COMAL COUNTY, Texas — In the Texas Hill Country, the setting sun is a signal for the hunt to begin.
Bracken Bat Cave, northeast of San Antonio, is home to the world's largest colony of bats.
Each night, an estimated 20 million Mexican freetail bats scour the countryside — 60 miles in each direction — for crop-destroying and people-pestering insects.
There are so many bats, it takes hours for them to leave the cave.
This colony alone eats 200 tons of insects every night. The positive impact on Texas agriculture is immeasurable.
Now, these mammals are being threatened by a mysterious and frightening disease.
"It's a really scary thing for us as biologists," said conservation biologist Katie Gillies. "What we're seeing is huge, huge die-offs."
A new fungus, called white nose syndrome, basically causes hibernating bats to starve. In caves where it's been found, 95 percent of the bats have perished.
Since it was detected in New York in 2006, nearly six million bats have died from white nose syndrome.
The disease is relentlessly marching south. It has been discovered in 16 states and Canada. It was discovered a few months ago in an Oklahoma cave. It has not yet been detected in Texas, though experts believe that is only a matter of time.
"We're looking at something that you and I have never seen in our lifetime," says Mylea Bayless, the director of conservation programs for Bat Conservation International, headquartered in Austin. "We're talking about a potential extinction event for bats, like we saw with bison and the passenger pigeon."
Scientists at Bat Conservation International are closely monitoring Texas bats.
Dianne Odegard, a bat rehabilitator in Austin, is on the front lines of inspecting the injured.
"I'm looking for signs on the wings; I'm looking for white spots, and certainly looking for the obvious white fungus on the nose or mouth," she said.
Texas Parks and Wildlife recently ordered that every bat turned in be examined for white nose syndrome. Experts are hoping to avoid an ecological disaster that affects everyone.
"It affects you if you eat; if you like to eat; if you like to pay reasonable prices for food," Bayless said. "Bats are such primary predators of agricultural pests that if we lose millions of bats across the landscape, we could see significant impacts on agriculture."
Because the body temperature of Mexican freetail bats doesn't drop low enough for the fungus to thrive, experts suspect freetails may instead become carriers that infect other bats.
No one knows for sure how freetail — or any other bat — will weather the fungus' impact. But experts have a very real fear that without man's intervention, a species that has survived for 50 million years could be wiped out in less than a decade.
Bracken Bat Cave is on private property. For the first time, Bat Conservation International is letting the general public see the spectacular bat emergence five nights a week to raise awareness of the problem.