DO BATS CONTROL MOSQUITOES?
Dr. Robert Corrigan
Often times, people are heard to say, "We should not harm bats because they control mosquitoes and other pests." But is this true? Do bats, through their feeding, control mosquitoes, or any pests for that matter? Let's take a closer look at bats and their feeding behavior.
Bats are members of the mammalian order Chiroptera, which means "winged hand." They represent our only true flying mammals. Bats are not flying mice or rats. In fact, they are not even closely related to rodents. With the exception of only a very few species of bats found in the Southwest that feed on nectar, pollen and fruit, the 40 different bat species of the United States feed exclusively on insects. The species that are most commonly found around urban communities, are the "colonial bats," which- include the big brown bat, Eptesicus fuscus, the little brown bat Myotis lucifugus, and the Mexican free-tailed bat, Tadarida brasiliensis. The big brown and little brown bats are our most commonly encountered bats around structures in most states, but the Mexican freetailed bat is very numerous in Texas and several other southwestern states.
WHAT'S FOR DINNER? Bats may be both opportunistic and selective in their feeding, and several factors are involved as to which specific insects may be consumed in the greatest quantity. In general, research has shown that the little brown bat feeds on softbodied insects such as moths, flies, midges, mosquitoes and mayflies. The larger big brown bat is opportunistic, and preys mostly upon beetles such as ground beetles, June bugs, cucumber beetles and other beetles and insects. The Mexican free-tailed bat consumes primarily moths and beetles.
Among the various types of insects consumed by bats, some are of obvious pest significance, such as the flies, mosquitoes and cucumber beetles mentioned previously. And it is true that a colony of bats can consume thousands, even hundreds of thousands, of insects over several weeks of feeding. But there are several reasons why this cannot necessarily be interpreted as bats control pest populations."
First, insect populations have no trouble compensating for their losses to bats (or to-'insectivorous birds, or even to our cars smashing them every night along our highways and interstates). The populations of many insect species, especially flies and mosquitoes, are measured in the millions, and commonly in the hundreds of millions. Second, relative to their relationship to people and the ecosystem, bats consume "bad bugs," "good bugs" and "neutral bugs." They are not selective in consuming only those insects that annoy people. Certainly, if a colony of bats consumes several hundred mosquitoes each night in an area, there will be less mosquitoes in that area. But does that mean we can sit out on our porches at night without using bug repellent? Don't count on it. There are many other factors at play.
This was clearly borne out to me when I was preparing my master's project on bats. I would stand outside of bat roosts (some containing up to 700 bats) and count the bats as they emerged to feed. The most uncomfortable aspect of that job was how the mosquitoes used to eat me alive while I counted the bats directly over my head. In jest, I used to murmur to the bats as they whizzed by, "Come on! Do your job!" But they had bigger and better goodies to eat over the streams and fields a mile out of town.
Third, we must consider the foraging strategy of the bat. Some people envision bats flying all over their neighborhoods, all night long, capturing and swallowing mosquitoes until dawn. How nice a thought when contemplating plans of sitting out at night during the summer. But the foraging strategy of bats is designed to provide the bats an efficient means of gathering food. Why would a bat spend lots of valuable energy "chasing down" mosquitoes if several larger insects can give it a faster nutritional return?
BATS PREFER "FAST FOOD." For example, research has shown that depending on the local availability of night flying insects, bats usually consume their nightly requirements within the first hour following their emergence from their roosts. In some cases, they feed for short periods, rest temporarily, and then resume feeding again before returning to their roost. Like most mammals, bats have energy budgets to maintain for flight activities, rearing and feeding their young, homeostasis. and fat storage preparation for the winter hibernation. Therefore, bats are usually as efficient as possible to limit their exposure time to predators and to gather more energy and fat than they lose. As an analogy, consider when you eat out. You don't normally waste your valuable energy and time eating your salad, entree and dessert in three different restaurants.
It is true that if the opportunity presents itself and a bat can collect many small insects in one area quickly and efficiently, it will do so. Many of us have probably witnessed and marveled at bats swooping and diving around a street lamp, consuming large numbers of bugs. In these cases, the bats may fly among the insect swarm capturing the bugs, or even feed with their mouths open -- almost as if it were "aerial plankton." In this way they can fill their stomachs quickly. But neither mosquitoes, nor any of the other major urban pest species, comprise the majority of insects at a street lamp.
BENEFICIAL ANIMALS. Do bats contribute with all the other insectivorous animals in providing
some type of check and balance of some insect populations (both good and bad bugs)? Yes. And this role is critically important in the overall scheme of our ecosystems. Therefore, bats are biologically useful mammals, and are a very important and unique part of our wildlife. People should protect and even encourage bat populations outside and away from of our buildings. I would even promote pest control associations nationwide to join and support bat conservation groups such as Bat Conservation International (BCI) to show as an industry we do as we say in our logos: that we protect and guard our environment and provide stewardship of important wildlife.
But relative to pest populations, whether or not the feeding of bats in our urban and agricultural communities provides any measurable benefit (or negative impact via consumption of good insects) is highly questionable. At best, their beneficial contributions are likely to vary tremendously depending on the complex local environmental conditions and particular ecosystem. Be that as it may, bats are not dependable natural pest controllers in our urban and agricultural communities.
In graduate school, I fell in love with bats. I am constantly wishing for bats to move into our big, old barn so I can enjoy their company on our farm. But should they do so, I also know that I will still need the insect repellent, and I better constantly monitor the crops and trees for devastating pests.
Reprinted with permission from the author Dr. Robert Corrigan, a contrlbuting editor to PCT magarine, is president of RMC Pest Management Consulting, 5114 Turner Road, Richmond IN 473741 317/939-2829.
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