Yes, Bats Really Do Eat a Lot of Mosquitoes

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A little brown bat, Myotis lucifugus, in a Missouri cave. Ann Froschauer/U.S. Fish and Wildlife Service

Bats make good neighbors, largely due to their outsized appetites for insects that bug us. American corn farmers save about $1 billion every year, for example, thanks to the free, nontoxic pest control provided by bats that eat corn earworm moths.

And aside from their agricultural benefits, bats are especially beloved for preying on some of the planet's most despised and dangerous insects: mosquitoes. This service is a major reason why many people set up backyard bat houses, especially amid the expanding threat of mosquito-borne diseases like malaria, dengue, chikungunya, West Nile and Zika.

Yet while it's common knowledge that many bats feast on mosquitoes, the science behind that knowledge is surprisingly fuzzy. One commonly cited study suggests a single bat can eat 10 mosquitoes per minute, for example, but those experiments were conducted in enclosures, so they don't represent natural conditions. In the wild, one little brown bat (pictured above) can reportedly eat hundreds of mosquito-sized flies per night, but how many of those flies turn out to be actual mosquitoes?

To find out, a team of researchers did the dirty work for the rest of us. They visited wild bat colonies, collected bat droppings — aka guano — and searched for signs of mosquito DNA. Their study, published in the Journal of Mammalogy, included 12 roosts of little brown bats (Myotis lucifugus) and 10 of big brown bats (Eptesicus fuscus), located in forests and farmland across Wisconsin. Since both species occupy large swaths of North America, the findings are likely relevant well beyond the study area.

After gathering enough guano, the researchers screened their samples using a recently improved molecular method for detecting arthropod DNA. They found mosquito DNA at 100% of little brown bat roosting sites, and in 72% of individual samples from those sites. For big brown bats, mosquito DNA turned up at 60% of sites and in a third of all samples.

big brown bat flying
Big and little brown bats are both widespread in North America, and both species seem to eat a lot of mosquitoes. Scott Heron [CC BY-SA 2.0]/Flickr

The DNA also revealed which kinds of mosquitoes the bats eat. Little brown bats, for example, preyed on nine mosquitoes species known to harbor West Nile virus, an insect-borne disease that can threaten humans as well as birds.

More research will be needed to clarify how this affects humans, the study's authors point out, but these findings suggest we'd be wise to keep investigating. "Our results show that bats eat more types of mosquitoes, and do so more frequently, than studies have shown in the past," says lead author Amy Wray, a doctoral student in forest and wildlife ecology at the University of Wisconsin-Madison, in a statement. "While this study doesn't tell us whether bats actually suppress mosquito populations, it does create a strong case for re-evaluating their potential for mosquito control through additional research."

Little brown bats are especially prolific mosquito hunters, possibly due to their smaller, nimbler frames. Big brown bats are no slouches, but may prefer meatier prey that's easier to catch and offers more calories to fuel their bigger bodies.

"Mosquitoes only constitute part of a larger diet that includes many other components," Wray says. "In future studies, we hope to explore the feeding interactions between bats and mosquitoes, particularly for different bat species across different regions."

This kind of research is increasingly urgent, Wray and her colleagues argue, amid the growth of existential threats like white-nose syndrome. "Bats continue to decline globally due to habitat loss, wind turbines and, in North America, white-nose syndrome," says co-author Zach Peery, a professor of forest and wildlife ecology at UW-Madison. "So it's critical that their potential role as mosquito-control agents, and thus their importance as a target for conservation, be re-examined thoroughly."