How do seed-applied insecticides affect bee health?

Pick an insecticide with a neonicotinoid active ingredient, say Thiamethoxam, and browse through the label to the point where you find the information on “Environmental Hazard”. It is that part of the label that tells you  whether the particular insecticide is toxic against non-target organisms. Under this headline, you’ll find something like “This product is toxic to wildlife and highly toxic to aquatic invertebrates.” with a standard precautionary note running like “do not contaminate water”. Interestingly, depending on the insecticide label you read, you may or may not find this line “This toxic is highly toxic to bees exposed to direct treatment on blooming crops or weeds”. Two insecticides with exactly the same neonicotinoid active ingredient may have differing labels when it comes to bee-related risks, what gives?

The underlying thought is that some insecticides are utilized as foliar application and thus have a very real risk of being sprayed on bees. Yet other insecticides, albeit containing the same active ingredient, are formulated as seed treatment and, consequently, assumed to pose low risk to bees due to low chance of exposure. Risk is, as the mantra goes, the meeting of toxicity and exposure.

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Photo: David Cappaert, Michigan State University, http://www.insectimages.org

A research by a team of entomologists at Purdue University may shed a light on this matter. In their open-access paper, the authors reported that “extremely high levels of clothianidin and thiamethoxam were found in planter exhaust material produced during the planting of treated maize seed.” Insecticide-treated seeds are sticky and to prevent the sticking of seeds in the planter talc are often added to the mixture. The talc that has been in close contact with insecticide are often blown away as a part of planter ‘exhaust material’, carrying traces of insecticides on them. Blooming weeds around planted fields, in this case dandelions, were found to contain neonicotinioid insecticides. The detectable level of neonicotinoids on these flowers may come from direct settling of insecticide-containing dust on them or by the weeds taking up the insecticides from the soil. Honey bees readily forage on these flowering weeds.

At the minimum, this result highlighted a possible route of exposure previously assumed not to exist: insecticide may still reach bees even if it is used to treat seeds and not as a foliar spray. Whether the amount of insecticide carried through planter exhaust poses significant poisoning risk to bees is yet to be convincingly proven. Yet, if further research shows that this route of exposure significantly increase the poisoning risk to bees during planting season, thought must be given on managing planter exhaust.

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