The connections are commonsense but the conclusion is shocking.

Bats eat insects. When a fatal disease hit bats, farmers used more pesticides to protect crops. And that, according to a new study, led to an increase in infant mortality.

According to the research, published Thursday in the journal Science, farmers in affected U.S. counties increased their use of insecticides by 31 percent when bat populations declined. In those places, infant mortality rose by an estimated 8 percent.

“It’s a seminal piece,” said Carmen Messerlian, a reproductive epidemiologist at Harvard who was not involved with the research. “I actually think it’s groundbreaking.”

The new study tested various alternatives to see if something else could have driven the increase: Unemployment or drug overdoses, for example. Nothing else was found to cause it.

Dr. Messerlian, who studies how the environment affects fertility, pregnancy and child health, said a growing body of research is showing health effects from toxic chemicals in our environment, even if scientists can’t put their fingers on the causal links.

“If we were to reduce the population-level exposure today, we would save lives,” she said. “It’s as easy as that.”

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