Routes of alfalfa leafcutting bee exposure to an insect growth regular under field conditions

Knowledge of lethal and sublethal pesticide effects on pollinators allows for prudent decision-making in cropping systems that need pollination services.  The chitin synthesis inhibitor novaluron is used to control certain coleopteran, lepidopteran, hemipteran, and dipteran crop pests.  Although novaluron is considered a reduced risk insecticide because it disrupts insect ecdysis and leads to death of only immature stages, some exposed adult beneficial insects produce fewer eggs and experience suppressed egg hatch.  Novaluron can be used to suppress pests in the alfalfa seed production system, but negative impacts on the reproductive success of the primary alfalfa pollinator Megachile rotundata F. (Hymenoptera: Megachilidae) were demonstrated.  This study was performed to better understand novaluron exposure routes for M. rotundata eggs and larvae and to assess the persistence of negative effects of novaluron on this bee species.  Bees were allowed to nest in field cages where they were exposed to no novaluron, recently-sprayed alfalfa, or alfalfa sprayed one- and two-weeks earlier.  Compared to control, we found greater proportions of cells with dead eggs and larvae and lower proportions of live prepupae regardless of whether nesting females bees were exposed to fresh or residual novaluron, revealing short- and long-term effects of this pesticide.  Two possible novaluron exposure routes were considered.  First, the mother bee may become contaminated internally through ingestion or direct contact.  Second, the pollen-nectar provision may become contaminated with novaluron that is on or within the leaf pieces that surround the provision or with novaluron carried on the mother bee’s body that may be transferred to the provision.  Both exposure routes appear possible in the scenarios we tested.  For M. rotundata as an alfalfa pollinator, understanding all possible routes of pesticide exposure will help in making the best decisions on how to maintain bee health and reproduction while protecting seeds from pests.