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It’s the diversity of pesticides, not the types or doses, that may be killing bees

New study complicates the policy landscape for limiting pollinators’ exposure to selected products.

The policy landscape for protecting honeybees from pesticides has just become a little more complicated, thanks to a new study suggesting that the sheer diversity of pesticides may be more of a problem than particular products.

Another key finding: Certain fungicides long thought to be harmless to the bees are in fact fairly toxic to them.

Published in Nature Scientific Reports, the paper by Kirsten Traynor of the University of Maryland is claimed to be “the first to systematically assess multiple pesticides that accumulate within bee colonies.”

Now, it is hardly the first to find that the bees are burdened with lots of insecticides, herbicides, fungicides, insect repellents and other potentially harmful compounds, often from off-farm sources. And discussion of how these chemicals might combine or even multiply their threats has been going on since colony collapse disorder first emerged as a national concern in 2006.

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But this work is the first to attempt to compare — and under field conditions, to boot — the lethal impacts at colony scale of three separate factors: the bees’ total exposure to pesticides, their exposure to pesticides beyond a “relevant” minimum dose, and their exposure to pesticides above a “hazard quotient” (HQ) reflecting a 50 percent risk of mortality.

All three showed high correlations with colony die-offs and also “queen failure” — the sickening of the colony’s most essential individual to the point where the other members must remove and replace her. If the replacement is unsuccessful, the colony collapses.

But it was the sheer number of different pesticide products that proved the most potent apparent driver of disaster. And they found, count ’em, 93 in the colonies they examined.

The university’s announcement of the study quotes Dennis vanEngelsdorp, the pioneering investigator of colony collapse and senior co-author of the Traynor paper, as saying:

Our results fly in the face of one of the basic tenets of toxicology: that the dose makes the poison. 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.

Examining an ‘exposome’

For this study, Traynor and colleagues borrowed from the field of cancer research the notion of an “exposome,” defined as an organism’s lifetime exposure to a wide array of possible carcinogens.

In contrast to a “bottoms up” approach that attempts to trace the effects of a single agent throughout a population, exposome research works backwards to associate disease incidence across a range of possible agents.

And in this case, the honeybee colony became a “superorganism” at the center of inquiry.

Data were drawn from 91 colonies kept by three beekeepers who transported them up and down the East Coast delivering pollination services. The study periods averaged about 300 days, essentially the full season, and included periods when the bees were off the job of pollinating in order to let them rest up and/or make honey.

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Pesticide residues were sought, and detected, in three places: the wax the bees produced, the “bee bread” of processed pollen they stored in the hives, and the bodies of “nurse bees” whose function is to turn bee bread into food for new larvae.

One example of exposure the Traynor team found: Every sample of bee bread the researchers examined exceeded the HQ for at least five pesticides, and some for as many as 20.

Somewhat surprisingly, perhaps, the team found very little exposure to the neonicotinoid insecticides that have been under intense scrutiny for their role in the health problems of bees and other pollinators.

“There were some trace residues of neonicotinoids in a few samples,” vanEngelsdorp explained, “but not nearly on par with other compounds. However, it’s possible we did not test the right matrix — we did not test nectar, for example — or that the product breaks down faster than others in the collection process, or that neonicotinoids are simply not very prevalent when crops are flowering.”

The other unexpected finding had to do with fungicides, which were also prevalent in the samples and showed a significant correlation with colony mortality, according to Traynor.

We were surprised to find such an abundance of fungicides inside the hives, but it was even more surprising to find that fungicides are linked to imminent colony mortality. These compounds have long been thought to be safe for bees.

We’re  seeing them at higher doses than the chemicals beekeepers apply directly to the colonies to control varroa mites. So that is particularly concerning.

Limitations of the study

There are some acknowledged limitations to this study. One is the age of the samples, which were collected in 2007; the paper notes that “crop-protection products used on different crops has almost certainly evolved in the interim.”

However, there is good reason to expect neonicontinoid exposure to have climbed since then, and little reason to imagine that overall pesticide use has declined. The mix may have shifted somewhat, but the key finding here is that the breadth of the mix may matter more than its specific membership.

[T]e high diversity of products found in different pollination environments suggests that more research is needed to understand how these products, many of which would not have been sprayed while bees were foraging, persist in the environment and become sequestered in honeybee colonies, with recent research suggesting the residues persist via uncultivated crops potentially exposed to pesticide drift.

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And of course much work remains on sorting out the interactions of those products, which lay beyond the scope of this study:

The three models implemented are imperfect estimates of pesticide burden, as they neither considers synergistic or antagonistic interactions among products nor the ability of bees to detoxify products.

The calculations also included the 171 products we tested and that occurred at levels above the limit of detection, and does not account for adjuvants and other products not included in the screening (such as glyphosate, which requires a separate, costly analysis).

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The full paper, “In-hive Pesticide Exposome: Assessing risks to migratory honeybees from in-hive pesticide contamination in the Eastern United States,” can be read here without charge.