Several bumblebee species have seen their ranges contract and some may face extinction due to several combined stressors, say ecologists Lynn Adler, professor of biology, and her former postdoctoral fellow Scott McArt, with others. Their recent analysis, one of the few to explore the relative importance of multiple factors, found unexpectedly that greater use of fungicides was the strongest predictor of range contraction in declining bumblebee species.
First author McArt, now at Cornell University, worked with his advisor, evolutionary ecologist Adler, and with others at Dartmouth College, North Carolina State University, Cornell and the Westchester Land Trust, New York. Their findings on the landscape analysis of factors predicted to cause bumblebee declines in the U.S. appear in a recent issue of Proceedings of the Royal Society B.
McArt says, “We threw everything but the kitchen sink at this analysis and the ‘winner’ was fungicides. It turns out that fungicide use is the best predictor of bumblebees getting sick and being lost from sites across the U.S. This is a surprising result since most people would probably guess insecticides or habitat loss. We hope that these results will put fungicides on the radar for growers who typically don’t worry too much about the non-target effects of these chemicals, especially in terms of pollinator health.”
For this work, the research team quantified 24 habitat, land-use and pesticide-use variables that might be associated with bee health across 284 sampling locations. Using machine learning model selection techniques, they assessed which variables predicted pathogen prevalence and range contractions. McArt and colleagues used one of the largest available datasets of bumblebee decline in North America to ask what factors predict pathogen prevalence in four bumblebee species and what factors predict range contraction in declining species.
To determine pesticide use levels at each sampling location, the researchers used the U.S. Geological Survey’s National Water Quality Assessment database, which captures pesticide use for sprays, treated seeds, soil drenches and all other application methods.
They found that greater use of the fungicide chlorothalonil was the best predictor of the prevalence of a pathogen, Nosema bombi, in four declining species ofbumblebeeslisted as vulnerable or critically endangered among the eight species once considered abundantand now stable. Further, they found “bumblebees in the northern USA experiencing greater likelihood of loss from previously occupied areas.” The pathogen load of N.bombiamong the “stable” bee populations was 1.3 percent compared to 18.3 percent among the “declining” species sampled.
Theresearchers add, “These results extend several recent laboratory and semi-field studies that have found surprising links between fungicide exposure and bee health. Specifically, our data suggest landscape-scale connections between fungicide usage, pathogen prevalence and declines of threatened and endangered bumblebees.”
A strength of this study, McArtand colleagues say, is that “landscape-scale analyses can provide broad insight into multiple competing factors that may be impacting bee health.” Further, “because bumblebees typically forage several hundred or thousand meters from their nests, landscape characteristics such as habitat composition and fragmentation, urbanization and agricultural intensification can impact foraging behavior, pathogen prevalence and hive performance.”
The authors suggest that further studies to investigate the response of bumblebees to many more commonly used fungicides are warranted. “With both landscape-scale analyses and controlled manipulative experimental data, conservation biologists and policymakers will have a broader information base upon which to make more informed decisions and improve pollinator health,” they point out.