Honeybees that consume pollen that contains amounts of commonly used fungicides at levels too low to kill the bees still may leave them more susceptible to infection by a gut parasite, according to U.S. Department of Agriculture and University of Maryland research published July 24 in PLOS ONE.
This research complements other recent USDA research on bees, including a comprehensive scientific report on honeybee health issued in May that found that multiple factors play a role in honeybee colony declines. Those factors include parasites and disease, genetics, poor nutrition and pesticide exposure.
Researchers from the University of Maryland and USDA's Agricultural Research Service (ARS) collected pollen samples from honeybees pollinating apples, watermelons, pumpkins, cucumbers, blueberries or cranberries. The scientists then analyzed the pollen to determine how much fungicide, insecticide, miticide and/or herbicide the bees were exposed to while pollinating each of the six crops.
In many cases, the pollen the bees brought back came primarily from plants other than the targeted crop. Some pollen samples contained very few pesticides, but the average for a pollen sample was nine different pesticides, which could include insecticides, herbicides, miticides and fungicides, ARS reported.
Fungicides were the most frequently found chemical substances in the pollen samples. The most common was the fungicide chlorothalonil, which is widely used on apples and other crops.
The most common miticide was fluvalinate, which beekeepers use to control varroa mites. Neonicotinoid insecticides were found only in pollen from bees foraging on apples.
"Honeybees that were fed pollen that contained the fungicide chlorothalonil and was collected at the hive entrance were almost three times more likely to become infected when exposed to the parasite Nosema compared with control bees," explained study author Jeff Pettis, research leader of the ARS Bee Research Laboratory in Beltsville, Md.
The fungicide pyraclostrobin, which was found less frequently in the pollen samples, also increased bees' susceptibility to Nosema infection.
"Our study highlights the need to closely look at fungicides and bee safety as fungicides currently are considered safe and can be sprayed during the bloom on many crops," co-author Dennis vanEngelsdorp with the University of Maryland said. "We also need to better understand how pesticides are getting into the hive. Clearly, it is not just from collecting pollen from the crops that bees are being used to pollinate."
These findings provide new information that will be useful in understanding the myriad problems affecting honeybees in the U.S., including colony collapse disorder, dwindling honeybee colonies and other health problems in managed bee colonies, Pettis added.
One unexpected finding was that honeybees collected relatively little pollen from blueberry and cranberry plants, which are both crops that originated in North America. Despite this low pollen collection, researchers know that bees do pollinate these plants.
Honeybees originated in Europe, along with crops such as almonds and apples.