Pesticides affect bumble bee health in complex ways

New research has uncovered a surprising wrinkle in our understanding of how pesticides affect bumble bees. 

While common sense tells us that chemicals meant to kill insects can only harm pollinators, it turns out there may be moments when a tiny amount of poison could actually help a bumble bee queen make it through winter – at least in the short term.

A team of researchers led by Pennsylvania State University’s College of Agricultural Sciences examined imidacloprid, a widely used neonicotinoid insecticide. 

Short-term benefits vs. long-term fitness

It’s already well-established that high doses of imidacloprid shorten bees’ lives and reduce their ability to reproduce.

But when the scientists took a closer look, they found something unexpected: at low, sublethal doses, imidacloprid seemed to help bumble bee queens survive through diapause, which is the bug-world version of hibernation.

This phenomenon is called hormesis – when a small dose of something that’s generally harmful turns out to be beneficial under just the right circumstances. 

The experts compared it to caffeine for humans. A small amount might provide a boost, but we all know it can also interfere with our sleep if we’re not careful. 

“Even low doses can have unintended effects, such as disrupting sleep,” said Etya Amsalem, an associate professor of entomology at Penn State and the study’s lead author.

According to Professor Amsalem, if hormesis goes unrecognized, it could lead to the mistaken conclusion that certain pesticides benefit bees. 

“Such a misconception is dangerous, given the well-documented negative effects of imidacloprid and the trade-offs associated with hormetic responses – where short-term benefits trade-off with long-term fitness.”

Winter struggle of bumble bee queens

One reason this discovery matters so much is that queen bumble bees spend a huge chunk of their life – often around 75% – in diapause. This is a dormant state that helps them weather harsh winter conditions. 

If a queen doesn’t make it through this frigid, food-scarce period, an entire future colony is lost in the process.

“Despite the fact that most pollinators undergo winter diapause, pesticide risk assessments typically focus on the bee’s active seasons, overlooking a significant portion of the bee’s life cycle,” Amsalem explained. 

So, while we know a fair bit about how chemicals impact bees during spring and summer, there’s a big gap in our knowledge about what happens to them over winter.

Pesticides and bumble bee survival

To figure out how imidacloprid might affect bees during both the active and dormant phases, the team fed bumble bees sugar water containing different concentrations of the insecticide. 

For worker bees and males, higher doses led to the usual bad news: shorter lifespans and fewer offspring. But when the scientists gave queen bees just a little bit of imidacloprid before sending them into diapause under controlled conditions, they noticed a puzzling effect. 

“Surprisingly, we found that sublethal imidacloprid exposure actually improved the queens’ survival,” Amsalem said.

Hormesis: A double-edged sword

While a longer winter survival time sounds like a good thing, that’s only one part of the bigger story. The team also saw that imidacloprid exposure lowered overall fitness, such as decreasing a colony’s total reproductive output. 

In other words, the queen might make it through the winter more often if she has just a tiny bit of pesticide in her system, but that same pesticide still saps her and her colony in other ways – perhaps making them less robust once spring arrives.

That’s the essence of hormesis: it’s a trade-off. In bees, these short-term gains don’t necessarily mean the insecticide is actually beneficial when you look at the big picture. 

Instead, scientists worry that not accounting for hormesis could lead us to think that certain pesticides are harmless or even helpful, when in fact they may still pose major threats to our pollinator populations.

Looking at the bigger picture

Bumble bees are critical pollinators for a variety of crops. Any shift in their survival rates – good or bad – could have huge implications for agriculture and ecosystems. If our standard methods of testing pesticide safety overlook what happens during diapause, we risk missing the full picture.

Professor Amsalem believes these findings should be woven into how we assess and regulate pesticides. She also said we need to dig deeper into the “why” of this hormetic response. 

“Most importantly,” she explained, “we must strive to conduct nuanced science that provides a holistic understanding of how stressors affect pollinators – ensuring better-informed conservation efforts.”

To truly protect bumble bees and other pollinators, experts will need to examine every part of their life cycle, not just those busy spring and summer months. 

That includes thinking about how warming and pesticides might affect them differently in different seasons – and whether certain amounts of chemicals that look safe on the surface can still be causing hidden problems.

Future research directions

The discovery that low doses of imidacloprid can help queens survive winter means we can’t simply dismiss all sublethal pesticide exposure as benign. It also means that if we don’t consider diapause in our risk assessments, we might jump to the wrong conclusions. 

Future research will no doubt explore exactly how these low doses increase queen survival and whether similar patterns show up in other insect species or other types of pesticides.

What’s clear is that pollinator health is rarely a simple matter of “just avoid poison.” From the vantage point of ecology, everything is connected. A small dose can have unforeseen benefits – until it doesn’t. 

By expanding our perspective to include times of the year we usually overlook, like the frigid months when queen bees are hidden away, we stand a better chance of keeping these vital pollinators humming along for generations to come.

The study is published in the journal Biology Letters.

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