-solutions that waste time and effort. For example, ultrasonic devices emit high-frequency sound waves intended to disrupt a bat’s echolocation and communication. Scientific testing has shown that these devices do not effectively change the behavior of wild animals, and the sound does not travel well through the irregular spaces of an attic or wall cavity.
Attempts to repel bats using strong odors like mothballs, which contain naphthalene, are also ineffective. Naphthalene is toxic to humans and pets, and the required concentration to bother a bat is impractical to maintain in a large attic space. Similarly, bright lights or air currents from fans are often suggested but typically encourage the bats to move deeper into a darker, more sheltered area of the structure. These deterrents only delay the necessary step of physical exclusion, allowing the colony to remain and the problem to worsen.
Safety and Legal Constraints
Before beginning any exclusion work, consider the safety and legal aspects of dealing with a bat colony. Most bat species are protected by state and federal laws, and the timing of removal is strictly regulated to ensure humane practices. Exclusion must not be performed during the maternity season, which typically runs from early April or May through late August, depending on the region.
During this period, flightless young, or pups, are present and dependent on their mothers. Sealing the entry points during maternity season will trap the young inside, leading to their death. The presence of bats also carries health risks, including the small chance of rabies transmission from direct contact and the risk of histoplasmosis, a respiratory disease caused by inhaling fungal spores found in accumulated guano. For these reasons, property owners should wear appropriate protective gear, including a respirator, when dealing with guano, and consider consulting a professional wildlife control specialist for safe and legal removal.