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North-facing, sloping ground with loose, sandy soil — if you’re a bumblebee queen on the market for a winter home, these features will have you racing to make an offer. But scientists were recently stunned to find there’s something else these monarchs like in a place to hibernate: pesticides.
In a paper published last month in the journal Science of The Total Environment, researchers described an experiment that gave common eastern bumblebee queens a choice: hibernate in clean soil, or in soil laced with pesticides. The insects behaved in a way that was the opposite of what was expected.
“Queens did not avoid any of the pesticides,” said Sabrina Rondeau, an ecologist at the University of Ottawa. “Even at high concentrations they didn’t, and still seem to prefer the soil contaminated with pesticides.” The finding, Dr. Rondeau admits, was “very surprising.”
One researcher described the study’s findings as “terrifying.”
“It wasn’t just one pesticide at one concentration, it was across the board,” said S. Hollis Woodard, a bee biologist at the University of California, Riverside. That’s scary, she said, because our soils are full of pesticides — many that traveled far from where they were originally used. Gravitating toward these pesticides may put queens at high risk for direct exposure, with potentially damaging consequences.
Most of the roughly 250 species of bumblebee have annual life cycles, where queens start new colonies in the spring that grow throughout the summer before collectively dying out in the fall. As winter nears, queens that have mated then burrow into the soil to sleep through the cold before restarting the cycle.
This period of solitude is poorly understood.
“There’s a huge bias in bumblebee research” that favors the social stage of a humming bumblebee colony, Dr. Woodard said. But “for much of the year, queens are living their own little solitary lives.”
Within mesh-covered hoop houses, Dr. Rondeau and Nigel Raine, a pollinator biologist at the University of Guelph, presented bumblebee queens with a range of soil options, including crates filled with pesticide-laced soil and others with soil that was chemical-free.
The duo tested five common pesticides at both low and high concentrations. They based the amounts of poison they used on measurements Dr. Rondeau had taken at farms and apple orchards in Southern Ontario.
The scale of the operation was immense. The researchers used small cement mixers to blend the soils and replicated the experiment in 10 different hoop houses. Within each hoop house, four bumblebee colonies were set up, with two on each end. After letting the bees do their business for five weeks, the team returned and dug into the soil to see where the queens had chosen to hibernate.
“We pretended to be archaeologists for a bit,” Dr. Raine said.
The queens overwhelmingly chose to rest their heads in the pesticide-contaminated soil. They were 1.3 to 2.4-times less likely to choose chemical-free soil, and when in a hoop house that used high pesticide concentrations, none of the queens chose the clean soil.
Pesticides are, on the whole, bad for bees, but what exactly they do to hibernating queens is a big unknown.
In a related experiment published in the same journal in July, Dr. Rondeau and Dr. Raine assessed which pesticides had negative consequences for the queens.
Exposure to one of the compounds, the insecticide cyantraniliprole, made some queens less likely to survive after hibernation, delayed their colony formation and made their daughters smaller. (Another recent laboratory study on a related bumblebee species, however, found the chemical had no negative effect on queen survival.)
But a fungicide, boscalid, actually improved queen survival and colony formation.
Dr. Woodard says it is difficult to study the effects of pesticides on queens all the way through their life cycle. Bumblebee queens might have no ill effects after emerging from hibernation in soil with pesticides, but later could struggle to forage for food once they start their colonies, for example. And in nature, bees contend with many other stressors like diseases, heat waves and starvation.
Dr. Rondeau is now working to understand how pesticide exposure and warmer winter temperatures caused by climate change may affect bumblebee queens. Why the queens choose pesticide-laced homes to hibernate in is also a mystery, but one idea is that the poisons kill fungi and parasites that the queens typically try to avoid.
Dr. Raine said that pesticide risk assessments for bees are often based on honeybees, which nest in cavities like rock crevices, tree hollows and, of course, wooden boxes, but not soil.
“Well over 65 to 70 percent of bee species, of the 20,000 or so that we have globally, nest in soil,” Dr. Raine said. With pesticides so thoroughly seeped into our soils, the world needs to better understand the risks faced by bees beneath the surface.
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