Fungal disease spurs bat population decline throughout Wisconsin

May 1, 2019

Sauk Prairie used to be home to 200-300 bats that used the area as a maternity site to raise young before they learned to fly. Today there are 30.

The decline in population is not isolated to Sauk Prairie. A mine in Grant County saw its bat population peak at over 1,200 bats. This year there were eight.

These population declines are being seen throughout Wisconsin, and throughout North America as a whole. The cause is a fungal disease known as white-nose syndrome, which has affected 33 states and seven Canadian provinces.

It was first noticed in the United States and North America in 2006, at a popular cave in New York state.

“It’s believed that it was an invasive species that was brought over inadvertently by people visiting that site,” said Wisconsin Department of Natural Resources Bat Team Lead Paul White. “It’s been in Wisconsin since 2014 (so) we’re now in our fifth year of infections.”

The fungal disease disrupts bat hibernation and often results in death by increasing exposure to the elements and depleting fat reserves.

“Bats have a strict energy budget that forces them to be underground for six to eight months while there’s no food and resources available,” White said.

During that period, bats wake up about every 15 days, but for those with White Nose Syndrome, the disruptions from hibernation are much more frequent at three to five days on average. This forces the bats to leave hibernation sites earlier and deplete energy more rapidly.

“Anywhere from February to March, you might see bats with White Nose Syndrome on the landscape, and they ultimately die because there aren’t any resources available and it’s much too cold,” White said.

The fungus persists long after bats are gone, which can make population recovery difficult. It thrives in cold and wet environments, exactly the kind bats need to survive the winter.

Solutions are being explored, including a vaccine, but a multifaceted approach will be necessary to adequately address the problem, White said.

“We haven’t found the silver bullet,” White said. “(The solution) is going to be a multitude of things.”

The reason some bats survive and most don’t is not immediately obvious to experts, but it is being studied.

As the primary predator of night flying insects, bats play a vital role in local ecological systems. Among other things, they maintain a necessary balance in the insect population.

“They are pretty much the wolves of the sky,” White said.

A 2011 study estimated bats save Wisconsin’s agriculture industry between $658 million and $1.5 billion every year with their penchant for devouring insects by the millions before the insects devour crops.

Without sufficient numbers of bats, more pesticides may have to be used to protect crops bats would otherwise protect naturally by preying on insects.

Wisconsin’s four species of cave-dwelling bats have been listed as threatened since 2011. The populations were beginning to stabilize at some sites prior to the arrival of white-nose syndrome, White said, but the disease wiped out that progress.

Concerned individuals can get involved to help nurture the Wisconsin bat population.

“What we need to do is encourage the roosting of bats in new places because many of the old places where they have roosted and hibernated are contaminated,” said Sauk Prairie Conservation Alliance Executive Director Charlie Luthin. “Putting up permanent bat houses on properties is one thing, not using pesticides on your yard and farm is another. Because if you kill insects, that means there are fewer insects available to the bat population.”

Completed bat houses are available for purchase at hardware stores, and the DNR offers information on how to build on on their website.

With many of the traditional roosting sites being contaminated by the deadly white-nose syndrome for the foreseeable future, bats will need new places to safely retreat for the winter.

“We’ve seen declines as much as 100%, where historically they had a couple hundred bats and now they don’t have any left,” White said.