Scientists at Purdue University said analyses of bees found dead in and around hives in Indiana showed the presence of neonicotinoid insecticides, commonly used to coat corn and soybean seeds before planting.
Researchers said those insecticides were present at high concentrations in waste talc exhausted from farm machinery during planting, a Purdue release said Wednesday.
The insecticides -- clothianidin and thiamethoxam -- were also consistently found at low levels in soil up to two years after treated seed was planted, on dandelion flowers and in corn pollen gathered by the bees, scientists said.
"We know that these insecticides are highly toxic to bees; we found them in each sample of dead and dying bees," Purdue entomologist Christian Krupke said.
"Given the rates of corn planting and talc usage, we are blowing large amounts of contaminated talc into the environment. The dust is quite light and appears to be quite mobile," Krupke said.
The United States is losing about one-third of its honeybee hives each year, Purdue behavioral genetics Professor Greg Hunt said.
No one factor is solely to blame, Hunt said, noting scientists believe multiple threats such as mites and insecticides are all working against the bees, important pollinators of food crops and wild plants.
"It's like death by a thousand cuts for these bees," Hunt said.