Volume 36, Issue 3, 2017

Pest Control

By Michelle Z. Donahue

A little brown bat (Myotis lucifugus) pursues a forest moth.
Photo: Micheal Durham / Minden Pictures.

As science unravels more and more about what bats are eating and where, their impact on agriculture becomes even more profound

Visitors who watch the nightly exodus of Mexican free-tailed bats (Tadarida brasiliensis) from Bracken Cave mainly come for the moment when millions of bats stream from the cave’s mouth. While they do know the bats are off foraging for insect prey, they are thunderstruck when they learn just how many bugs the bats can eat in a night.

“Something that always gets a lot of ‘wows’ from our visitors is when we tell them just how many tons of insects the bats are eating—mostly agricultural pests,” said Fran Hutchins, BCI’s director of the Bracken Cave Preserve. These bats will munch their way through 140 to 147 tons of insects—nearly 300,000 pounds of nocturnal insects each and every night during the growing season.
In crops from corn to cotton and soy to sorghum, bats provide a huge, yet mostly hidden, service to the United States’ agricultural communities.

Following the Feast

In south-central Texas, bats’ nightly foraging occurs over huge tracts of land planted with corn, cotton and sorghum. Their prey: primarily moths, especially adult corn earworm and cotton bollworm moths. With each female moth capable of depositing up to 1,000 eggs, every moth consumed by a bat represents a major reduction in the millions of dollars of potential damage that could occur.

A 2006 study found that just in the cotton fields of Texas alone, Mexican free-tailed bats saved farmers an annual average of $724,000 in pest control costs and losses from insect-related damages. Extrapolating that to the country as a whole, a follow-up study in 2011 estimated that bats are worth around $23 billion in pest suppression services. 

Agriculture feeds into many sectors of the nation’s economy, equaling $992 billion in 2015, or 5.5 percent of the nation’s gross domestic product. Farm output alone contributed $136.7 billion that year, or 1 percent GDP. In that light, the economic services bats provide are a real and quantifiable sum to the overall U.S. economy.

Specialized equipment is used to monitor the interaction between insects and
bats at various levels in the atmosphere.
Photo: United States Department of Agriculture.

To refine what we know about where the bats are and what insects they’re eating, scientists are looking more closely at the night skies. In the relatively new field of “aeroecology,” researchers study how bats—and birds, and other sky-bound critters—live and thrive in the hazy boundary between earth and space.

After scientists realized that Doppler radar could detect swarms of bats as well as insects, they started teaming up to try to better understand how high-altitude bat foraging flights, about 1,500 feet in the air, were related to seasonal migrations of crop pests.

“Our discovery that it’s a feeding frenzy up there was new for understanding where bats are getting their resources,” said Gary McCracken, an ecologist at the University of Tennessee who has been studying bats and their interactions with insects for decades.

Bats aren’t just eating a couple of species of insects, but a plethora. Dr. Jennifer Krauel, who workd with Gary McCracken and USDA meteorologist John Westbrook over three seasons in Uvalde, Texas, found that bats were eating 44 different agricultural pests, 20 of which were migratory. Krauel’s work investigated which insects bats were eating during the autumn migration, showing that migratory insects are necessary to fatten up bats before their own migration and lean winter months.

It’s an especially important food source for bats born that summer, and for mother bats who have worked hard and used up fat reserves while nursing and raising their young.

Moth pheromone trap.
Photo: United States Department of Agriculture.

Though a 2015 study by Josiah Maine and Justin Boyle at Southern Illinois University Carbondale showed that bats play a significant role in combating corn crop pests—saving farmers an estimated $1 billion worldwide in crop damages by the corn earworm moth—it can be tough to demonstrate to farmers in one region that they’re impacted by bats far away.

“Part of the issue is that the bats can be removed in distance from the farms because it’s high-altitude, so making that connection to the farmer is more difficult,” Westbrook said. “They want to see direct impacts on their farms, but bats are intercepting these vast migrations that have a great impact on downwind crop areas, places that may never see bats.”

“Where I live in South Dakota is a really agricultural area, but we’re far away from the big bat colonies where I worked in Texas,” Krauel added. “The idea of bats controlling insects for agriculture is not at all well understood, or even really thought of in many parts of the country.”

Making Connections

Some growers, however, are very aware of their airborne partners’ labors.

John Worth Byrd, a fourth-generation pecan grower in San Saba, Texas, is one farmer who says he does everything he can to encourage bats to forage among his trees at night. As well as never spraying for insects, he builds and erects bat houses around the orchard to help combat pecan nut casebearer moths, whose larvae are a serious pest of his crop.

“A bat eats one moth, and that’s 100 more pecans, in theory. If they’re doing that every night, that amounts to something,” Byrd said.
Leatherwings, which Byrd says is the old-timers’ name for Mexican free-tails, were a good sign, according to people whose families have grown pecans for generations. An old pecan man told him if an orchard had leatherwings, that was good for the pecans, Byrd said, and he knew they were there, zipping in and out of his pecan stands.

With the help of then-Ph.D. student Liz Braun, Byrd also came to realize that there were many other types of bats roosting in the trees and bat houses he’d erected around the farm, including cave myotis and eastern red bats. Though the smaller, more agile bats hunted down pecan casebearer moths in the orchards, the free-tails headed out to the surrounding field crops.

Bat houses are the orchard help to combat pecan nut casebearer.
Photo: John Worth Byrd.

"They knew more than we do now about pecans, back in the old days," Byrd said. "If you live there, you understand what’s going on, because you walk through it, you sit and see it."
Enabling more people—farmers as well as the general public—to “see” bats more often is one thing Krauel said she’s interested in working on, hopeful that technology can help make more of those kinds of connections. She’s studying how real-time or recent radar visualizations can be integrated with something like a mobile app so people can see bat emergences from caves, or nightly bat, bird, and insect activity in their local areas.

Part of that involves helping a broad swath of people understand the threats bats face from loss of habitat and how to protect it—including doing anything possible to slow or halt the spread of the fungal pathogen causing White-nose Syndrome—so that bats can remain on the landscape and continue to provide the myriad benefits that they do.

For his part, Byrd says that though it’s a slow process, the word is gradually getting out among farmers and growers that bats are a critical part of a farm’s success. Neighbors using sprays and other chemical pest control, once doubtful of Byrd’s approach, have started to come around, he said.

“We’ve done a lot of miles together, and I guess they trust me,” Byrd said. “But all my life, you can go out at night and there have always been bats. I don’t know how it would be without them.”

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