VERTEBRATE PESTS: Are Hibernating Pests Driving You Batty?

As most pest professionals know, the insectivorous and “colonial bats,” such as the big brown bat (Eptesicus fuscus), the little brown bat (Myotis lucifugus) and the Mexican free-tailed bat (Tadarida brasiliensis), commonly establish roosts inside buildings within their ranges from April through September or October.

In most cases, summer structural infestations involving more than just a few bats are “nursery colonies.” That is, the bats move into structures to give birth and rear their young within the protective spaces of a warm attic or some other type of structural void. When colder nights arrive, bats in temperate regions must hibernate (or migrate) because their food source (flying insects) is not available.

The onset of winter is a dangerous time for most colonial bats. Bats must find acceptable quarters that provide the right environmental conditions for successful hibernation. These include proper ambient temperatures, a suitable resting surface or harborage, low airflow and a certain degree of humidity. Bats must also store enough fat reserves to sustain their metabolisms while inactive. Successful bats evacuate their summer roosts before the onset of the coldest temperatures of autumn. This allows the bats sufficient remaining time before the winter season to migrate, or disperse to a winter roost, mate, and build up fat reserves before entering hibernation. Mortality can be high for first-year bats, especially if the winter is severe.

Let’s take a look at each of the “urban bats” and their selection of winter roosts.

The big brown bat is the most common structure-infesting bat in the U.S. They typically establish summer nursery roosts ranging from a few to 200 bats, depending on how long the roost has been repeatedly used over the years. Come winter, most big brown bats leave their summer roosts in fragmented clusters and hibernate in caves, mines, storm sewers and some specific buildings. The clusters range in size from a solitary bat to groups of four to six. Slightly larger groups do occur, but are not typical. This species tends to remain within a 50- to 75-mile radius of summer roosts.

Little brown bats are less likely to be found inside buildings during the winter even though they commonly establish large (500 or more) nursery colonies inside buildings. Little brown bats typically migrate to caves to hibernate in large clusters. These winter caves may be nearby, or several hundred miles away from the nursery roosts, and are characterized by cool, stable temperatures, high humidity and slow air currents.

The Mexican free-tailed bat is a common structure-infesting bat in the South. Free-tails, generally, are not considered a true hibernating species. During temporary cold snaps within their region, they may enter short periods to torpor. Free-tails usually spend winters by shifting their home ranges to match those areas warm enough to accommodate insects.

ROOSTING BATS. Winter reports of bats inside buildings in temperate regions of the country are most likely caused by big brown bats. When big brown bats use buildings as hibernation sites, the building may be the same as the summer roost, or it may be an entirely different building within the bat’s dispersal radius. This is an important point, because both homeowners and some pest professionals alike mistakenly assume winter sightings of bats are probably being used as summer roosts also. Too, for structures where a summer roost has been identified, and then a few bats are noted to be flying around during the winter, it is often also incorrectly assumed that the colony never left the structure and overwintered in the same structure.

Scientists have found that bats require temperatures above freezing, but cold enough to allow the bat to remain in the physiological state of hibernation — at least for a certain number of prolonged periods. Environmentally speaking, the location has to be specific to serve as good hibernacula.

In one study, the average temperature of the hibernating spaces for bats utilizing buildings was about 41°F. Obviously, the building must be heated. But somewhere in this heated building, the bat must locate a suitable spot where the environmental conditions conducive to hibernation exist. Hibernation for bats is not a continual prolonged sleep until spring. Most bats awaken periodically throughout the winter, and relocate to either colder or warmer areas of the building depending on the fluctuating temperatures. This explains why you receive stray bat calls during extremely cold days in winter, as well as during temporary warm-ups.

When hibernating bats inside buildings awaken, they may begin squeaking inside the walls, or if they fully awaken, they may begin flying about interior rooms. If it is still winter or early spring before insects are active, and these bats do not find a suitable space in which to re-enter hibernate, they are likely to die as a result of using fat reserves and a lack of water to sustain their now active metabolism.

IS A VISIT NECESSARY? When a client calls requesting information about hibernating bats inside their buildings, or requesting a service visit to remove a stray winter bat, you have several options. First, in the majority of cases, awakened bats tend to find their way back out of active rooms, or they succumb to thirst and weakness in some out-of-the-way cranny and are never seen again. Some may be successful in re-entering hibernation, but how many are successful at this is likely to vary widely and cannot be generalized.

When the whereabouts of the stray bat is not specifically known to the client, it is not likely you will find the stray bat should you make a service visit, especially if it was a previous night’s bat. Conducting an exhaustive search of the home in these cases is occasionally productive, but many times the bat is never found.

Still, some clients will gladly pay to have you conduct a thorough inspection and confirm that a bat is not “lurking” under their beds. If the bat is flying around when you arrive, wait until the bat lands. Slowly approach the bat with a coffee or similar size can and a piece of cardboard. Slowly cover the bat with the can and slip the cardboard underneath the can to trap the bat inside. During the visit, the client should be questioned as to whether or not intermittent sightings of bats in their building is common. If the answer is yes, the structure is probably being used as a regular roost either as a summer nursery colony or as a winter hibernaculum.

Regarding structural overwintering bats, accurate and timely information is the best services a pest professional can offer. For additional information on managing stray bats refer to my column in the September 1999 issue of PCT.

The author is president of RMC Pest Management Consulting and can be reached at rcorrigan@pctonline.com or 765/939-2829.