A new way to assess the danger that pesticides can pose to bees

By Matt Kelly

Image of female hoary squash bee on leaf.
A female hoary squash bee. | University of Guelph

Discussing the hazards that different pesticides might potentially pose to bees can be a frustrating and tricky thing. The problem is that risk assessments are done with honey bees. And the honey bee (domesticated, highly social and cavity-nesting) is by no means representative of the roughly 3,999 other bee species in North America (the majority of which are wild, solitary and ground-nesting). Sure, a certain dose of a certain chemical under certain conditions might not kill off a perennial honey bee colony with tens of thousands of individuals. But what would the effect be on a single bee who is alive for only six weeks, raising her brood of eight? A study that was recently published in August might have an answer for us.

Researchers from the University of Guelph and the University of Ottawa looked at the concentrations of three commonly-used neonicotinoid insecticides in the soil at 18 different cucurbit farms in Ontario, Canada. They evaluated the probability that ground-nesting bees – hoary squash bees (Peponapis pruinosa) in this case – would be exposed to a lethal dose of those insecticides while constructing their nests.

“The results were sobering,” said Susan Chan, Ph.D. student and lead author, by email. Under many of the scenarios they evaluated, the team found that a hoary squash bee will be exposed to a killing dose of neonicotinoids 80 percent of the time.

“This is really important because we know that bee populations are declining globally but we have never assessed the contribution that pesticides in soil may be making to that decline,” said Chan.

Quote: We can no longer glibly ignore soil exposure when it comes to bees.

Sixty percent of neonicotinoids are applied through seed coatings or as soil applications. Residues can persist in the environment for years, and they’ve been found in the pollen and nectar that bees consume as both adults and larvae, which is how honey bees can be exposed. However, Chan and her colleagues found the greatest amount of neonicotinoids in the soil, posing a far greater risk than exposure through pollen and nectar combined. 

“Our study shows that ground nesting bees are literally risking their lives by excavating their nests in agricultural soil because of the high concentrations of neonicotinoids there,” said Chan. “It is important to remember that not all ground-nesting bees nest in agricultural soil, but many do and those ones are likely at risk.”

The toxicity of a substance and the risk it could pose to different forms of life is measured and discussed in terms of median lethal dose (LD50): the average amount of said substance that is required to kill 50 percent of a population in a given amount of time, under specific circumstances. The lower the LD50, the more poisonous it is. Knowing this helps determine how different substances should be handled and used in real-world situations. And this raises an important question regarding pesticides and bees: if the current LD50s for neonicotinoids were determined based on honey bee populations, how accurate and helpful can they be when it comes to assessing the danger these same chemicals pose to other types of bees when they’re used on a farm, in a park or around our homes?

Chan thinks the current LD50 for neonicotinoids could indeed be a fair measure: honey bees and hoary squash bees are similar in size, and toxicity through contact varies much less among species than toxicity through consumption. However, because not all ground-nesting bees are as big as a squash bee, and because some pesticides are simply more poisonous to one species than another, Chan recommends dividing the LD50 by ten as a best practice to accommodate the range of bees that exist.

The obvious next step with this new knowledge would be to evaluate any insecticide that is applied to soil for its toxicity on bees that make their homes there. Chan and her colleagues are also in the final stages of completing research into the sublethal effects of contact with pesticides through the soil – because messing up a bee’s foraging or reproductive behaviors can be just as problematic for survival as outright death.

“We can no longer glibly ignore soil exposure when it comes to bees,” she said.