Study: Dead bats are linked to a spike in farm pesticides and infant mortality

There are a number of well-known ways to keep babies healthy — wash your hands often, get them vaccinated, don’t smoke inside, and so on.

But there’s one thing you probably haven’t heard of: protecting bats. Like literal flying bats.

That’s one takeaway from a remarkable new study, published in the journal Science, that links the decline of bats to a rise in newborn deaths in the United States.

By compiling and analyzing a huge amount of government data, environmental economist Eyal Frank, the study’s sole author, discovered that in regions where white nose syndrome, a wildlife disease that impacts bats, has hit particularly hard, the rate of infant mortality increased by nearly 8 percent.

There’s a clear reason for this, according to the paper. Most North American bats eat insects, including pests like moths that damage crops. Without bats flying about, farmers spray more insecticides on their fields, the study shows, and exposure to insecticides is known to harm the health of newborns.

“When bats that eat insects go down, farmers compensate by using more insecticides,” Frank, an assistant professor at the University of Chicago Harris School of Public Policy, told Vox. “That has adverse health consequences — full stop. The damages from their absences appear to be substantial.”

Frank’s study adds to a growing body of research that supports the idea — which perhaps should be obvious by now — that healthy ecosystems are important for human well-being.

Earlier research has found that wolves help limit car accidents by keeping deer off the road. Other research, also led by Frank, links the sudden decline of vultures in India to an increase in human death rates. Vultures eat animal carcasses that, if left to rot, can pollute waterways and feed feral rats and dogs, a source of rabies.

When the link between human and environmental health is overlooked, industries enabled by short-sighted policies can destroy wildlife habitats without a full understanding of what we lose in the process. This is precisely why studies like this are so critical: They reveal, in terms most people can relate to, how the ongoing destruction of biodiversity affects us all.

Not everyone finds bats cute — they are! — but they are undeniably impressive. They’re the only mammal on earth that can truly fly. Plus, they eat astounding quantities of bugs. A single bat can catch several hundred insects an hour, and thousands in a single night.

This is good for us: Many of the critters that bats consume during their nightly hunt are insects that we don’t like, such as blood-sucking mosquitos and crop-eating moths and beetles. Bats are, essentially, a natural pest control.

So it stands to reason that without bats, farmers have to use more insecticides on their crops; agrochemicals do the job that bats do for free.

There hasn’t been a great way to test that theory, until somewhat recently, when bats across North America began dying en masse. In 2006, a fungal disease called white nose syndrome appeared in New York state and began spreading among bat colonies, killing an average of more than 70 percent of the bats within them. It’s been brutal. WNS invades their skin, producing fluffy white growths around their noses, and wakes them up during hibernation when they should be resting. Infected bats burn off vital energy stores and either freeze or starve to death.

Devastating as it may be, the rapid loss of bats has provided researchers with a rare opportunity to test what happens when these animals disappear from the landscape. In the new study, Frank — who works at the intersection of economics and conservation — analyzed data on pesticide use across US counties with and without WNS, which until recently were mostly in the eastern US. Where there’s WNS, there are presumably far fewer bats.

His results were astonishing: Farms in regions hit by WNS used 31 percent more insecticides on their crops, compared to counties without the disease. That suggests that when bats disappear, farmers compensate by using more chemical bug killers.

First, there is a cost to farmers. According to Frank’s study, the decline of bats has cost the agriculture industry nearly $27 billion between 2006 and 2017, as shown by a drop in revenue in regions with white nose syndrome. The reason for this loss is not clear, though it might be that bat-free regions produce lower quality crops, Frank said.

A study published in 2022 supported a similar conclusion, linking the spread of WNS to a drop in the rental price of farmland. The idea is that farmers have a lower yield or have to spend more money to grow crops — such as on purchasing insecticides — when there are no bats providing free pest control. (I interviewed one of the study co-authors, Amy Ando, for an episode of the Vox science show Unexplainable. You can listen here.)

Then there is the serious cost to human lives.

It’s well-known that when farmers spray their fields with insecticides, those chemicals can leach into the environment, where they pose a risk to public health. One recent review links pesticide exposure among newborns, for example, to life-long abnormalities and diseases. With this in mind, you might expect regions with no bats, where farmers are using more insecticides, to have more health issues.

Frank tested this theory too, using government data on infant mortality, overlaid with the spread of white nose syndrome. The results of his analysis were alarming: The rate of internal infant mortality — babies who have died by causes other than accidents or homicides — increased by nearly 8 percent in counties following WNS outbreaks. Put another way, when insecticide use increases by 1 percent, infant mortality increases by a quarter of a percent, which is comparable (though slightly lower) to the impact of ambient air pollution.

“I was surprised that the signal [in the data] was so strong,” said Dale Manning, an environmental economist at the University of Tennessee Knoxville, who was not affiliated with the study. “They’re big, big numbers in terms of monetary impacts, but we’re also talking about human lives, right? And so those impacts are pretty substantial.”

Manning and Ando, an environmental economist at Ohio State University who also was not involved in the study, said the paper’s conclusions were sound. (Ando and Manning were both involved in the 2022 study, mentioned above.)

While the research doesn’t definitively prove that bat declines cause insecticide use and infant mortality to increase, the study ruled out many other potential forces behind this trend. Frank also found that when bat declines were more severe — when more bats died, more caves were infected, or the decline was steeper — the rate of infant mortality was higher.

Studies like this make addressing the ongoing collapse of bat populations ever more urgent. In North America, more than half of all bat species “are at risk of populations declining severely in the next 15 years,” according to a 2023 report by the North American Bat Conservation Alliance, a coalition of groups including government agencies and Bat Conservation International. This trend is mirrored globally.

WNS continues to spread west, invading new regions. Climate change is harming these animals, too. Bats’ flight-adapted physiologies make them highly susceptible to severe droughts and heat waves, as I previously reported. Plus, wind turbines — an important climate solution — are killing hundreds of thousands of bats each year in North America alone. Typically, the bats, most of which are migratory species, die from colliding with turbine blades, though it’s not clear why these animals are drawn to them.

It’s not all bad news; there are ways to help bat colonies survive. Scientists have been testing a vaccine for WNS, for example. And research shows that slowing down wind turbines at night during certain times of year reduces collisions.

But these approaches can be costly — underscoring the value of studies that reveal, with more clarity, the payoff of investing in conservation, in both dollars and human lives.

“At the end of the day, scientists and policymakers have to justify allocating resources” to things like fixing bridges and fixing schools, or to “fixing” bats, Manning said. “All of those have different returns associated with them.”

“And if we don’t make an effort to show what the benefits are of ‘fixing’ the bats,” he said, “those benefits will be discounted.”