Lethal and sublethal effects of imidacloprid on Osmia lignaria and clothianidin on Megachile rotundata (Hymenoptera: Megachilidae)

The authors examined lethal and sublethal effects of neonicotinoid insecticides on larval development and adult emergence of two native bee species in British Columbia, Canada. They tested imidacloprid for effects on Osmia lignaria and clothianidin for effects on Megachile rotunda.

For O. lignaria, a laboratory study as well as a field study were conducted with imidacloprid in highbush blueberry fields. In the laboratory, there were effects among females on the time to darken a cocoon, and among males, control bees weighed significantly less than bees in all other treatments. The authors noted challenges with their lab experiments that made it difficult to measure life stages and reduced their sample size, which made these findings uncertain. In the field, the authors found significant effects of the time between the egg and the last larval stage and to complete darkening their cocoon for both males and females, as well as for spinning cocoons for males. O. lignaria that were started earlier in the season generally had longer development times and larger weights.

For M. rotundata, a laboratory study and a field study in a wild clover field were conducted. Treatment affected the time that females took to spin a cocoon, but most of the other qualities were only affected by initiation date of the egg.

Overall, the authors did not find lethal effects on either species at any of the concentrations tested, nor did they see increasing mortality with increasing pesticide concentration. They note that it is possible that the pesticides could have degraded between the time they were injected into the pollen and the time the larvae ingested it, however the sublethal effects seen in O. lignaria suggest that the imidacloprid was still active. They suggest that the minor changes in development time seen in this study could impact individual bees if the changes are substantial enough to effect the timing of diapause initiation. The effects they found from initiation date indicated how closely M. rotundata’s development is linked to environmental conditions and resource availability.