Four common pesticides used on crops and livestock also kill honeybee larvae within the hive, according to recent research by Penn State University and the University of Florida.
In addition, the scientists found a common inert ingredient in pesticides—N-methyl-2ppyrrolidone—also is toxic to honeybee larvae, according to a news release.
The pesticide active ingredients examined were fluvalinate, an insecticide/miticide marketed under several names including Apistan; coumaphis, a miticide/insecticide used as a livestock dip, dust or spray; chlorothalonil, a fungicide marketed under several names including Bravo; and chlorpyrifos, an insecticide marketed under several brands including Lorsban.
Fluvalinate and coumaphis also have been used by beekeepers to treat varroa mite and can persist in a beehive for up to five years, according to the release.
The Environmental Protection Agency currently looks at pesticide sensitivity on adult honeybees, and the agency does not look at pesticide combinations, either.
Based on the results, Jim Frazier, a Penn State entomology professor, said the EPA should change its risk assessment process to also include immature, or larval, bees and chemical mixtures.
Earlier work by the research group found that forager bees bring back an average of six different pesticides with the pollen they collect.
Nurse bees within the hive use this pollen to make beebread, which is then fed to honeybee larvae.
As part of their study, the researchers reared the bee larvae in their laboratory. They then applied the pestcides alone or in combinations to the beebread.
What they found were the combinations were much more toxic than individual chemicals.
"We found that mixtures of pesticides can have greater consequences for larval toxicity than one would expect from individual pesticides," Frazier said in the release.
Among the four pesticides, the larvae were most sensitive to chlorothalonil.
They also were negatively affected by a mixture of chlorothalonil and fluvalinate.
The larvae also were sensitive to a combination of chlorothalonil with coumaphos.
By the addition of coumaphos significantly reduced the toxicity of the fluvalinate-chlorothalonil mixture.
The pesticides may affect honeybee larvae two ways. They may directly poison the immatures or they may indirectly kill them by disrupting the beneficial fungi essential for nurse bees to process pollen into beebread.
"Chronic exposure to pesticides during the early life stage of honeybees may contribute to their inadequate nutrition or direct poisoning with a resulting impact on the survival and development of the entire bee brood," Penn State entomology professor Chris Mullin said in the release.
The researchers also looked at the effects of NMP on larvae by adding seven different concentrations of the chemical to a royal jelly diet.
NMP, an inert ingredient, enhances the spread and penetration of the pesticide active ingredient.
As the concentration of NMP increased, so did larvae mortality.