Sometimes dozens of pesticides turn up in a single sample of wax or pollen

By Janet Raloff

Web edition: March 21, 2010

SAN FRANCISCO — For years the news has been the same: Honey bees are being hammered by some mysterious environmental plague that has a name -- colony collapse disorder — but no established cause. A two-year study now provides evidence indicting one likely group of suspects: pesticides. It found “unprecedented levels” of mite-killing chemicals and crop pesticides in hives across the United States and parts of Canada.

Scientists here at the American Chemical Society spring annual meeting, which kicked off today, will report on the findings of this study later in the week. But if you want an early peak at their results, or can’t make it to the meeting, check out a 19-page synopsis of the data that has just been published online in the March PLoS ONE.

In it, Christopher Mullin of Pennsylvania State University in University Park and his colleagues describe widespread pesticide tainting in 749 samples of bee-dom, some of those chemicals at levels that would be toxic if they occurred alone. Except that most bees aren’t exposed to just a single pesticide.

In beeswax, they report, “87 pesticides and metabolites were found with up to 39 different detections in a single sample.” The average number of pesticides identified per wax sample (and they analyzed 259 samples): eight. Among 350 pollen samples retrieved from hives, each harbored an average of seven such chemicals – but at times up to 31 pesticide contaminants (or their breakdown products, some of which are far more toxic to bees than the parent chemical would have been).

Overall, the 140 bees they analyzed tended to be less contaminated. Their bodies contained, on average, a little over two pesticides. At least one poor bug hosted 25.

The researchers have several suspicions why the bees looked cleaner than their dwellings. In some cases, detoxifying systems within the bees might have broken down the chemicals, fostering their excretion. But an even likelier explanation: The sampling focused primarily on live bees extracted from the hives. These tended to be the queens, brood nurses and adolescents – hive residents that aren’t on the chemical frontlines, foraging in pesticide treated fields. Indeed, the fact that researchers found so few healthy worker bees in many of the hives from which they received samples suggests that sickened foragers probably die before they get home.

In fact, some of the pesticides that were detected in hive materials can disorient bees. Which suggests many foragers that had been unwittingly carrying home such contaminants at last become too confused to find their front door.

Mullin’s team didn’t just sample hives clobbered by colony collapse disorder. They also analyzed all aspects of hives and their inhabitants from ostensibly healthy communities of bees as well. And that’s what makes the next stat so troubling. Out of all of those many hundreds of samples analyzed, “Only one of the wax, three pollen and 12 bee samples had no detectable pesticides.”

In general, the most commonly occurring pesticides were those that may have been intentionally applied to hives in hopes of killing mites, a bee parasite. However, some of these miticides may, when paired up with other classes of pesticides, act synergistically to poison insects. The top 10 most frequently found pesticidal chemicals: fluvalinate and coumaphos – both mite killers -- followed by chlorpyrifos, chlorothalonil, amitraz (another miticide), pendamethalin, endosulfan, fenpropathrin, esfenvalerate and atrazine – the last an herbicide.

Overall, mite-killing chemicals accounted for most pesticide residues in comb wax and bees. Fungicides dominated the pollen contamination

In some instances, the researchers note, “unprecedented” parts-per-million levels of individual chemicals were found. And the real problem, Mullins’ group argues, is that “The biological impacts of these materials at their dietary levels on other honey bee larvae or adults remains to be determined.”

It may even be that direct mortality is not the primary risk, they note. “Laboratory studies have clearly indicated sublethal impacts [of some pesticides] on honey bee learning, immune system functioning, and synergism of insecticide toxicity by fungicides.” To date, they note, no data on bee-toxicity risks exist for interactions by three or more pesticides.

Currently, $14 billion worth of U.S. crops depend on pollinators. Mullin and his colleagues call “for emergency funding to address the myriad holes in our scientific understanding of pesticide consequences for pollinators.” In their PLoS ONE paper, they argue that relying on agricultural workers to protect those pollinators on the basis of warning labels on chemical packaging – warnings developed on the basis of studying exposures to bees by a single chemical in isolation – together with “the underestimation of systemic pesticide hazards to bees in the [pesticide] registration process may well have contributed to widespread pesticide contamination of pollen, the primary food sources of our major pollinator.”