Research published in scientific journal PLOS ONE suggests the variety of agricultural chemicals to which honeybees are exposed during their foraging activity is taking a toll on the bees' health.

Across the Northern Hemisphere, bee populations are in decline. Lending support to the dominant hypothesis that the cause of these losses is complex and multifaceted, involving the interplay between human actions and external factors such as disease, researchers from the United States found that commonly used pesticides may play a significant role.

Commercial honey bees used to pollinate crops are exposed to a wide variety of agricultural chemicals during their pollinating forays, including common fungicides which, according to a study backed by the US government, impair the bees' ability to fight off a potentially lethal parasite.

Researchers at the University of Maryland and the U.S. Department of Agriculture said theirs is the first study to analyse "real-world conditions encountered by honey bees" as their hives pollinate a wide range of crops, from apples to watermelons and bees' activity was monitored across large areas. The lack of field data conducted in 'real-world' scenarios was used earlier this year by the UK government as its excuse for inaction in the face of a growing weight of evidence showing certain neonicotinoid chemicals are having an effect on bee health in the EU.   

In the event, the EU placed tight regulations on the use of these chemicals in May, and Defra leaders came under fire for their abandonment of the precautionary principle, which forms the basis of environmental regulation around the world.

In the United States, researchers collected pollen from honey bee hives in fields from Delaware to Maine. They analyzed the samples to find out which flowering plants were the bees' main pollen sources and what agricultural chemicals were commingled with the pollen. The researchers fed the pesticide-laden pollen samples to healthy bees, which were then tested for their ability to resist infection with Nosema ceranae – a fungal parasite of adult honey bees that has been linked to 'colony collapse disorder' wherein worker bees disappear from colonies en masse.

On average, the researchers said, pollen samples contained nine different agricultural chemicals, including fungicides, insecticides, herbicides and miticides. Sublethal levels of multiple agricultural chemicals were present in every sample, with one sample containing 21 different pesticides. Pesticides found most frequently in the bees' pollen were the fungicide chlorothalonil, one of the most widely used fungicides in the United States, and the insecticide fluvalinate, used by beekeepers to control Varroa mites, common honey bee pests.

Chemical affects bees' resistance to parasites

The study yielded some incredibly "surprising" results, according to the USDA researchers; bees that were fed the collected pollen samples containing chlorothonatil were nearly three times more likely to be infected by Nosema than bees that were not exposed to these chemicals. Jeff Pettis of USDA's Bee Research Laboratory and the study's lead author said evidence suggests the miticides used to control Varroa mites also harmed the bees' ability to withstand parasitic infection.

University of Maryland researcher Dennis vanEngelsdorp, himself an author of the study, said beekeepers know they are making a trade-off when they use miticides. The chemicals compromise bees' immune systems, but the damage is less than it would be if mites were left unchecked, he explained. But the team's finding that common fungicides can be harmful at real world dosages is novel and, the scientists believe, a cause for concern. According to Dr vanEngelsdorp this points to a gap in existing regulations.

"We don't think of fungicides as having a negative effect on bees, because they're not designed to kill insects," vanEngelsdorp continued. He added that, although in the US, federal regulations restrict the use of insecticides while pollinating insects are foraging, "There are no such restrictions on fungicides, so you'll often see fungicide applications going on while bees are foraging on the crop. This finding suggests that we have to reconsider that policy."

The researchers also found that most of the crops being pollinated by bees provided bee hives with little nourishment. Honey bees gather pollen to take to their hives and feed their young. But when the researchers collected pollen from bees foraging on native North American crops such as blueberries and watermelon, they found the pollen came from other flowering plants in the area, not from the crops. This is probably because honey bees, which did not evolve in the US, are not efficient at collecting pollen from the continent's native crop species, even though they can pollinate these crops.