Several studies have linked the spring sowing of maize seed to the lethal poisoning of bees and have found that worker bees coming in contact with the exhaust from the drilling machines became contaminated with insecticides and rapidly died. In previous studies with still caged bees and free bees, the correlation between the particles from the drilling machines and the poisoning of bees was not clear. To specify how close the bees have to be to be poisoned, Girolami et al. incorporated the distance from the drilling machines, the type of drilling machine, and the number of times the bees had to pass by the machines before they were killed. The authors also ran the drilling machines with 200 g of Talc added to the seed containing hoppers during three trials in order to capture the extent of the exhaust cloud. After being exposed to high humidity, bees that were moved alongside the machines were lethally poisoned. No significant difference in bee mortalities was found between modified and unmodified corn drillers. The exhaust cloud extended approximately 20 feet around the drilling machine. —Lia Metzger
Girolami, V., Marzaro, M., Vivan, L. Mazzon, L., Giorio, C., Marton, D., Tapparo, A., 2013. Aerial powdering of bees inside mobile cages and the extent of neonicotinoid cloud surrounding corn drillers. Journal of Applied Entomology 137, 35-44.
In order to mimic the movement of foraging bees, ten cages, each containing one bee, were attached to a four meter aluminum bar, held 2.5 meters high, and walked by the side of the drilling machine. In some trials, the bees were held at 2, 4, and 6 meters from the still drilling machine, and then placed in either lab humidity or high humidity to test for the lethality of the neonicotinoid cloud. In addition, the bees were walked 1—5 meters or 5—9 meters from the right side of the mobile drilling machine and parallel to its movement for 30 seconds. Then the caged bees made a U-turn around the drilling machine and were walked along the left side, reflecting the movement of bees making a round-trip around the sowing field. To test for the exposure to insecticides in the exhaust, the bees underwent chemical analysis and were only exposed on one side of the drilling machine in the same way. This allowed for the authors to differentiate the levels of contaminants on each side of the drilling machines and the lethality of the exhaust at varying distances from the machines.
The authors found bees that had been exposed at 2, 4, and 6 meters by passing by the drilling machine rapidly died from clothianidin if they were placed in high humidity afterward. Mapping the deaths of the bees between 2 to 12 meters on the right side and up to 8 meters on the left side of the drilling machine, the extent of the neonicotinoid cloud was measured as approximately 10 meters on each side and 2 meters high. Possible change in the shape of the cloud was considered, but the proposed elliptical shape formed by wind and movement would not reduce the extent of the cloud. Even though the modified drilling machines direct their exhaust at the ground instead of at a 45° anglethere was no significant difference between the mortality of bees passing by modified or unmodified drilling machines. Machines sowing seeds that were coated in fungicide did not cause significant poisoning to the bees, but the coatings clothianidin, imidacloprid, and thiamethoxam all caused more than 50 % of the bees that passed by to die. Furthermore, the chemical analysis revealed that all of the bees contained very large quantities of insecticide, with the highest levels of insecticide in bees passing by at 1 meter and decreasing in insecticide levels at greater distances.
The poisonous exhaust can account for the mass deaths of bees in sowing season from corn drillers that are used during this time. Although there were differences in levels of poisons in the bees, the majority of the bees that passed by the drilling machines in the way that bees normally forage were killed. The authors showed that, even with modified drillers or insecticides not affecting the bees directly, the airborne contaminants are extremely lethal to bees.