Bat die-off led to more insecticide use and more infant deaths in US

The decline of bats in the US due to the spread of a fungal disease reduced farmers’ earnings and led to an additional 1300 deaths in children under 1 year old, a study has reported.

In counties where the bat disease is present, farmers increased their use of insecticides by 31 per cent to make up for the reduction in insect predation by bats, Eyal Frank at the University of Chicago in Illinois found. Between 2006 and 2017, farmers in affected counties lost $27 billion as a result of lower crop sales and higher insecticide costs, he calculates.

What’s more, in the affected counties, there was also an 8 per rise in the number of children dying before 1 year of age, which Frank attributes to the increase in insecticide use.

“Insecticides are toxic by design,” he says. “Even when they are used at regulatory levels, there seems to be a health cost.”

In 2006, hibernating bats with a white fungus on their muzzles were seen in a cave in New York state, with many dying. White-nose syndrome, as it is known, has been spreading across North America ever since, killing millions of bats.

When Frank read about white-nose syndrome, he realised it provided a way to directly assess the benefits that bats provide to farmers. “This is a very good approximation for an ideal experiment where one would go out and randomly manipulate populations,” he says.

He has used data from agricultural censuses to compare counties where white-nose syndrome has been detected with those where it wasn’t yet present up to 2017 – stopping that year because the census data is only released every few years.

In affected counties, insecticide use rose higher every year after the detection of the disease, but it remained broadly constant in other areas.

White-nose syndrome affects only 11 of the around 50 bat species in the US and has killed around 70 per cent of those species, on average, in affected areas, so the total value of bats to farmers in the US is far greater than the numbers Frank has calculated.

He then looked at data on infant mortality, excluding deaths due to accidents and murders. The 8 per rise in affected counties would have resulted in an extra 1300 infant deaths by 2017, he calculates.

Frank thinks his findings go beyond correlation to show that the die-off of bats is the cause of both higher insecticide use and higher infant mortality. He says the compelling thing is that the trajectory of counties changed in the same way once the disease reached them, whatever the year the disease arrived.

However, exactly how higher insecticide use led to higher infant mortality isn’t clear. “I can’t say anything about the direct exposure mechanism, only that my results are not consistent with the idea of exposure through food,” says Frank.

That leaves breathing in insecticides or contaminated water supplies as the most likely routes.

“Frank convincingly demonstrates that in counties affected by white-nose syndrome, insecticide use increases compared to counties that are not affected,” says Roel Vermeulen at Utrecht University in the Netherlands.

However, Vermeulen says that the loss of income would lead to more stress in farming communities, and that this could also contribute to higher infant mortality. “Therefore, it is questionable whether the observed effects on infant mortality can be solely attributed to increased insecticide use,” he says.

“This study shows that bats can save human lives just by doing what they do best – eating insects,” says Jennifer Raynor at the University of Wisconsin-Madison.

“Many wild animals are important for human health and well-being, and we are now beginning to understand that technology cannot always replace these benefits when they are lost,” she says.

Vermeulen thinks the study also shows that we need to broaden the way we think about human health. “It emphasises the need to move from a human-centric health impact analysis, which only considers the direct effects of pollution on human health, to a planetary health impact assessment,” he says.