Honeybees have been dying in droves lately across North America and Europe, a problem known as colony collapse disorder that has been at least partly linked to pesticides.

But honeybees aren't the only buzzworthy victims of this mystery. Many wild bumblebees are also in decline, and some have shown vulnerability to neonicotinoids, a class of insecticides often implicated in colony collapse. And according to a new study, even low doses of common, plant-inspired pesticides like pyrethroids may harm bumblebees more than previously thought, yet another twist in the plight of these vital native pollinators.

Published in the Journal of Applied Ecology, the study reveals that pyrethroids stunt the growth of young bumblebees, resulting in smaller workers that may be less efficient foragers. Pyrethroids are synthetic imitations of biotoxins produced by certain kinds of chrysanthemum flowers, and their low toxicity to mammals and birds has made them a popular alternative to harsher insecticides. Pyrethroids are now widely used in a variety of environments, from fields of flowering crops to mosquito-prone yards.

No previous study had examined these chemicals' effects across the full lifecycle of bumblebees, so three researchers from Royal Holloway, University of London, decided to investigate. Working with bumblebee colonies in a lab, they exposed half the insects to pyrethroids and left half untreated as controls, then tracked all the colonies' growth during a four-month period. The researchers not only counted the number of bees per colony and recorded body sizes over time, but also weighed individual bees on micro-scales.

Worker bees exposed to pyrethroids grew less robust and hatched at smaller sizes than bees in the control group, the researchers report, a troubling outcome since body size is known to correlate with foraging efficiency among bumblebees in the wild.

"We already know that larger bumblebees are more effective at foraging," lead author Gemma Baron says in a statement about the research. "Our result, revealing that this pesticide causes bees to hatch out at a smaller size, is of concern as the size of workers produced in the field is likely to be a key component of colony success, with smaller bees being less efficient at collecting nectar and pollen from flowers."


Photo: Vladimir Yaitskiy/Flickr

The European Union recently adopted a two-year ban on neonicotinoids, based on studies suggesting the doses bees get from flowers are enough to kill or impair them. The U.S. Environmental Protection Agency has no plans to limit the nicotine-related toxins, but it does say they're being re-evaluated. Either way, public scrutiny for one class of pesticides tends to boost the popularity of others, and the study's authors warn pyrethroid use may increase amid growing anxiety about neonicotinoids' effects on bees.

And while that concern usually focuses on Western honeybees, thanks to their high-profile role in commercial beehives around the world, many native bumblebees struggle behind the scenes as wild pollinators, lacking the visibility of their domesticated cousins.

"Bumblebees are essential to our food chain, so it's critical we understand how wild bees might be impacted by the chemicals we are putting into the environment," says study co-author Mark Brown, a professor of evolutionary ecology and conservation at Royal Holloway. "We know we have to protect plants from insect damage, but we need to find a balance and ensure we are not harming our bees in the process."

"Our work provides a significant step forward in understanding the detrimental impact of pesticides other than neonicotinoids on wild bees," adds co-author and animal behaviorist Nigel Raine, who's speaking this week at the U.K. Bee Health Conference in London. "Further studies using colonies in the field are essential to understand the full impacts, and conducting such studies needs to be a priority for scientists and governments."

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