The research, conducted by U.S. Department of Agriculture and University of Maryland scientists, was published in the journal PLOS ONE, according to a news release.
Researchers collected pollen samples from honeybees pollinating apples, watermelons, pumpkins, cucumbers, blueberries and cranberries.
They then analyzed the pollen to determine how much fungicide, insecticide, miticide and/or herbicide the bees were exposed to during pollinating each of the six crops.
Although some samples contained very few pesticides, the average number seen in a pollen sample was nine different chemicals.
Fungicides were the most frequently found substance, which chlorothalonil being the most common fungicide.
Fluvalinate, which is used to control varroa mites in bees, was the most common miticide, and neonicotinoid insecticides were only found in pollen from bees foraging on apples.
Honeybees that fed on pollen that contained chlorothalonil were three times more likely to become infected when exposed to the parasite Nosema, compared to untreated control bees.
The fungicide pyraclostrobin, though found less frequently in pollen samples, also increased bees' susceptibility to Nosema.
"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 in the release. "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."