Scientific Method / Science & Exploration

Pesticides hit non-pests: exposed bees forage poorly, die more often

Study also shows the long-term impact takes weeks for bees to feel.

The recent demise of many commercial bee hives has focused attention on the health of pollinators. Both domesticated and wild insects play key roles in making sure a variety of agricultural plants are pollenated and productive, so the sudden decline in domesticated bees was worrisome. Although initial evidence pointed to parasites, more recent evidence pointed the finger at insecticides.

A new study, released over the weekend by Nature, doesn't do much to clarify the cause of colony collapse. But it does show that insecticides can influence the health of a bee colony in subtle ways that could be hard to detect with standard study approaches. And it indicates that natural pollinators, which generally don't form the massive hives that domestic bees do, may be even more sensitive to these effects.

The work relied on a careful tracking of the bees, using small, indoor colonies of bumble bees, a common natural pollinator. The bees were fitted with small radio tags that could be read as the bees passed through a tube on their way in and out of the colony. On their way out, the bees would go through a chamber where they were exposed to pesticides. Ten colonies were exposed to a topical insecticide that's normally dusted on plants (λ-cyhalothrin, a pyrethroid); another 10 were exposed to a neonicotinoid called imidacloprid, which is ingested. A 10-colony control group went unexposed (except, possibly, in the environment), and another group got both insecticides.

A description of the experiments that shows how the bees were housed and tracked.

Although the number of colonies was a bit low, the ability to track individual bees raised the statistical power of the study considerably. The biggest limitation of the study design, however, is that the authors didn't try to track actual exposure by testing for the levels of these chemicals in the bees.

By just about every measure, the bees exposed to imidacloprid (either alone or in a mixture) were worse off. The number of dead workers found in the hives were up. Deaths in the colony exposed to both insecticides was four times that of the rate in the control colony. In addition, the exposed colonies replaced workers at a slower rate than the controls. Combined, these mean that the insecticide-treated colonies suffered a higher rate of worker loss than the controls.

In the end, however, the colonies treated with imidacloprid ended up with more workers over the course of the study. But those workers took fewer foraging flights and ended up coming back to the hive with less pollen. They also spent more time looking for food outside of the hive.

There were a couple of odd things in these results. The first is that the insecticide that wasn't ingested (λ-cyhalothrin) didn't seem to cause problems in any of the assays the authors used, but they still ended up with a high rate of worker loss. In the same way, the population that received both chemicals didn't appear to be very different from the one that got just imidacloprid but suffered the highest level of worker loss. These differences seem to be explained largely by the death of workers outside the lab, but the authors don't really know what's happening to those bees.

The key thing that the authors note is that these differences weren't obvious until several weeks into the experiment. Thus, shorter term studies, or those that don't use a sufficient number of colonies/bees, could miss this entirely. And the most severe impact would only be seen if the research used more than one pesticide.

The general significance of this work isn't clear at this point—we'll need to look at more species, additional insecticides, and a careful tracking of the actual exposure before we really understand how insecticides are impacting our pollinators. That's not to say this study is flawed, just that it wasn't designed to answer every question imaginable.

But even though it doesn't answer everything, it helps us identify many things we should be asking. For one, the use of bees that form smaller colonies may tell us a lot more about the wider impact of insecticide use. It also tells us we should start thinking in terms of multiple exposures, both to insecticides and parasites, that accurately reflect what the bees see in the real world. Finally, it tells us that some of the effects won't be apparent during short exposures, which means we have to start thinking in terms of longer experiments. Future studies will undoubtedly attempt to track many of these factors.

Listing image by National Park Service

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