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Planting honeybee forage amid crops may not help much, S.D. study says

Drift and persistence of a neonicotinoid product suggest that planting clean forage alongside crops may not help the bees much, and could hurt them further.

A new study of honeybee exposure to pesticides finds that a widely used neonicotinoid drifts so far, and lasts so long, that planting “pollinator strips” of untreated forage may not be of much help to the bees – and might even cause harm.

That is just one of several findings with big policy implications in the research, which was conducted in the region around Brookings, S.D., for the U.S. Department of Agriculture and published earlier this month in the journal Nature.

The analysis focuses on clothianidin, a systemic insecticide it says has become “ubiquitous” in South Dakota corn and soybean production. Like other “neonics,” it is widely touted as being safer for farmers — and better for the environment — than other insecticides because it is typically applied as a seed treatment instead of, say, being sprayed on rows of growing corn.

Although the neonics are widely blamed as a leading (though not only) cause of beekeepers’ surging colony losses over the past decade, manufacturers and their allies have countered that there is very little direct evidence connecting these products with the problem.

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Honeybees don’t forage much on corn, they point out, and even if they did there’s no reason to think they’d consume enough insecticide to cause serious harm. Laboratory studies, they say, test exposure levels that are unrealistically high compared to actual field conditions.

Enter Christina Mogren, now at Louisiana State University, and her examination of honeybees’ exposure to clothianidin in 16 pollinator strips planted alongside cornfields – some conventional, some organic – for the 2014 and 2015 growing seasons, when she was working for USDA’s Agricultural Research Service.

Because “only 2-20% of the active ingredient on treated crop seeds is taken into the developing plant, and recent data suggests that the remaining majority of the compound is not staying within cropland,” she wanted to test the effectiveness of creating islands of clean forage.

Initially, the aim of this study was to determine whether increasing forage by planting pollinator strips in a corn and soy dominated region would serve to buffer against harmful effects of plant-incorporated pesticides, specifically the neonicotinoids, with organic corn fields serving as controls.

However, when it became apparent that an unintended consequence of planting pollinator strips adjacent to treated cropland meant that they became a source for neonicotinoid exposure, the goal shifted to determine whether pollinator strips were themselves harmful to the bees.

Persistent poisons

Like other pesticides, neonics move to unintended places in unpredictable ways and can be quite persistent. They are found in the wetlands and waterways of agricultural regions; they are found in soils, with half-lives ranging from as little as 25 days for some compounds to as long as 1,155 days for clothianidin; they are found in plants that nobody would bother treating, like the mustard, buckwheat and phacelia that became the forage for Mogren’s winged subjects.

It wasn’t a big surprise to see the neonic show up in the forage strips; more disconcerting was the finding that leaf-tissue concentrations in samples from strips planted next to organic fields were about the same as in strips next to fields where the insecticide was heavily used.

The proportion of forage plants contaminated by drift was about the same, too. And time of year for sampling didn’t matter, indicating the drift problem wasn’t a short-lived feature of spring planting periods).

When the bees’ honey was tested for clothianidin, levels were similar regardless of pesticide practices near their forage strip; they were about seven times higher than the contamination levels in the plant nectars themselves. However,

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In contrast, bee bread (pollen returned to the hive and stored for food) recovered from the hives located on organic farms had significantly lower levels of clothianidin than bee bread from hives near seed-treated fields, with bee bread from seed-treated farms containing nearly twice as much clothianidin. Pesticide residues in bee bread were also unaffected by sampling date and there was no significant interaction between sampling date and treatment [statistics omitted here and below].

Although both studies deal with honeybees’ exposure to toxins from foraging on non-crop plants, Mogren’s differs in some key ways from the June paper out of Purdue University, also published in Nature, which looked at a wide range of bee-burdening poisons adrift in the environment – not all of them significant in agriculture, and many not even intended as insect-killers.

Health impacts assessed

One big distinction is that Mogren considered the here-and-now health effects of what she was seeing. Even though the exposures were sublethal, they were hardly innocuous in the way neonics’ promoters would have us believe.

Increasing concentrations of clothianidin in the bee bread collected by honey bees were associated with significant declines in glycogen and lipids in the worker bees at those respective sites, with a negative association in total proteins.

Bees from the hives at organic and treated farms had a similar nutritional status. Clothianidin concentrations of in-hive honey had no effect on honey bee nutritional status, although a negative trend was observed between honey contamination levels and bee protein quantities.

Acute mortality at the levels observed in our study is unlikely. Nevertheless, clothianidin concentrations in bee bread were associated with declines in nutritional status of the bees under field conditions.

Among possible impacts of sublethal exposure, in Mogren’s view: stress-induced vulnerability to other pesticides or disease; diminished reproductive capacity; a variety of behavioral disruptions; reduced ability to survive through the winter, which depends heavily on stored lipids.

Excerpts from the paper’s overall conclusions:

The paradox of the present study is that pollinator strips intended to enhance honey bee health in a heavily developed agricultural landscape resulted in declining bee health due to unintended accumulation of clothianidin from adjacent treated corn fields at both organic and conventional farms.

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Although pollinator strips on organic farms were generally more than 140 meters from the nearest treated crop, this was not far enough to fully isolate these strips from negative seed treatment effects.

It has been suggested that any harmful exposures resulting from non-target plant uptake of neonicotinoids would be diluted by the fact that honey bees visit numerous flowers on a single foraging expedition. Dilution would be difficult in highly developed agricultural areas like eastern South Dakota where neonicotinoid seed treatments are ubiquitous.

While pollinator strips and uncropped areas have the potential to serve as buffers to pesticide exposures for bees, our results indicate that their placement within the landscape needs to be carefully considered.

In all likelihood, reducing bee exposures to these pesticides will require reductions in their use across the landscape and a movement away from prophylactic applications towards more integrated pest management strategies, as has been suggested elsewhere.

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The full Mogren paper, “Neonicotinoid-contaminated pollinator strips adjacent to cropland reduce honey bee nutritional status,” can be read here without charge.