Honey bee colonies in the United States have been dying at high rates for over a decade, and agricultural pesticides are often implicated as major culprits.
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Until now, most scientific studies have looked at pesticides one at a time, rather than investigating the effects of multiple real-world pesticide exposures within a colony.
A new study is the first to systematically assess multiple pesticides that accumulate within bee colonies. The researchers found that the number of different pesticides within a colony -regardless of dose -closely correlates with colony death.
The results also suggest that some fungicides, often regarded as safe for bees, correlate with high rates of colony deaths. The study appeared online September 15, 2016, in the journal Nature Scientific Reports.
"Our results fly in the face of one of the basic tenets of toxicology: that the dose makes the poison," said Dennis vanEngelsdorp, an assistant professor of entomology at UMD and senior author on the study.
"We found that the number of different compounds was highly predictive of colony death, which suggests that the addition of more compounds somehow overwhelms the bees' ability to detoxify themselves."
The researchers followed 91 honey bee colonies, owned by three different migratory commercial beekeepers, for an entire agricultural season.
The colonies began their journey in Florida and moved up the East Coast, providing pollination services for different crops along the way. They also spent time in locations meant for honey production, as well as "holding areas" where beekeepers prepare large numbers of colonies for upcoming pollination contracts.
A total of 93 different pesticide compounds found their way into the colonies over the course of the season, accumulating in the wax, in processed pollen known as bee bread and in the bodies of nurse bees.
At every stop along the beekeepers' itinerary, the researchers assessed three different parameters within each colony: the total number of pesticides; the total number of "relevant" pesticides (defined as those above a minimum threshold of toxicity); and each colony's "hazard quotient," a measure devised by other researchers to integrate the total hazard posed to each colony by the cumulative toxicity of all pesticides present.
All three measures correlated with a higher probability of colony death or queen failure. In addition, the researchers found between five and 20 different pesticide residues in every sample of bee bread that exceeded a hazard quotient's safety threshold.
The highest number of pesticides accumulated in the colonies early on, shortly after beekeepers placed colonies into early season flowering crops like apples and blueberries. Honey production stopovers and holding areas offered the bees some respite from further contamination.
The study results also suggest that some fungicides, which have led to the mortality of honey bee larvae in lab studies, could have toxic effects on colony survival in the field. In the current study, pesticides with a particular mode of action also corresponded to higher colony mortality.
For example, the fungicides most closely linked to queen deaths and colony mortality disrupted sterols -compounds that are essential for fungal development and survival. ■