Mass bat deaths lead to increased pesticide use in the US, and increased infant deaths

A decline in bat populations in the United States due to the spread of a fungal disease has led to reduced farm incomes and an additional 1,300 deaths of infants under the age of one, a study has found.

In bat disease-hit counties, farmers increased their use of pesticides by 31% to compensate for the decline in bat predation, according to Eyal Frank of the University of Chicago in Illinois. Between 2006 and 2017, farmers in affected counties lost $27 billion as a result of reduced crop sales and higher pesticide costs, Frank calculates.

Additionally, affected counties saw an 8 percent increase in the number of infants dying before the age of one, which Frank attributes to increased pesticide use.

“Pesticides are designed to be inherently toxic,” he says, “and even when used at regulated levels, they appear to pose health hazards.”

In 2006, hibernating bats were found in a cave in New York state with white fungus on the tips of their noses, many of them dead. The disease, known as “white-nose syndrome,” has since spread across North America, killing millions of bats.

When Frank read about white-nose syndrome, he realized it was a way to directly assess the benefits bats provide to farmers. “It’s a pretty good approximation of an ideal experiment, where you go out and randomly manipulate bat populations,” he says.

He used agricultural census data to compare counties where white-nose disease had been detected by 2017 with those where it hadn’t yet been detected. Because census data is only released every few years, he stopped looking that year.

In affected counties, insecticide use increased each year after the disease was discovered, while in other areas it remained roughly constant.

Because white-nose syndrome affects only 11 of the roughly 50 bat species that live in the United States, and because it kills an average of about 70 percent of those species in affected areas, the total value of bats to American farmers is much greater than Frank’s calculations.

He then looked at data on infant mortality rates, excluding deaths from accidents and homicides, and his calculations showed that an 8 percent increase in the affected counties translated into 1,300 more infant deaths by 2017.

Frank believes his findings go beyond correlation and show that bat deaths are the cause of both increased pesticide use and higher infant mortality. He says what’s compelling is that the disease’s arrival changed counties’ trajectories in the same way, regardless of which year the disease arrived.

But it’s not clear how increased pesticide use could lead to higher infant mortality. “I can’t say anything about a direct exposure mechanism, but my results are not consistent with exposure through food,” Frank says.

This makes inhaling pesticides or contaminated water the most likely route of infection.

“Frank convincingly demonstrates that counties affected by white-nose disease experience increased pesticide use compared to unaffected counties,” says Roel Vermeulen of Utrecht University in the Netherlands.

But Vermeulen says that reduced incomes could increase stress in rural areas and also lead to higher infant mortality rates. “We therefore question whether the observed effects on infant mortality can be attributed solely to increased pesticide use,” he says.

“This study shows that we could save humans’ lives simply by eating insects, a food that bats specialize in,” said Jennifer Rayner of the University of Wisconsin-Madison.

“We are beginning to understand that many wildlife species are important to human health and well-being, and that technology cannot necessarily compensate for the loss of these benefits,” she says.

Vermeulen also believes the study shows the need to broaden how we think about human health: “This study highlights the need to move from human-centric health impact analyses that only consider the direct effects of pollution on human health to global health impact assessments,” he says.