Pesticides sprayed on crops could be making honey bees susceptible to a fatal parasite and contributing to recent declines in bee populations, according to a study.
Researchers found 35 pesticides, some at lethal levels, in the pollen collected from bees servicing major food crops in five states, including California, according to the study published online Wednesday in the journal PLOS One.
Levels for two chemicals were above the dose that would kill half a population within two days, according to the report. Pesticide residue was found on all the pollen samples, including those that the bees apparently collected from nearby wildflowers, according to the report.
The report highlights the diverse cocktail of agricultural chemicals to which domesticated bees are regularly exposed, some of which have been linked to weakened immune system responses in the insects crucial to the world's food supply. Most studies of domestic honey bees have examined exposure to a single chemical at a time.
"Bees are getting exposed to a lot of different products, including fungicides," said Dennis vanEngelsdorp, an entomologist at the University of Maryland and lead author of the report. "What's surprising is that it seems to weaken the bee's ability to fight off infection."
Pesticides, along with climate change, habitat destruction and handling practices that expose bees to exotic pathogens, are among the factors blamed for the catastrophic collapses of colonies of domesticated honey bees worldwide.
Of particular concern to the researchers was the presence of fungicides, two of which (chlorothalonil and pyraclostrobin) were associated with increased risk of infestation with fatal Nosema gut parasites. Two other chemicals commonly used by beekeepers to control mites (2,4 dimethylphenyl formamide and fluvalinate) also were associated with significantly greater risk of infection with Nosema spores, according to the report.
Fungicides generally do not carry the warnings found on packaging of other agricultural chemicals that suggest farmers not apply them while blossoms are present and bees are foraging, vanEngelsdorp noted.
The report linked eight pesticides with increased the risk of Nosema infection, and another 14 with decreased risk. Neonicotinoids, a class of pesticides that has been banned in Europe as a potential suspect in colony collapse, were detected only on pollen from a single apple orchard, vanEngelsdorp said.
"It certainly wasn't very common," vanEngelsdorp said. "It suggests there are other products that should be getting attention from the regulatory agencies."
How pesticide residues found their way to non-crop pollen also remains a mystery.
"It could be drift from when they sprayed their crop, but it also could be that the bees are picking it up and contaminating the pollen on the forage trip," vanEngelsdorp said.