A scientific breakthrough has shed a sliver of light on the mystery of the beleaguered honeybee. These crucial colony-dwellers play major ecological and economic roles pollinating both wild plants and crops, but in recent years they’ve been dropping dead for their troubles — at 100 to 1000 times the previously recorded rates in commercial colonies.

This is a serious problem, and not just for the bees. Food production worldwide relies on insect pollinators, so any factor that adds to the widespread, accelerating collapse of colonies has grave implications for everything from the canola fields of Canada’s Prairies to fruit orchards in the Mediterranean. There are many factors at play, but as a study published today in the journal PLOS ONE suggests, certain common agricultural chemicals — generally thought to be harmless to insects and other animals — make bees more susceptible to deadly pathogens.

A fungus called Nosema ceranae was linked to the untimely deaths of honeybees and the collapse of entire populations not long after it appeared in Canada and the United States between 2003 and 2006. The pathogen attacks the intestinal lining of workers, queens and drones. Nosema ceranae-infected cells that would normally produce digestive enzymes instead release fungal spores into the bees’ digestive tracts, which can kill their hosts in a matter of days.

Researchers identified sublethal levels of agricultural chemicals, like fungicides, herbicides and other pesticides, in pollen samples from beehives in fields in the northeastern United States. They fed that pollen (laden with an average of nine chemicals per sample) to healthy bees and tested their ability to fight off Nosema ceranae infections.

The greatest surprise of the study came when the test bees were fed pollen containing one of the most common chemicals, a fungicide called chlorothalonil, which is used widely throughout Canada and the United States. In Canada, the substance is sold as the active ingredient in products such as Bravo, Daconil and Ridomil Gold, though it can also be used as a paint, adhesive and wood preservative; in its fungicidal form it does not bioaccumulate in mammals and degrades rapidly in soil. The ingestion of real-world amounts of the stuff, nevertheless, made honeybees roughly three times more likely to be infected by Nosema than healthy bees.

University of Maryland researcher Dennis van Engelsdorp said in a press release that this finding may point to a need for stricter regulations on fungicides, perhaps more in line with restrictions on insecticide use, which limit spraying while pollinating insects are foraging. “We don’t think of fungicides as having a negative effect on bees,” he said, “because they’re not designed to kill insects.”