A new UC Davis study shows the combined threats of food loss and pesticide use reduced blue orchard bee reproduction by 57% and produced fewer female offspring when reproduction did happen.

The study, published in the journal Proceedings of the Royal Society B, found that pesticide exposure had the greatest impact on nesting activity and the number of offspring the bees produced. Pesticide exposure reduced bee reproduction 1.75 times more than limiting their food.

“Just like humans, bees don’t face one single stress or threat,” said lead author Clara Stuligross, a Ph.D. candidate in ecology at UC Davis. “Understanding how multiple stressors interplay is really important, especially for bee populations in agricultural systems, where wild bees are commonly exposed to pesticides and food can be scarce.”

The study was ducted by exposing bees to the neonicotinoid insecticide imidacloprid by treating some wildflowers with the product. Some environments also had a more limited amount of wildflowers available to the bees. The research found that pesticide-exposed and resource-deprived female bees delayed the onset of nesting by 3.6 days and spent five fewer days nesting than unexposed bees.

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Co-author Neal Williams, a pollination ecologist and professor in the Department of Entomology and Nematology at UC Davis, said that’s a substantial delay considering bees only nest for a few weeks. The production of female bees is also crucial to determining the health of future bee populations.

“In the bee world, males don’t matter so much,” said Williams. “Male numbers rarely limit population growth, but fewer females will reduce the reproductive potential of subsequent generations.”

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