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Do bats serve as an effective mosquito control?
Recently the public has shown increased interest in the value of insectivorous species of bats in controlling mosquitoes. Although untested lately, this is not a new idea. During the 1920's several bat towers were constructed near San Antonio, Texas, in order to help control malarial mosquitoes. Mosquito populations were not affected and the project was discontinued. Bats in temperate areas of the world are almost exclusively insectivorous. Food items identified in their diet are primarily beetles, wasps, and moths. Mosquitoes have comprised less than 1% of gut contents of wild caught bats in all studies to date. Bats tend to be opportunistic feeders. They do not appear to specialize on particular types of insects, but will feed on whatever food source presents itself. Large, concentrated populations of mosquitoes could provide adequate nutrition in the absence of alternative food. However, a moth provides much more nutritional value per capture than a mosquito. M.D. Tuttle, a world authority on bats, is often quoted for his anecdotal report that bats effectively controlled mosquito populations at a popular resort in New York State. While there is no doubt that bats have probably played a visible, if not prominent, role in reducing the mosquito problems in many areas, the natural abatement of mosquito populations is an extremely complex process to study, comprising poorly known ecological relationships. Tuttle attempts to underscore the bats role by citing an experiment in which bats released into a laboratory room filled with mosquitoes caught up to 10 mosquitoes per minute. He extrapolated this value to 600 mosquitoes per hour. Thus, a colony of 500 bats could consume over a quarter of a million mosquitoes per hour. Impressive numbers indeed, but singularly unrealistic when based upon a study where bats were confined in a room with mosquitoes as their only food source. There is no question that bats eat mosquitoes, but to utilize them as the sole measure of control would be folly indeed, particularly considering the capacity of both mosquitoes and bats to transmit diseases.
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Do Purple Martins help reduce mosquitoes?
It has been known for many years that bird species like purple martins consume large numbers of flying insects. Proponents of their use in mosquito control are quick to cite J. L. Wade, an amateur ornithologist, who reasoned that an average 4 oz. adult purple martin, due to its rapid metabolism, would have to consume its body weight (14,000 mosquitoes) per day in order to survive. Wade recognized that the purple martins diet includes many other types of insects, but this appears to have been lost on many individuals searching for a natural means of control. In fact, during daylight, purple martins often feed voraciously upon dragonflies, known predators of mosquitoes. At night, when mosquitoes are most active, purple martins tend to feed at treetop level, well above most mosquito flight paths. Ornithologist James Hill, founder of the Purple Martin Conservation Association (PMCA), writes, "The number of mosquitoes that martins eat is extremely insignificant, and they certainly don't control them. In-depth studies have shown that mosquitoes comprise no more than 0 to 3 percent of the diet of martins". They eat only flying insects, which they catch in flight. Their diet is diverse, including dragonflies, damselflies, flies, midges, mayflies, stinkbugs, leafhoppers, Japanese beetles, June bugs, butterflies, moths, grasshoppers, cicadas, bees, wasps, flying ants, and ballooning spiders. Martins are not, however, prodigious consumers of mosquitoes as is so often claimed by companies that manufacture martin housing. An intensive 3-year diet study conducted at PMCA headquarters in Edinboro, PA, failed to find a single mosquito among the 350 diet samples collected from parent martins bringing beakfuls of insects to their young. The samples were collected from martins during all hours of the day, all season long, and in numerous habitats, including mosquito-infested ones. Purple Martins and freshwater mosquitoes rarely ever cross paths. Martins are daytime feeders, and feed high in the sky; mosquitoes, on the other hand, stay low in damp places during daylight hours, or only come out at night. Since Purple Martins feed only on flying insects, they are extremely vulnerable to starvation during extended periods of cool and/or rainy weather. Rather than erecting martin houses to specifically attract insect-eating birds for mosquito control, we should at least promote them for their aesthetic and educational value.