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April 2002

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Bat Patrol

By Gary F. McCracken and John K. Westbrook
Each summer evening in south-central Texas one of nature's great spectacles unfolds. From the long, narrow mouth of Bracken Cave, near San Antonio, a stream of Mexican free-tailed bats begins to emerge a few hours before sun down. For more than two hours they leave the cave and spiral higher and higher, an immense cloud of 20 million bats, the world's largest known colony. Finally the columns disappear beyond the view of our binoculars.

For many years the two of us have watched free-tailed bats emerge from Bracken and other caves nearby. We wondered: Where are these bats going? How high do they fly? Are they feeding up there, and if so, on which insects and how many?

We suspected that they might be headed to dinner above the Winter Garden agricultural region, several hundred thousand acres of corn, cotton, and vegetables southwest of San Antonio. Those fields are infested with billions of insects, many of which cause immense damage to the crops. Eventually we decided to see for ourselves—by flying with the bats.

Back in the spring of 1995 we had learned of another party privy to the bats' nocturnal flights, a sort of visual eavesdropper only 19 miles from Bracken Cave. There, in the community of New Braunfels, the National Weather Service had built a Doppler radar facility.

Doppler radar tracks moving objects such as raindrops through the atmosphere by bouncing electromagnetic energy off them and measuring the amplitude as well as the change in frequency. But the radar doesn't distinguish between bats and hailstones. To the radar the millions of bats emerging from their caves look like a huge storm that starts at a point on the ground—a cave—and spreads rapidly up and over the landscape.

"It didn't take long for word to get around among bat researchers that we could view bat colonies on the new radar," recalls Jim Ward, science and operations officer at New Braunfels. "We saw bats flying as high as 10,000 feet."

Ward alerted Merlin Tuttle, director of Bat Conservation International in Austin, about the radar's bat-tracking ability. Merlin contacted us, and a project was born: We realized that Doppler radar could help us determine how the airborne bats interacted with insects. The nightly flights in spring and early summer involve an estimated 100 million Mexican free-tailed bats emerging from a dozen major caves in south-central Texas. Perhaps another 50 million live in caves in Oklahoma, New Mexico, and Arizona. These bat populations are maternity colonies, huge congregations of females that migrate into the southwestern United States each spring from wintering grounds in Mexico. In June each female gives birth to a single pup. She leaves the cave twice each night to feed, returning to nurse the pup. By late July the young bats have grown enough to hunt with their mothers, doubling the number of bats flying from the caves.

To feed herself and her growing pup, a female bat must eat the equivalent of up to 70 percent of her body weight each night.

Although each bat weighs only half an ounce or so, calculations show that a million bats can devour about ten tons of insects nightly. That means the 100 million free-tailed bats in south-central Texas must eat an incredible 1,000 tons—two million pounds—of insects in a single night.

Since bats in other locales pursue insects close to the ground, we wondered why the free-tails were flying as high as 10,000 feet. Is this where they find the billions of insects needed to sustain their huge populations?

Again, Doppler radar offered clues by detecting the billions of insects that swarm high above Texas. Since the 1980s researchers from the United States Department of Agriculture have used radar to map the flight patterns of some of North America's most destructive agricultural pests—fall armyworms, beet armyworms, tobacco budworms, and, the worst of all, corn earworms.

Corn earworms and tobacco budworms alone cost U.S. farmers over a billion dollars annually. To control them, Texas cotton growers spread nearly 30 million dollars' worth of insecticides between 1995 and 1997 but still lost an annual average of 148,000 bales of cotton (each worth $350).

"Some years we've had to spray three to five times at a cost of $30 to $50 an acre," says Ray King, a farmer in the Winter Garden. "I'll never forget June of 1995 when the beet armyworms ate our lunch. We'd have been better off if we'd just let them have the crop."

During the first weeks of June each year—about the time the bats give birth—up to seven billion corn earworm moths, plus a similar number of fall armyworm moths and other pests, emerge from the cornfields of the lower Rio Grande Valley. After dusk they ascend anywhere from hundreds to thousands of feet and ride the prevailing winds north to the Winter Garden. They can make the 250-mile flight in one night. Each female then lays as many as a thousand eggs in corn, cotton, and other crops, where the larvae grow fat as they mature.

Texas farmers are only the first in the United States to suffer. In late June and early July the next generation of moths hatches and flies north, eventually invading crops across the central United States and into Canada. If we could prove that bats were eating corn earworms in Texas, we could show that they're an unrecognized first line of defense against invading insect hordes and that without the bats the damage to farmers in Texas and farther north could be even worse.

Conventional analysis of the bats' digestive remains told us that they ate moths—but not what kind. During much of the summer moths make up 30 to 40 percent of the bats' diet. That figure rises to an astounding 90 percent during the peak of moth migration from Mexico into Texas. Ya-Fu Lee, a researcher at the University of Tennessee, had examined bat feces from several colonies and discovered a striking relationship between the moths' arrival and their proportion in the bat droppings.

But we still didn't know exactly what the bats were doing high above those Texas fields. What we did know is that a cruising bat emits a search call as it looks for food. When it attacks, its call changes to a rapid repetition known as a feeding buzz. We can identify different bat species by their distinctive calls. Though these calls are above the limits of human hearing, electronic bat detectors make them audible to us. The detectors' range is limited, so the trick is to get them airborne, near the bats.

We had first tried in 1996 by attaching bat detectors and radio transmitters to helium balloons. As they rose, we chased them across Texas in a van, tracking the signals with a radio receiver. It was primitive, but it worked: We picked up transmissions of freetail search calls at 3,900 feet and feeding sounds at 2,400 feet.

The next year we attached radio microphones to kites used to monitor atmospheric pollutants. We heard the bats feeding as high as we could raise the kites—4,000 feet. And we heard them in great numbers. Clearly, at times there's a feeding frenzy up there.

Based on that experience, we decided we needed to witness their aerial feeding. There was only one way to do that.

"Hot-air balloons are the safest mode of aviation," our pilot assured us as we prepared to ascend in the predawn August darkness near Uvalde. Launching and flying at night isn't a problem—but landing can be. In the dark, power lines and barbed wire fences create invisible hazards during descent.

So it would be a quick two-to-three-hour flight, then hopefully a safe landing soon after dawn. It was still pitch-black when we rose from Ray King's farm near Frio Cave. We drifted silently, save for occasional blasts from the propane burners, which kept us aloft. To the bats we must have looked like a giant lightbulb. As the sun began to rise, we could see deer scattering below us in the dim light and the faint outline of a hawk in a tree.

When we rose to several thousand feet, we began to hear bats. Our pilot pointed to a group close to the balloon, apparently investigating the large object that had invaded their airspace. They were Mexican freetails. As we approached 5,000 feet—the limit of our flight plan—we saw and heard more freetails and recorded their feeding buzzes with bat detectors. But we saw only a few moths, mostly in the distance.

Already it was daylight. Shortly after dawn we bumped down, exhilarated but a bit disappointed. Where was the feeding frenzy? We had flown in August, well after the June moth migrations from Mexico, so a subsequent generation of moths was already leaving en masse from the patchwork of crops far below us. Billions of these insects should have been flying at our altitude. Why had we seen so few?

Simple math eased our disappointment.

Although Doppler radar shows billions of moths as a huge cloud, we later calculated that the density—from our vantage point in the balloon—translated to perhaps one moth per 30,000 cubic feet of airspace. With 20/20 hindsight we realized that we shouldn't have expected to see swarms of moths.

Later DNA analysis of the insect remains in bat feces by Sunitha Vege, a graduate student at the University of Tennessee, finally confirmed our suspicion: The insects in the freetails' droppings are in fact corn earworms and tobacco budworms.

Our project now involves more than a dozen scientists in the U.S. and Mexico. Together we are trying to determine the dollar value of the bats. How much crop damage would there be without the freetails? How many more tons of insecticides would farmers need to spray if it weren't for the bats? Even before these questions are answered, however, it's clear that we need to safeguard our free-tailed bats, which cost us little to protect but could cost us a great deal if we lose them.


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