Pesticides might be worse for bees than we thought

Increasingly, scientists are documenting the decline of bees and butterflies, evidence that the loud hum of buzzing insects on many landscapes is turning to a whisper. For bees, the threats are numerous, including habitat loss, climate change, and intensive agriculture. And when insects forage in farms, they suffer from poor nutrition due to a lack of diverse food sources and become exposed to agricultural chemicals. It turns out, cocktails of agricultural chemicals may have a synergistic effect on bee mortality.

In other words, more bees die than would have if the effects of the chemicals simply added to each other. The authors of the paper analyzed 90 studies that in total documented 356 effects from interacting bee stressors, such as combinations of chemicals, nutritional problems, and parasites. Each study included at least two factors harming bees. For example, if one pesticide used alone caused 10 percent of bees to die, and another pesticide killed 15 percent, the two combined would have a synergistic effect if more than 25 percent of bees died.

Across the studies, the researchers repeatedly found that when bees were exposed to multiple agrichemicals, the combination had a synergistic effect on mortality. In other research, however, scientists have found that certain pesticides can weaken a bee’s immune system, potentially making them extra vulnerable to other chemicals or pathogens. There are also numerous other processes that may be responsible for the compounding effect, says Elizabeth Nicholls, an ecologist studying bees at the University of Sussex who was not involved in the analysis. “It also might be that their detoxification pathways might be impaired if they’re being bombarded with lots of chemicals at one time.

Studies have found that bees are exposed to a range of pesticides, both from crops and nearby wildflowers. “The actual commercial formulas that are used on farms often have multiple chemicals in them. Especially with bees tending to forage across many plants, their chances of getting exposed to multiple toxins are high, says Nicholls. Importing extra honey bees to fill the gap isn’t an option, either.

And wild bees are probably even more sensitive to threats, because they tend to be solitary and lack the robust social networks of honey bees. “Wild bees are really important, and those are the bees that are doing really badly,” says Siviter. “If you don’t consider the interactions, you’re underestimating the impact of environmental stressors on bees.