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Honey bees rented to out pollinate crops from apples to watermelons return to their hives with pollen containing an array of agricultural chemicals that make the insects more vulnerable to infection by a lethal parasite, according to a new study.

While other research has shown certain pesticides, including insecticides known as neonicotinoids and others used to fight parasitic mites, can compromise bee health, the new study shines a light on the impact of sprays used to kill fungi and molds.

"Fungicides, which we didn't expect to harm insects, seem to have a sub-lethal effect on bee health," Dennis vanEngelsdorp, an entomologist at the University of Maryland and senior author of the new study, told NBC News.

"And that is important, of course, because there is not a lot of regulation for fungicides when they are being applied to flowering plants … so this suggests that we need to rethink and reevaluate how we write label laws for some fungicides," he added.

The study, published online Wednesday in the journal PLoS One, is the first to analyze the real-world conditions honey bees encounter as they pollinate crops. The analysis revealed which flowering plants were the bees' main pollen sources and what agricultural chemicals were commingled with the pollen.

On average, the pollen samples contained nine different agricultural chemicals, including fungicides, insecticides, herbicides and miticides. Sub-lethal levels of multiple agricultural chemicals were present in every sample. One contained 21 different pesticides.

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The most common pesticides were the fungicide chlorothalonil, which is used on apple and other crops, and the insecticide fluvalinate, which is used by beekeepers to control Varroa mites, a honey bee pest that a government backed study released earlier this year fingered as the leading cause of honey bee decline.

"Varroa mites are beekeepers number one problem and so the treating with miticides … is a little bit like chemotherapy; it is bad for the bees, we know it is bad for the bees, but we know that not treating is much worse for the bees," vanEngelsdorp said.

The researchers only found neonicotinoids, which other studies have shown to be toxic to pollinators, in honey bees that were pollinating apples, suggesting the insecticide is just one part of a complex problem.

This chemical-laden pollen was fed to healthy bees, which were then tested for their ability to resist infection with Nosema ceranae, a parasite of adult honey bees that has been linked to the overall problem of honey bee decline.

The biggest surprise, according to the researchers, is that bees fed pollen containing the fungicide chlorothonatil were nearly three times more likely to be infected by Nosema than bees that were not exposed to these chemicals.

"It suggests that we really have to look broadly at pesticides and look at ways of reducing pesticide exposure, including fungicides," vanEngelsdorp said.

The finding jives with earlier studies pointing to interactions between various factors as a cause for honey bee decline, noted Olav Rueppell, a biologist and bee expert at the University of North Carolina, Greensboro, who was not involved in the new research.

"But this study is particularly nice because it takes actually the pollen that bees bring into the hive … to compromise honey bee immunity," he told NBC News in an email. "Therefore, many substances are involved and seem to have an effect."



And since the problem of honey bee decline appears multifaceted, added vanEngelsdorp, "the solutions are probably multifaceted."

John Roach is a contributing writer for NBC News. To learn more about him, visit his website.