Sowing the seeds of death for bees

honey bees (photo credit: Mike Sheridan)

Neonicotinoid insecticides are known to have a devastating impact on bees and a new study just published has shown another way in which these chemicals are damaging the worlds pollinators. Neonicotinoid coatings around the seeds are released into the environment at planting time and find their way to the honeybee hives.

A study by Purdue University scientists have shown that when treated corn or soy seeds are planted there is a high level of neonicotinoids in the dust – or talc – emitted from the farm machinery.

Seed coating insecticide lasts in soil for up to 2 years.

Two of the neonicotinoid insecticides - clothianidin and thiamethoxam – were also found to be present in the soil up to 2 years after the seeds had been planted. The chemicals were also found on dandelion leaves at the field margins and were found in corn pollen that was collected by honeybees.

It stands out as being an enormous source of potential environmental contamination, not just for honeybees, but for any insects living in or near these fields.

“We know that these insecticides are highly toxic to bees; we found them in each sample of dead and dying bees,” said Christian Krupke, associate professor of entomology and a co-author of the findings that has been published in PlosOne.

A third of honeybee colonies lost each year in the US.

The United States is losing about one-third of its honeybee hives each year, according to Greg Hunt, a Purdue professor of behavioral genetics, honeybee specialist and co-author of the findings.

“It’s like death by a thousand cuts for these bees,” Hunt said as he explained that no one thing is clearly standing out as the cause of the mass bee deaths.

The study team, which included Brian Eitzer from the Connecticut Agricultural Experiment Station who undertook toxicological assays, looked into the reported deaths of honeybees at planting time. One common chemical group present at all the sites with bee deaths was the neonicotinoids.

Surviving bees at the affected hives exhibited behaviour well established with insecticide poisoning such as tremors, un-cordinated movement and convulsions.

Protective coating of seeds a risk to honeybees.

The team explained that all corn seeds and almost half of all soy seeds are coated in neonicotinoids insecticides to protect them. The coating though makes the seeds sticky and in order to aid planting the seeds are mixed with talc. Excess talc is ejected from the farm machinery during planting and during equipment cleaning.

“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.

Concentrations in seed dust 700,000 times greater than needed to kill on contact.

This dust can have serious consequences for bees if they become contaminated or take it back to the hives. Studies have shown that pollen taken back to hives by the bees can have quite low levels of neonicotinoids – about 100 parts per billion - and well below a toxic dose for bees. The emitted talc however has much higher levels of insecticide concentrations - about 700,000 times the lethal contact dose for a honeybee.

“Whatever was on the seed was being exhausted into the environment,” Krupke said. “This material is so concentrated that even small amounts landing on flowering plants around a field can kill foragers or be transported to the hive in contaminated pollen. This might be why we found these insecticides in pollen that the bees had collected and brought back to their hives.”

Call to end seed talc exhaust emissions.

The researchers are now calling on action to be taken to reduce or even eliminate the talc emissions during planting. Krupke stated that, ”It stands out as being an enormous source of potential environmental contamination, not just for honeybees, but for any insects living in or near these fields. The fact that these compounds can persist for months or years means that plants growing in these soils can take up these compounds in leaf tissue or pollen.”