U.S. study links bumblebee declines to fungicide use

Western bumblebee. (Stephen Ausmus photo courtesy ARS/USDA)

A new look at the environmental factors around declining bumblebee populations and ranges points to a less-than-usual suspect: fungicides.

“Insecticides work; they kill insects. Fungicides have been largely overlooked because they are not targeted for insects, but fungicides may not be quite as benign — toward bumblebees — as we once thought,” Scott McArt, assistant professor of entomology at New York’s Cornell University, said in a release Wednesday. “This surprised us.”

McArt is the lead author on the study, published Wednesday in the Proceedings of the Royal Society.

“While threats to bees are numerous, few analyses have attempted to understand the relative importance of multiple stressors,” the study team wrote. “Such analyses are critical for prioritizing conservation strategies.”

This analysis, McArt said, shows how fungicides — particularly chlorothalonil, a general-use fungicide often found in bumblebee and honeybee hives — may negatively affect bee health.

The study team gauged 24 habitat, land-use and pesticide-use variables across 284 sampling locations, looking at which variables predicted pathogen prevalence and range contractions, by way of machine learning model selection techniques.

“We found that greater usage of the fungicide chlorothalonil was the best predictor of pathogen prevalence in four declining species of bumblebees,” particularly nosema (Nosema bombi), a gut infection that can be fatal in bumblebees and honeybees.

Chlorothalonil, in Canada, is an active ingredient in crop fungicides such as Syngenta’s Bravo, among others. Bravo, for one, is registered to manage diseases such as early blight and late blight in potatoes and ascochyta blight in peas, chickpeas and lentils.

Chlorothalonil has been linked to stunted colony growth in bumblebees and an increased vulnerability to nosema, the study team said.

Bees, McArt said, can pick up the fungicides’ residue when foraging for pollen and nectar.

“While most fungicides are relatively nontoxic to bees, many are known to interact synergistically with insecticides, greatly increasing their toxicity to the bees,” he said.

“Since fungicide exposure can increase susceptibility of bees to nosema, this may be the reason we’re seeing links between fungicide exposure, nosema prevalence and bumblebee declines across the U.S. in this data set,” McArt said.

Nosema, the team wrote, has previously been found in greater prevalence in some declining U.S. bumblebee species compared to “stable” species.

“Greater usage of total fungicides was the strongest predictor of range contractions in declining species, with bumblebees in the northern (U.S.) experiencing greater likelihood of loss from previously occupied areas.”

The team said its results “extend several recent laboratory and semi-field studies that have found surprising links between fungicide exposure and bee health.”

Specifically, they wrote, “our data suggest landscape-scale connections between fungicide usage, pathogen prevalence and declines of threatened and endangered bumblebees.”

The team noted its study builds on a large data set collected by Sydney Cameron, professor of entomology at the University of Illinois. — AGCanada.com Network


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