Neonic soil treatment hurts ground-nesting bees, 1st of its kind study finds

A new study from researchers at the University of Guelph looks at the impact of pesticides applied to seeds and soil before planting on squash bees. While other studies have looked at the role of neonicotinoids on honey bees and bumble bees, this is the first to look at ground-nesting bees.

'The extent of it was quite surprising. Shocking,' says research Susan Chan

New research from the University of Guelph looked at the impact pesticides have on ground-nesting bees, like the squash bee. (Nigel Raine/University of Guelph)

A new study shows the behaviour and reproduction of ground-nesting bees, like those that pollinate squash and pumpkins, is severely impacted when farmers treat the soil with neonicotinoid insecticide at the time of planting.

While other studies have looked at the impacts neonicotinoid insecticide, or neonics, have had on honey bees and bumblebees, this study from University of Guelph researchers is the first to look at ground-nesting bees. 

It's crucial to understand how these particular bees are impacted, because they're often the ones pollinating those very crops, says Guelph professor and researcher Nigel Raine.

"We know that they nest in farm fields so the likelihood is that they might be exposed to pesticides if they're applied to those crops," said Raine, who also holds the Rebanks Family Chair in Pollinator Conservation. 

The research was led by student Susan Chan over a three-year period. The researchers grew squash plants and applied three treatments: a neonicotinoid insecticide called imidacloprid applied to the soil, a neonicotinoid called thiamethoxam that coated the seeds of the plants and a chlorantraniliprole spray applied when the plant had five leaves, but before they flowered. There was also a control group.

Each treatment had three hoop houses dedicated to each treatment and the control group. Eight bees were introduced to each hoop house when the plants started to flower.

"We didn't know which treatments were assigned to which hoop house but boy, oh boy, you could quickly tell something was going on in three of those hoop houses," Chan said. "The extent of it was quite surprising. Shocking, as a matter of fact."

Raine said the most significant impact was in bees where the neonics were applied as a soil treatment.

"We found that those bees initiated 85 per cent fewer nests, so they dug fewer nests, they collected much less pollen, they left more than five times as much pollen … unharvested on the male flowers in our hoop houses and over the three years of the experiment, we found they produced 89 per cent fewer offspring than the untreated controls," he said.

The researchers set up hoop houses to grow squash plants, applied pesticides, then introduced the bees into the hoop houses just as the plants were beginning to flower. The testing was blind, so researchers didn't know which treatments were in which hoop houses, but researcher Susan Chan says it became clear pretty quickly that bees were negatively impacted by one particular treatment. (Nigel Raine/University of Guelph)

Information farmers need

While educating the  public on how neonics impact bees is important, Chan says she hopes her research reaches regulators, such as the Pest Management Regulatory Agency, as well as farmers.

"This is really information that farmers need to have and I totally trust farmers to make good decisions once they have good information and often they're not given good information," she said.

Regulators, she added, "haven't paid attention to the idea that bees could be exposed to pesticides in soil."

Chan says previously, farmers who have been made aware of her research have changed the way they use insecticides.

"It won't surprise me at all if they make changes," she said.

Raine says regulators tend to look at risk assessments involving honey bees, but honey bees are not representative of most bee species. About 75 per cent of bee species are solitary, ground-nesting bees while honey bees will live in a hive or other opening above ground.

He added farmers are in a difficult position: They need to grow crops, they need to control pests, but they rely very heavily on squash bees to pollinate those crops.

"They don't want to be doing unintended harm to their pollinators so being able to pass out the information that certain pesticides may be more detrimental to those pollinators than others, then that may be informing their choices," he said.

Regulatory review

Geoffroy Legault-Thivierge, a spokesperson at Health Canada of which the Pest Management Regulatory Agency is part of, says they are aware of the study and are reviewing the research conducted by Chan and Raine.

Health Canada began a special review in 2014 of three neonicotinoids: thiamethoxam, clothianidin and imidacloprid.

In 2019, Legault-Thivierge said the agency re-evaluated the three neonics.

"Health Canada cancelled the soil uses of these pesticides on cucurbits [gourd crops], which the research article identified as posing a risk to the bees. The seed treatment uses did not pose a significant risk to squash bees in the Guelph research or in Health Canada's 2019 pollinator re-evaluation decisions," he said.

He noted pesticides are registered for use in Canada "only if the science-based assessments indicate that they do not pose risks to human health or the environment."

When there are "reasonable grounds" to believe the pesticides may pose a risk to the environment or human health, they can initiate a special review.

Chan and Raine's research was published in the journal Scientific Reports in February.