Researchers recently found what they believe may be an unexpected cause of bee decline in the United States — fungicides.
Looking at sites across the country where bee populations were falling, Scott McArt, assistant professor of pollinator health at Cornell University, and his team looked at a range of factors that might put stress on bees. What they found surprised them. Fungicides, especially the commonly used Chlorothalonil, were the best predictor bees for getting sick with Nosema, a pathogen that sickens bees, as well as for range contractions and bee decline.
McArt suspects this may indicate that fungicides are the main cause of bee declines.
“This is just the tip of the iceberg here,” he said. “This is definitely something going forward that we’ll be pursuing.”
McArt knew citizen science data on bumblebees stretched back a couple hundred years, and he wanted to take advantage of that data, especially in light of the recent large-scale bee declines. He met the author of a 2011 paper that showed range detractions and declines in a few species of bumblebees, and she agreed to share her data to help him explore why these declines might be occurring.
“We were looking at all of these sites across the U.S. where bees were known not to do that well,” said McArt, lead author of the study in Proceedings of the Royal Society B. They decided to “throw everything in the kitchen sink,” McArt said, taking into account a wide range of factors, including longitude and latitude, habitat fragmentation, agricultural intensification, urbanization, human population density and pesticides. They wanted to see if any of them could predict the bees’ range contraction or the prevalence of the Nosema pathogen.
They looked at these effects in declining species such as the rusty patched bumblebee (Bombus affinis), the yellow-banded bumblebee (Bombus terricola), the western bumblebee (Bombus occidentalis) and the American bumblebee (Bombus pensylvanicus).
McArt found most of the range contractions occur in the northern United States, where fungicides are used the most. They may be altering the bees’ microbiota, which can influence pathogen dynamics, he said, or the combination of fungicides and insecticides together may increase the toxicity for bees.
Fungicides may also be acting as a barrier to bee population movement, he said, possibly explaining past research that has shown that U.S. bumblebees are not responding to climate change by going north and up in elevation, as some other species are.
“I would say we are not necessarily at the stage yet to say this particular fungicide is something you should stop using,” McArt said. “But that is definitively the goal.”