Pesticides poison honey bee larvae in the hive

Pesticides may directly poison honeybee larvae or they may indirectly kill them by disrupting the beneficial fungi that are essential for nurse bees to process pollen into beebread, says Chris Mullin, professor of entomology. (Credit: Tamaki Sono/Flickr)

Four pesticides commonly used on crops to kill insects and fungi also kill honey bee larvae in the hive, a new study reports.

The findings also show that N-methyl-2-pyrrolidone (NMP)—an inert, or inactive, chemical commonly used as a pesticide additive—is highly toxic to honey bee larvae.


“We found that four of the pesticides most commonly found in beehives kill bee larvae,” says Jim Frazier, professor of entomology at Penn State. “We also found that the negative effects of these pesticides are sometimes greater when the pesticides occur in combinations within the hive.

“Since pesticide safety is judged almost entirely on adult honey bee sensitivity to individual pesticides and also does not consider mixtures of pesticides, the risk assessment process that the Environmental Protection Agency uses should be changed.”

Previous research has demonstrated that forager bees bring back to the hive an average of six different pesticides on the pollen they collect. Nurse bees use this pollen to make beebread, which they then feed to honey bee larvae.

To examine the effects of four common pesticides—fluvalinate, coumaphos, chlorothalonil, and chlorpyrifos—on bee larvae, researchers reared honey bee larvae in the lab. They then applied the pesticides alone and in all combinations to the beebread to determine whether these insecticides and fungicides act alone or in concert to create a toxic environment for honey bee growth and development.

The researchers also investigated the effects of NMP on honey bee larvae by adding seven concentrations of the chemical to a pollen-derived, royal jelly diet. NMP is used to dissolve pesticides into formulations that then allow the active ingredients to spread and penetrate the plant or animal surfaces onto which they are applied. The team fed their treated diet, containing various types and concentrations of chemicals, to the laboratory-raised bee larvae.

Toxic beebread

“We found that mixtures of pesticides can have greater consequences for larval toxicity than one would expect from individual pesticides,” Frazier says.

Published in PLOS ONE, the research shows that among the four pesticides, honey bee larvae were most sensitive to chlorothalonil. They also were negatively affected by a mixture of chlorothalonil with fluvalinate. In addition, the larvae were sensitive to the combination of chlorothalonil with the miticide coumaphos. In contrast, the addition of coumaphos significantly reduced the toxicity of the fluvalinate and chlorothalonil mixture.

These pesticides may directly poison honey bee larvae or they may indirectly kill them by disrupting the beneficial fungi that are essential for nurse bees to process pollen into beebread, says Chris Mullin, professor of entomology.

“Chronic exposure to pesticides during the early life stage of honey bees may contribute to their inadequate nutrition or direct poisoning with a resulting impact on the survival and development of the entire bee brood.”

Honeybee mortality

Fluvalinate and coumaphos are commonly used by beekeepers on crops to control Varroa mites, and are found to persist within beehives for about five years. Chlorothalonil is a broad-spectrum agricultural fungicide that is often applied to crops in bloom when honey bees are present for pollination because it is currently deemed safe to bees. Chlorpyrifos is a widely used organophosphate in crop management.

“Our findings suggest that the common pesticides chlorothalonil, fluvalinate, coumaphos, and chloropyrifos, individually or in mixtures, have statistically significant impacts on honey bee larval survivorship,” Mullin says. “This is the first study to report serious toxic effects on developing honey bee larvae of dietary pesticides at concentrations that currently occur in hives.”

The team also found that increasing amounts of NMP corresponded to increased larval mortality, even at the lowest concentration tested.

“There is a growing body of research that has reported a wide range of adverse effects of inactive ingredients to human health, including enhancing pesticide toxicities across the nervous, cardiovascular, respiratory, and hormone systems,” Mullin says.

“The bulk of synthetic organic chemicals used and released into US environments are formulation ingredients like NMP, which are generally recognized as safe. They have no mandated limits on their use and their residues remain unmonitored.

“Multi-billion pounds of these inactive ingredients overwhelm the total chemical burden from the active pesticide, drug, and personal-care ingredients with which they are formulated. Among these co-formulants are surfactants and solvents of known high toxicity to fish, amphibians, honey bees and other non-target organisms. While we have found that NMP contributes to honey bee larvae mortality, the overall role of these inactive ingredients in pollinator decline remains to be determined.”

Other authors on the paper include graduate research assistant in entomology Wanyi Zhu and Daniel Schmehl, postdoctoral associate in entomology and nematology at the University of Florida.

The National Honey Board and the US Department of Agriculture supported the research.

Source: Penn State