In Spite of Mosquito Feasts, Bat Populations in Serious Decline

bat population

Story at-a-glance -

  • Bats really are bona fide “skeeter” eaters, researchers reported in 2011 when guano found under bat maternity roosts contained several mosquito varieties, including those carrying the West Nile Virus
  • The role of bats in helping to decrease mosquito populations may also play a role in human health, but over the last several years, bat populations have been in serious decline
  • Between 2007 and 2011, 1 million bats died from white nose syndrome (WNS) and just as many have died since while scientists have been working with scant information to help reverse the virus that causes a deadly upsurge in hibernating bats’ immune systems
  • Researchers in the U.S. and Canada have found what may help eliminate the virus causing the infection, but how to implement what they now know may be a monumental task, and it may be a race against time to preserve the species

By Dr. Karen Shaw Becker

On clear evenings, just after dusk, you may see one, two or several swooping silhouettes of what appears to be birds in the light of the moon or nearby streetlamps. Well, they may look like birds, but they’re often bats, which are technically mammals — the only flying mammals, and one of my favorite creatures on earth.

Bats are capable of eating mosquitoes by the millions, which in some peoples’ minds makes bats worth their weight in gold. Researchers at the University of Wisconsin (UW) found that the fecal matter of bats (known as guano) indeed contained not just one but several varieties of the pesky insects capable of making the great outdoors more of a pain than a pleasure.

While previous studies state that bats consume an average of 10 mosquitoes per minute, those experiments were from controlled, enclosed and contained labs. The UW-Madison study, conducted in 2014 and published in the Journal of Mammalogy, indicates that bats may not necessarily eliminate mosquitoes from a given area, but they can certainly curtail it. As The Journal Times reported:

“The UW-Madison research team used volunteers to collect bat feces in 2014, from areas beneath a dozen little brown bat and 10 big brown bat maternity roosts in farm and forest land across the state (Wisconsin). DNA was extracted from the samples to screen for mosquitoes using new techniques that could analyze insectivore diets, such as bats eating mosquitoes.

The results showed at least one mosquito in all of the little brown bat sites and in 60 percent of the big brown bat sites … Bat populations have declined because of a number of factors, including white nose syndrome in North America, so the researchers feel it’s important to re-examine the bat as a mosquito control agent and how important it is to keep bat populations from shrinking.”1

Not just ordinary mosquitoes, but mosquitoes carrying the West Nile Virus were found inside the bats’ feces. The scientists’ work may resolve speculation regarding whether bats make any real difference in quelling mosquito populations, and also highlights the importance of bats and their role in human health. What’s become a serious quandary related to bats, however, is their serious decline over the last several years.

Where Have All the Bats Gone?

According to scientists, white nose syndrome (WNS) has been reported as far south as Texas and Florida and as far west as Washington in the U.S., and has destroyed 90 percent of the bats in some populations. They agree that bat populations do seem to be in decline in North America, and evidence points to WNS as a potential culprit. Smithsonian Magazine approached the topic in 2011, when the problem was first described as “catastrophic,” and “the worst epidemic in years.”2

Spores of the fungus Pseudogymnoascus destructans (previously known as Geomyces destructans3) seem to be both the source and cause of the devastating disease, especially for bats hibernating by the millions in the Northeastern U.S. In the four years prior — from around 2007 to 2011 — 1 million bats had already died from it.

The virus was first detected in upstate New York in early 2006 when bats started behaving “oddly,” flying into neighborhoods by day, even though they’re nocturnal creatures, and not only far from their caves, but in brutally cold weather, which is not typical at all, according to Al Hicks, then a wildlife biologist for the New York State Department of Environmental Conservation.

“There would be [3] feet of snow and it would be 20 degrees — not bat-flying weather — and you’d see bats flying out and taking off into the distance. You’d know every darn one of them was going to die. It was awful.”4

Up until 2011, the virus had been present in a cave located in Tennessee, but not 80 miles south where Kentucky’s Mammoth Cave, the largest cave in the world, is visited by around 500,000 people every year, the virus was also present. Then officials at the national park released reports in early 2013 that the fungus killing off bats had reached there, too.

In light of this disease, Zach Peery, Ph.D., a professor of forest and wildlife ecology at UW-Madison, believes it’s time to step up and implement more vigorous strategies for preventing their waning populations. Peery explains:

“Bat declines resulting from white nose syndrome and other factors may compromise potential mosquito suppression, but they also provide opportunities to test the hypothesis that bats limit mosquitoes through a natural experiment.”5

A recent study published in the Journal of Wildlife Diseases6 said the WNS fungus striking bats in such great numbers isn’t necessarily fatal, but if they survive the initial infection, it may still negatively impact the ability of females to reproduce. It also leaves them with significantly more damaged wing tissue and higher metabolic rates compared to uninfected bats coming out of hibernation. According to The Washington Post:

“A white-nose death is grisly. The fungus hits when the bats’ breathing is low and their tiny heartbeats are at an ebb in hibernation. Scientists at the U.S. Geological Survey and the National Institutes of Health theorize that the bats die when they awaken from hibernation, and their reactivated immune systems go overboard in an attempt to eliminate the intruding disease, destroying the illness but also tissue bats need to live. Ligaments in the wings of bats appear scorched.”7

Facts About Bats: What Makes Them Vulnerable

Scientists and amateurs alike find bats to be fascinating creatures for a number of reasons. Bats have many more options for movement in flight than your average bird or insect; their flexible skin allows them to catch air and reduce drag better, their wing surfaces curve to give them greater lift with less energy, and they can make 180-degree turns in less than half a wingspan’s distance. Boston University’s BU Blogs notes:

“Bat wings are highly articulated, with more than two dozen independent joints and a thin flexible membrane covering them. Their wings are similar in structure to the human arm and hand … One important point about the wings of bats is that they are not designed for take off, and in order to take off, they have to fall from a high location. This feature of bat wings might be the reason why bats sleep upside down.”8

A group of Canadian scientists began studying the movements of small brown bats in 1989 and found that both males and females can fly between 6 and 500 miles during “relocation events.” This knowledge has become crucial since the discovery of white nose syndrome, although when the Journal of Mammalogy reported its migration findings in 2013, it wasn’t clear how the fungus killing the bats was spreading, but the little brown bat had the most significant population declines.9

Recent Information on WNS — and a Possible ‘Achilles Heel’

However, in 2017, another Canadian study in mSphere observed that big brown bats (Eptesicus fuscus) are resistant to the disease or tolerant of infection, so they don’t seem to have suffered mass mortality like the small bats.

“Our results raise the issue of how further accumulation of variability will impact the evolution of virulence in the fungus and, in turn, of resistance of host bats. The answer here will depend not only on the ongoing population dynamics of the fungus but also on those of the bats …

Of particular concern is that new introductions of P. destructans may yet add variability to the North American population of the pathogen and increase the potential for recombination by enabling sexual recombination between mating types; sexual recombination would presumably proceed at a much higher rate than parasexual recombination.

Such changes in the fungus population could affect the durability and strength of newly appearing resistance in bats and might even lead to a renewed or expanded epidemic.”10

Nature Education explains that DNA recombination “involves the exchange of genetic material either between multiple chromosomes or between different regions of the same chromosome.”11

A more recent study12 in Nature found that greater mouse-eared bats (Myotis myotis), one of the larger European species, also seem resistant to the disease except in areas where the infection is highest, which results in detectable health deterioration. The scientists, from the Czech Republic, Slovakia and Poland, also discovered a relationship between the bats’ body temperature, their body mass index (BMI), the number of skin lesions and the severity of the WNS infection.

However, while preventative measures have been unsuccessful, there has been a recent breakthrough. Some of the most recent information is that the fungus grows on not just the noses but also the ears and wings of bats, striking them as they hibernate and causing them to wake up and burn off the fat stores needed for survival, especially in winter. In addition:

“P. destructans is known to thrive only in cold, dark environments (such as caves) with a strict temperature range of 39 to 68 [degrees] F, so it can only affect bats during hibernation. P. destructans evolved alongside bat species in Europe and Asia for millions of years, allowing Eurasian bats to develop defenses against it while leaving their counterparts in North America vulnerable.”13

A combined team of researchers from the University of New Hampshire, the U.S. Forest Service and the U.S. Department of Agriculture (USDA) may have found a possible “Achilles heel,” as the fungus is highly sensitive to UV light. It seems the fungus lacks a key DNA repair enzyme. When exposing the fungi to different UV light intensities, even for a few seconds, and observing how each behaved, Jon Palmer, Ph.D., a research botanist in Madison, Wisconsin, wrote:

“It is unusual that P. destructans appears to be unable to repair damage caused by UV light. Most organisms that have been found in the absence of light maintain the ability to repair DNA caused by UV light radiation. We are very hopeful that the fungus’ extreme vulnerability to UV light can be exploited to manage the disease and save bats.”14

Those scientists and many others have speculated on the best way to use the new information in order to save infected bats and those still vulnerable. Treating hibernating bats with UV light through the winter would be an impossibly large-scale task, and it would disrupt the bats’ cycles.

However, while it may be a race for time as scientists continue to study how the bat-killing fungus can be reversed, slowed or eliminated, they now have a much clearer picture of what they can do. Follow-up research funded by a grant from the National Fish and Wildlife Foundation is said to be already underway. According to Tony Ferguson, director of the USDA’s Forest Service’s Northern Research Station and the Forest Products Laboratory:

“This research has tremendous implications for bats and people. Bats play a key role in the health of forests as well as the production of food in the United States, and developing an array of tools with which we can treat bats for white-nose syndrome is important to preserving these very important species.”15