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Studies show neonicotinoid levels drop as crop season progresses

The Mid-South Extension entomologists who work in cotton, corn and soybeans have been conducting extensive tests to determine how long neonicotinoid seed treatments persist in those crops.

The persistence of those insecticides – thiamethoxam, clothianidin and imidacloprid – has become an important issue in the debate over the declines in honey bee populations. Environmental activists have asked EPA to cancel the registrations of those products in those crops.

But the entomologists’ studies show the insecticides in the neonicotinoid seed treatments generally drop to extremely low levels by the time honey bees would normally be foraging in those plants.

Scott Stewart, Extension entomologist with the University of Tennessee, talked about those studies during a presentation at the Consultants Conference at the Beltwide Cotton Conferences in San Antonio. Stewart’s work involved the application of Poncho (clothianidin) 250, Poncho 500 and Poncho 1250 as seed treatments on corn.

“The point I want to make is that during the seedling stage we got very high concentrations – hundreds or thousands of parts per million,” he noted. “By the time we get to the flowering stage when bees might be foraging on pollen, in this case, we had over a 97 percent reduction in the concentrations of clothianidin.”

“The reduction is even more dramatic when you get into other crops. Some of Jeff Gore’s work with an Aeris seed treatment in Mississippi shows that in that early seedling stage we’re talking about hundreds of parts per million of imidacloprid in cotton. By the time, we get to first flower it’s 2.5 parts per million. That’s over a 99 percent reduction.”

Other tests in Mississippi and in Arkansas have shown similar reductions between the seedling stage and the flowering stage of the different crops.

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“I think this provides a lot of context,” said Stewart. “You know we’re losing efficacy in cotton by the third-leaf stage, and we’re at hundreds of thousands of parts per billion. It doesn’t seem very likely to me that one part per billion a month down the road is having any significant measurable effect on bees.”

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