Trick or Treat? The Frightening Threats to Bats

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Written by Marisa Lubeck and Ethan Alpern

Pollinating bat. Courtesy Ami Pate, National Park Service
Bats perform valuable services, including pollinating plants and crops. This bat is covered in pollen.

Iconic Halloween animals, bats have long suffered a spooky reputation. They’ve been accused of harboring vampiric spirits, making nests in piles of ratty hair, and are often associated with witches, warlocks and Halloween. Few other mammals seem to spook us with so many misunderstandings. But bats, because of their incredible echolocation abilities, rarely fly into or touch people, and serve amazing and essential ecological roles in our country.

Unfortunately, white-nose syndrome (WNS), a fatal fungal growth in the wings and muzzles of hibernating bats, has killed over six million bats since 2006, and may well lead to the extinction of certain bat species. In addition, bats are susceptible to being killed or injured by wind turbines.

“People often ask why we should care about bats, and new evidence strongly suggests that bats are saving us big bucks by gobbling up insects that eat or damage our crops,” said Paul Cryan, a bat ecologist with the U.S. Geological Survey. “It is obviously beneficial that insectivorous bats are patrolling the skies at night above our fields and forests, and these bats deserve help.”

Unlike the dreaded vampire bat typically associated with Halloween, insect-eating bats perform services valuable to humans. Research by Cryan and his colleagues shows that insect-eating bats, through their free pest-control services, save the agricultural industry billions of dollars each year. A single little brown bat, which has a body no bigger than an adult human’s thumb, can eat four to eight grams (the weight of about a grape or two) of insects each night. The loss of millions of bats in the Northeast has likely resulted in between 1.4 and 2.9 million pounds (equivalent to about two to three full Boeing 747-8F airliners) of insects no longer being eaten each year by bats in the region.

For more information about the economic value of bats, listen to this podcast.

An Invasive, Emerging Killer: White-Nose Syndrome

Image: Bat with White-nose Syndrome
This hibernating little brown bat shows the white muzzle that is typical of white-nose syndrome. (Greg Turner, Pennsylvania Game Commission)

U.S. bat populations have been declining at an alarming rate since the 2006 discovery of WNS in New York state. To date, the disease has been found in 29 states and five Canadian provinces and has killed more than six million bats. The Northeast, where bat population declines have exceeded 80 percent, is the most severely affected region in the country.

In March 2016, USGS scientists confirmed WNS in a bat from Washington state, about 1,300 miles from the previous westernmost detection in Nebraska. The fungus that causes WNS was subsequently found on one other Washington bat that was being tested for rabies and in bat guano (feces).

“The high number of bat deaths and range of species being affected far exceeds the rate and magnitude of any previously known natural or human-caused mortality event in bats, and possibly in any other mammals,” said Cryan.

WNS is caused by a deadly fungus called Pseudogymnoascus destructans (formerly Geomyces destructans), according to research by USGS scientists and partners. True to its ominous name, P. destructans causes a powdery white growth on the muzzles and wings of most infected bats (the telltale sign of a life-threatening WNS infection), wing damage and abnormal bat behavior. WNS is not known to pose a threat to humans, pets, livestock or other wildlife.

The disease is spread by bat-to-bat contact during hibernation, bat contact with a P. destructans-contaminated environment, and possibly by humans carrying the fungus from infected caves to uninfected sites. For this reason, many caves in affected states have been closed to recreational use, and people visiting open sites are urged to follow specific decontamination procedures.

The abrupt spread of WNS has impacted seven bat species so far, and if the current rate continues, WNS could threaten several of these species—including the federally endangered Indiana bat—with extinction. There is no known cure for WNS, and diseases among free-ranging wildlife are difficult to stop once they’ve become established in wildlife populations.

However, studies by USGS scientists and collaborators provide critical information about WNS, which may help decision-makers preserve ecologically and economically valuable North American bat populations.

“Increased understanding of WNS through ongoing collaborative research has greatly accelerated efforts to develop strategies, including vaccination, to limit the impacts of this emergent disease on North American ecosystems,” said USGS scientist David Blehert.

Bats and Wind Energy

This surveillance video from a temperature-imaging camera shows a bat interacting with a wind turbine at about 3 a.m. on a brightly moonlit summer night. (Paul Cryan, USGS)

Wind energy is one of the fastest-growing sources of renewable energy in the U.S. today. Land-based wind turbines can reach more than 425 feet above ground with a rotor-swept area of one to sometimes 2.5 acres.

Though wind turbines play an important role in the nation’s energy portfolio, bats and birds have been injured or have died from collisions with turbines or from the physiological damage caused by the massive turning blades. It is estimated that tens if not hundreds of thousands of bats die at wind turbines each year. As our nation’s energy portfolio continues to grow, it is critical that development be guided by the best science available so that it happens in the right way and in the right places. USGS researchers are assessing not only how and why a bat interacts with the blades of a wind turbine at night, but possible methods to reduce the numbers of bat and bird fatalities.

The USGS is creating new applications of innovative technologies, like employing radar to track flight patterns of bats; using low-light surveillance cameras to discover the underlying causes of bat-turbine encounters; developing models to predict fatalities; recording flight calls of bats and birds to determine the distribution of migrants in time and space; and experimenting with new ways of keeping bats away from the blades of wind turbines. Together, this information may help reduce the harmful effects of wind energy on bats by providing information needed for better turbine design, operation and placement.

To learn more, please listen to this podcast on bats and wind energy.

About Bats

Bats remarkably similar to the ones we have today first appeared on Earth more than 50 million years ago. No other mammal has ever achieved the ability to sustain flight. There are more than 1,000 species of bats, some the size of a human thumb and others with a six-foot wingspan. Most eat insects, many eat fruit and nectar from plants, some eat rodents, and yes, some eat blood. All are primarily active at night. Bats rely on echolocation (locating objects by reflected sound), as well as incredible dim-light vision, to navigate through the night and in the caves and tree-roosting sites many kinds of bats inhabit.

“Many people think bats are blind, but they actually have really sensitive vision, which helps them see in conditions we might consider pitch black,” Cryan said. “They don’t have the sharp and colorful vision we do, but they don’t need that. Think a dark-adapted Mr. Magoo.”

Bat Colony
While mother bats are out foraging, the young bats huddle together in groups that biologists call a cuddle. (Alan Cressler, USGS)

During the winter, many species of bats hibernate in dark and moist caves or mines, playing an important role by bringing important resources into cave ecosystems. Hibernation is an adaptation for bat survival during the cold winter months, when there are no insects available for bats to eat. Bats must store energy in the form of fat prior to hibernation. One of the consequences of WNS is that the hibernation of many afflicted bats is interrupted, often causing them to depart their winter roost and eventually starve to death.

Bat reproduction begins with mating in the fall before hibernation, yet new USGS research uncovered a surprising amount of mating during winter hibernation. Female bats store sperm through the winter and become pregnant in the spring soon after emerging from caves or other winter roosts. In spring, bats migrate to their summer areas, often in wooded locations with lots of trees and vegetation. Females usually roost together in maternity colonies under the peeling bark or in cavities of dead and dying trees, and in caves in groups of up to 100 or more. Each female in the colony typically gives birth to only one pup per year. Young bats are nursed by the mother, who leaves the roost only to forage for food. While mothers are out foraging, the young bats huddle together in groups that biologists call a cuddle. The young stay with the maternity colony throughout most of their first summer.

Bats remain a frontier of wonder and discovery. Scientists recently discovered that bats are among the longest-lived mammals for their size and may hide secrets to longevity. We also now know that bats are more closely related to horses, dogs and cats than any other mammals.

“These mysterious creatures will undoubtedly continue to benefit us as they fly above our heads in the dark, and science can help us discover and help protect those free and irreplaceable benefits,” Cryan said.

More Information:


Bat necropsy at NWHC
A USGS pathologist and a technician necropsy (animal autopsy) a little brown bat at the USGS National Wildlife Health Center. (USGS)
Image: Bat Wing Damage
This little brown bat has wing damage from the P. destructans fungus. (Kim Miller, USGS)
Image: The Effects of Geomyces Destructans Infection on Bat Wings
These are back-lit photographs of wings of white-nose syndrome-positive little brown bats, one with subtle circular and irregular pale areas (arrows) indicating areas of fungal infection (A) and another bat (B) with areas of relatively normal tone and elasticity (black arrow), compared to a WNS affected area that looks like crumpled tissue paper with loss of elasticity, surface sheen and areas of irregular pigmentation (white arrow). (C) Microscopic section of wing membrane from a little brown bat showing extensive infection with the fungus (magenta structures), P. destructans. (Carol Uphoff Meteyer, USGS)
Image: Side View of Insect-Eating Spotted Bat (Euderma maculatum) in New Mexico
This spotted bat, native to western North America, may be at risk as the disease white-nose syndrome moves westward. (Paul Cryan, USGS)
Image: Hawaiian Hoary Bat
A Hawaiian Hoary fits in the palm of one's hand. (Frank Bonaccorso, USGS)