In the last 20 years, a fungal disease called white-nose syndrome (WNS) has killed over six million bats in the US, causing a severe decline in the population of many bat species. About 90 percent of the long-eared, little brown, and tri-colored bat populations that once inhabited North America are now dead because of this infection. 

What’s even more shocking is that the decrease in bat population has been correlated to the deaths of thousands of human babies. So, what could be the connection? A new study reveals that as bats declined in number, the use of chemical pesticides on farms across North America increased dramatically (although still within regulatory limits).

“I found that the internal infant mortality rate (IIMR) increased in the years after WNS detection.” For instance, “When farmers increased their use of pesticides, the infant mortality rate rose by almost 8 percent. This corresponds to an additional 1,334 infant deaths. Or, for every 1 percent increase in pesticides, there was a 0.25 percent increase in the infant mortality rate,” the study author notes.

Bats feed on insects that harm crops, acting as natural pesticide. So, for years, farmers in North America relied on bats to deal with the pests on their farms, thus decreasing their dependence on chemical pesticides.

However, since the rise of white-nose syndrome in 2006, the bat population has been steadily decreasing. And, in response, farmers use more and more pesticides in areas with declining bat populations. Numerous studies show the harmful chemicals that make up these pesticides eventually make their way into our food and water.

Frank tested several alternative explanations for the rise in infant mortality—including economic downturns, weather changes, and the opioid crisis—but none of them accounted for the increase. The one consistent factor was the rise in pesticide use.

What is white-nose syndrome?

When insect-eating (insectivorous) bats undergo hibernation in dark cold caves, they may come in contact with Pseudogymnoascus destructans. This pathogen starts growing on the wings and muzzles of the bats, causing the hibernating animals to lose their fat reserves and energy.  

As the infection grows on the bats, it develops a white, fuzzy appearance around their noses, hence its name. It interferes with the hibernation cycles of these bats, causing them to wake up more frequently. So, they end up using up their fat reserves more quickly. At the end of hibernation, they wake up with very low energy levels and are starving. 

Even worse, they begin to lose energy at twice the rate of healthy bats. With each passing day, their bodies weaken, and this eventually leads to death. Moreover, since bats can catch the disease simply through physical contact, the infection spreads quickly, killing entire bat colonies at once.

Government agencies and scientists have come up with solutions ranging from vaccines and probiotics to UV treatments. However, all these methods have their limitations and will take time to demonstrate effectiveness.  

“Having a variety of tools gives us opportunities to work on multiple scenarios. However, the dream treatment would be something that can be used in almost any situation and benefits bats for years after a single application,” Jonathan Reichard, assistant national white-nose syndrome coordinator at the US Fish & Wildlife Service, said.

Pesticides can’t compensate for bats

According to environmental economist Eyal Frank, after large declines in bat populations, farmers began using more insecticides in the hope that it would protect their yields and prevent them from suffering any losses in their agricultural income. 

However, the study shows that in addition to causing adverse health consequences and possibly higher infant mortality rates, chemical pesticides also decreased farmer revenues. 

For instance, while the use of insecticides rose by 31 percent after the WNS outbreak, the revenue of farmers decreased by 29 percent. This is because pesticides may have protected the yield but caused a sharp decline in the quality of crops. 

“Combining this revenue loss with the expense of pesticides, farmers in communities that experienced the bat die-offs lost $26.9 billion dollars between 2006 and 2017. Adding onto those losses the $12.4 billion in damages from infant mortality, the total societal cost from the bat die-offs in these communities amounted to $39.6 billion,” Frank said.

“These findings provide empirical validation to previous theoretical predictions about how ecosystem disruptions can have meaningful social costs,” he added.

It shows that society has to pay a large cost when biological pest control systems such as bats start to disappear, and this cost is much higher than what is required to protect bats in the first place. So the priority of farmers, government agencies, and scientists should be to invent and adopt practices that could save the bat population.

The study is published in the journal Science.