As the Environmental Protection Agency faces a lawsuit over its policy to allow use of a pesticide class implicated in global honeybee die-offs, new evidence that the compounds may damage the brains of bees could convince the agency to reconsider.

Scientists are investigating many possible causes of honeybee deaths. Neonicotinoid pesticides are under particular scrutiny, however. A growing body of field research studies links these compounds to behavior changes that affect bee survival, including impaired memory and navigation.

Until now, neonicotinoids’ effects on bees’ brains have been unknown. A new report finds that neonicotinoids and another pesticide class, the organophosphates, inactivate brain cells that help bees learn (Nat. Comm., DOI: 10.1038/ncomms2648).

Researchers already knew these pesticide classes target acetylcholine signaling to kill bugs.

But honeybees are not killed right away. To find out whether the compounds’ modes of action translate to effects on bee brain function, a team led by Christopher N. Connolly at the University of Dundee, in Scotland, monitored nerve signals at the learning centers of intact honeybee brains as the bees were exposed to the pesticides.

At doses commonly found in plants treated with the pesticides, the neonicotinoids imidacloprid and clothianidin each inactivated bee brain nerve cells after 20 minutes. The organophosphate coumaphos, which beekeepers use to prevent mites, led to epilepsy-like brain activity in bees before it, too, shut down learning center nerve cells.

“If bees can’t learn efficiently, then they can’t forage efficiently and the whole colony is likely to struggle and weaken,” Connolly says.

But Christian Maus, a safety manager at Bayer CropScience, which makes clothianidin, cautions that it’s tough to determine what happens to bees in nature from this study, because it was conducted on isolated bee brains in direct contact with insecticides, without any of the normal protective barriers or metabolism.

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It’s great to know the underlying mechanism for the behavioral impairments scientists have observed in the field, says David Goulson, who studies bees at the University of Stirling, in Scotland. But he’s not sure this result changes the wider debate about whether to ban neonicotinoids.

That debate came into renewed focus on March 21, when a coalition of environmental groups and beekeepers filed suit against EPA, claiming the agency rushed approval of two neonicotinoids—clothianidin and thiamethoxam—without addressing how they impact bees and other pollinator species. The groups are demanding that EPA ban the pesticides.