The true value of bats to agriculture is in some ways inestimable because their value extends well beyond the farm into the woods and wetlands and even cities. A U.S. Geological Service survey in 2011 illustrated the difficulty in putting a price tag on their beneficial contributions. The study estimated the economic benefit of bats to agriculture to be anywhere from $3 billion to $53 billion a year — that’s right, a $50 billion spread. (The number most cited is around $24 billion.) And the survey didn’t account for the ill effects that pesticides might have on ecosystems or the benefit of bats suppressing insects in the forest.
Few places in the country are as blessed with bats as Texas, especially Central Texas. There are about 100 million Mexican free-tail bats concentrated in approximately 20 Hill Country caves. The biggest concentration of Mexican free-tail bats in the world is at Bracken Cave near Fredericksburg, where 20 million bats eat several times their own body weight in insects every night from March through October. The Congress Avenue Bridge in Austin is home to another 1.5 million bats, all of them voracious consumers of insects.
Included in their nightly feast are a high number of agricultural pests like the corn earworm moth and the cotton boll moth. Studies in South Central Texas have shown a savings of as much as $173 per acre because of the bats, which includes the reduced cost of pesticide applications that are not needed because the bats are patrolling skies thousands of feet in the sky, at the same altitude that the moths fly. The savings to the area cotton harvest is estimated at $6.4 million annually.
Merlin Tuttle, a veteran bat biologist with Bat Conservation International in Austin, says that each of the 10 million bats from Bracken Cave can eat up to 40 earworm moths a night, and each one of those moths could be carrying as many as a thousand eggs. He said a lot of organic farmers have come to rely on bats as a big part of an Integrated Pest Management (IPM) plan to control insects. They build houses for up 2,000 bats at a time, and some orchards have quit using pesticides by successfully attracting bats.
Nor do bats limit themselves to the immediate neighborhood.
“They’ll fly thousands of feet in the air at 35 to 45 miles per hour, catch a 45 to 50 mile per hour tailwind and make some real time,” Tuttle said. “Doppler weather radar indicates that on a single night every one of these bats may go all the way down to the coast to feed.”
But bats, so prolific for so many years, are facing new threats. A virus called whitenose syndrome (WNS) has killed millions in the eastern U.S. and is moving south. It was discovered most recently in South Carolina and Georgia. Wind power facilities have been identified as being hazardous or fatal to the world’s only flying mammal. Bats may be injured by direct impact with turbine blades — thoe blades in those gigantic, modern stand-alone windmills — towers or transmission lines, and recent research shows that bats may also be killed when suddenly passing through a low air pressure region surrounding the turbine blade tips.
Even Bracken Cave is facing pressure from development. Galo Properties of San Antonio owns 1,500 acres south of the cave, and plans are in place for the company to build a 3,800-home subdivision there called Crescent Hills. BCI is worried that the plan puts bats and humans on a collision course, with both parties certain to be adversely affected.
The development is opposed by BCI and the Greater Edwards Aquifer Alliance. House Bill 36, authored by State Rep. Lyle Larson (R-San Antonio) and filed in the special session of the Texas Legislature, would put a moratorium on development around the cave until the impacts can be studied. That bill is not expected to be heard during the special session of the legislature, however.
Larson told the San Antonio Express-News that he wants time to study the possible health and environmental problems of having so many people living so close to 10-20 million bats.
“The bottom line is if we don’t do this right, we are setting up a really bad Johnny Depp movie,” Larson said. “The moral would be that humans did not understand the unintended consequences.”
BCI officials have said that beyond educating the public about the subdivision there is little they can do to oppose it. But the organization has certainly received more than token support. In May, BCI presented to the San Antonio City Council a petition with more than 13,000 signatures opposing the development. More than 60 speakers, including representatives from Texas Parks and Wildlife, Audubon Texas and the Sierra Club, were among the people that testified in support of and about the importance of Bracken Cave to the council.