Honey bee safety of imidacloprid corn seed treatment

In 2000, beekeepers from the Italian region of Friuli reported losses of a high number of bee hives during the spring season. At the time of this bee incident farmers in Friuli had started to drill their corn fields and some of the seeds had been treated with the seed dressing product Gaucho registered (active ingredient: imidacloprid). Reports by some French beekeepers on suspected impacts of seed dressed sunflowers on honeybees made some Italian beekeepers to believe that there might be a link between their bee losses and the use of Gaucho registered seed dressing on corn seed. During drilling, dust particles are emitted by the pneumatic seed drilling machines which may also contain abraded seed dressing particles. In response to this concern, further investigations were conducted to examine whether corn crops, seed-treated with Gaucho registered will pose a risk to honey bees. In October 2001 and June 2002, two field trials were conducted in Germany with Italian and German commercial maize seeds to investigate the possible abrasion of Gaucho registered from seed-treated maize during sowing with a pneumatic drilling machine. The results show an average emission rate of only 4% on the seed drilling rate with a standard dressing formulation. Formulations with adjuvants diminished abrasions even more. In parallel, a replicated cage test with honey bees was conducted to examine whether the recorded abrasion rates could adversely affect honeybee colonies foraging on flowering plants with simulated deposits of emitted dust particles. From the findings of this replicated cage study it can be concluded that deposition rates which were much higher than those potentially emitted from pneumatic corn drilling machines will not have detrimental effects on honeybees. In 2001, no hive losses were recorded in the region of Friuli. This observation in combination with the results of the presented replicated cage study make it very unlikely that the bee colony losses in 2000 were linked to the drilling of Gaucho registered dressed corn seeds. Accordingly, the real causative factors (e.g. climatic conditions, bee diseases) of the bee hive losses in 2000 remain to be elucidated in order to develop appropriate preventive measure against losses in future.