Researchers from Purdue University, Indiana, have added to the weight of evidence linking neonicotinoid insecticides with bee deaths. Neonicotinoids are among the most widely used insecticides in the world though their use is becoming increasingly controversial.

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The students analysed remains of bees found dead in and around hives from several apiaries over two years and found the presence of neonicotinoid insecticides, commonly used for coating corn and soybean seeds before planting. The researchers also found the insecticides were present at high concentrations in waste talc exhausted from farm machinery during planting.

The Purdue scientists, whose study has been published in scientific journal PLoS One, found insecticides clothianidin and thiamethoxam were also consistently found at low levels in soil up to two years after treated seed was planted. The chemicals were also found on nearby dandelion flowers and in corn pollen gathered by the bees.

Neonicotinoids have gained increasing notoriety as their effect on pollinators and the environment and their use has been subject to increasing scrutiny. Studies conducted in Europe in the 1990s showed that the pesticide’s residues can remain in plats’ pollen and nectar for years after treatment. These studies led to strict controls and suspension on the use of neonicotinoid insecticides in France, Italy and Germany.  

Bees face “death by a thousand cuts”

Although the researchers said the insecticides were not the only factor responsible for the bee deaths, Christian Krupke, associate professor of entomology at Purdue, said, "We know that these insecticides are highly toxic to bees; we found them in each sample of dead and dying bees." The research team said bees were effectively facing “Death by a thousand cuts” as pesticides, mites and other factors result in hive collapses.

The researchers began their investigation after receiving reports that bee deaths were occurring at planting time in hives near agricultural fields. Toxicological screenings performed by Brian Eitzer, a co-author of the study from the Connecticut Agricultural Experiment Station, for an array of pesticides showed that the neonicotinoids used to treat corn and soybean seed were present in each sample of affected bees. Krupke said other bees at those hives exhibited tremors, uncoordinated movement and convulsions, all signs of insecticide poisoning.

In the USA, all corn seed and about half of all soybean seed is treated with neonicotinoid insecticide at planting. The scientists believe excess talc, mixed with the sticky insecticide coating in order to keep seeds flowing freely in the vacuum systems used in planters, and released during planting and routine planter cleaning may be affecting nearby hives.

Professor Krupke commented, "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."

The researchers found that, while corn pollen brought back to the bees’ hives tested positive for neonicotinoids at levels around 100 parts per billion, not acutely toxic but enough to kill bees if sufficient amounts are consumed, the talc released at planting showed levels of the insecticides up to 700,000 times the lethal contact dose for a bee.

The U.S. Department of Agriculture estimates the value of honeybees to commercial agriculture at $15 billion to $20 billion annually. The estimated value of insect pollination for European agriculture is €22 billion a year. The Purdue scientists said that even non-lethal doses of the pesticides could potentially affect bees’ homing abilities or their resistance to mites and disease.

Professor Krupke believes efforts to limit or eliminate talc emissions during planting are essential. He said, "That's the first target for corrective action. 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."

Neonicotinoids affect the central nervous system of insects, causing paralysis and death. They are deemed less toxic to mammals as they block a specific neural pathway, which insects have in abundance compared to warm blooded creatures.

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