In the January 2012 issue of PLoS One, an online science journal, a report is published that describes in more detail the routes of exposure for honey bees for neonicotinoid insecticides, which are widely used and highly toxic to honey bees. While it has been widely acknowledged that this class of insecticides is found inside the hive in pollen and wax, the means by which the chemicals enter the hive has been less clear. Neonicotinoids are suspected to play a role the decline of pollinators world wide.
An interesting factoid was described in the paper. Scientists use a dosage level, designated LD50, when quantifying exposure to a known toxic chemical. “LD50” means that 50% of the population will die at that dose, or “lethal dosage”. For clothianidin, a type of neonicotinoid widely used in treating corn seed, the oral LD50 is 2.8 ng/bee. A single corn seed is typically coated at the rate of 0.5 mg/kernel. So that a single kernel has enough active ingredient to kill over 80,000 honey bees. But since the seed is planted below ground it was unclear how honey bees would be exposed to the chemical.
The research identifies high levels of neonicotinoids in the planter exhaust material produced when planting treated corn seed. If you have ever watched a corn planter at work, you perhaps have noticed a certain amount of “dust” raised by the operation. It is this dust which carries high levels of the insecticide. Presumably, the dust drifts to other areas and be taken up by soil and plants in adjacent areas, as well as in the target fields.
During the course of the study the researchers collected dead bees near the hive entrances in the spring and found the bees contained neonicotinoids. It was unclear if the exposure to chemicals was oral (consuming pollen) or by contact with the soil or dust. However, they found pollen inside the hive contained neonicotinoids and measured this same chemical in pollen from corn that was planted with treated seed.
This study helps clarify that there are multiple avenues throughout the growing season by which honey bees can come in contact with, and be contaminated by, neonicotinoid insecticides.