Minnesota’s bat population, devastated by a fungal disease known as white-nose syndrome, has become so shrunken and fragile that state biologists are giving up their annual December “bat count” as a lost cause.

Researchers believe that even if their cave count did turn up a few survivors, the naturalists might risk disturbing bats when they’re most vulnerable, said Gerda Nordquist, mammalogist for the Minnesota Department of Natural Resources. “Best to just leave them alone,” Nordquist said. “There’s just nothing left to find.”

The bats’ plight is especially troubling because of their far-reaching impact on the region’s economy and environment. The four species at risk of extinction here — the little brown bat, northern long-eared bat, tri-colored bat and big brown bat — all eat their weight in mosquitoes, beetles and crop-damaging moths every night. Scientists fear that their extinction would result in significant crop damage and increased use of chemical pesticides, to say nothing of the increased nuisance for people.

The DNR is placing its hopes on artificial bat houses and boxes for the state’s four hibernating bat species. This spring, they’ll study if the bats can hibernate successfully without caves and try to discover why some of the animals — albeit very few — survive the destructive fungus.

White-nose syndrome was discovered in Minnesota in 2015. Since then, the disease is believed to have killed more than 90% of bats in important hibernation spots such as Mystery Cave in southeastern Minnesota and the Soudan Underground Mine on the Iron Range. It is not known to affect other animals, humans or livestock. States and federal agencies have been trying to find a treatment or antidote since the fungus was first discovered in North America, in upstate New York in 2007. Logistically, that’s proving impossible, Nordquist said.

The fungal spores can cling to just about anything and are now so firmly established in North America that there is little hope they’ll ever be removed. “Even if you had the time and money to sterilize a cave or any one place, new bats will come in every year and just reinfect it,” she said. “It’s frustrating.”

The fungus is estimated to have killed more than 6 million cave bats in 33 states and Canada. The spread shows no signs of slowing.

The best, and perhaps only, hope lies in the few individual bats that have been able to survive a winter with white-nose syndrome. In some Eastern states that have been infected longer than Minnesota, a few bats tagged by researchers were found to live through multiple winters while the rest of their populations were eliminated.

Scientists don’t know if that’s because they are resistant to the fungus in some way or were just big and fat enough to have the energy to fight off the disease.

Researchers have discovered that what makes the fungus fatal to bats is the energy they expend trying to clean it off their fur.

Even in normal circumstances, to survive winter without the normal diet of insects, bats need to shut down their bodies almost completely. When the fungus sprouts on the tips of their wings or across their faces, the bats wake up periodically and try to lick or scratch themselves clean, much like a dog or cat. That alone can consume enough energy to cause starvation.

The fungus may also block the release of carbon dioxide, which is done through the bat’s wings during hibernation.

That again causes bats to wake up to exhale, expending precious energy. Sometimes bats die right in the cave, other times they fly off in a hopeless search for insects to eat in the dead of winter.

If biologists can build artificial habitats where the bats can feast sufficiently to build up their fat stores, perhaps some segment of the species can survive, Nordquist said.

“The idea is let’s give them the most optimum opportunity to get fat and healthy before winter and see if they can make it,” she said.

The DNR will start this summer, trying to determine if any of the boxes, roosts or artificial habitats already built are working and if there is anyway to augment habitat or insect populations around them.

Link to West Nile?

Whether the species are eliminated or cut to just a fraction, the consequences aren’t entirely clear, but biologists fear the worst. A U.S. Geological Survey study found that bats contribute insect control worth at least $3.7 billion for farmers in the United States each year. More moths in the environment will damage crops, leading to an increase in pesticide use, which could further harm already endangered bee, butterfly and other pollinator populations.

The loss of bats could also cut into an important side benefit for both humans and wildlife — the control of West Nile virus. State naturalists in Minnesota, Wisconsin and Michigan are trying to get a handle on how the mosquito-borne West Nile virus is killing grouse. The virus has proved to be particularly deadly to the birds, and may be a cause of their population decline throughout the Upper Midwest, placing an even greater premium on the ability of bats to control mosquito populations.

But it’s difficult to make precise predictions about the impact of bat die-offs. Booms and busts in insect populations are so complex, multifactored and weather dependent it’s just about impossible to track the role of any one species or predator, said David Blehert, a white-nose syndrome expert at the USGS National Wildlife Health Center.

“You really need to look at the loss as one more adverse impact to the environment,” Blehert said. “It’s difficult to quantify the specific impact. It’s cumulative.”

There is some hope that hibernating bat populations can stabilize after white-nose syndrome, said Richard Geboy, regional white-nose syndrome coordinator for the U.S. Fish and Wildlife Service.

More than a decade after the fungus was found in New York, some banded adults are still surviving year after year, and some juveniles are making it to maturity.

It’s too soon to say if that’s because the bats are resistant and can start to rebuild their populations, or if the survivors are just temporarily taking advantage of less competition for insects.

“But the fact that they’re still reproducing is good news,” Geboy said.

Greg Stanley • 612-673-4882