Developing an approach for testing systemic pesticides on solitary bees: Effects of sulfoxaflor on Megachile rotundata

Background/Question/Methods: Systemic pesticide application is effective against a wide range of pests as it is absorbed by and translocated through various plant tissues, including tissues developing after initial application. Because pesticide residues can be found in pollen and nectar, systemic pesticides potentially pose a risk to non-target organisms that consume floral resources. Previously, testing for pesticide effects on bees primarily focused on Apis mellifera, the European honey bee, but there is increasing recognition that negative effects of pesticides may vary among bee species due to differential sensitivity or to different exposure pathways associated with life history. Sulfoxaflor is a new systemic insecticide that acts on the insect nervous system, similar in effect to neonicotinoids. We performed experiments to test for negative effects of sulfoxaflor on the alfalfa leafcutter bee, a key alfalfa pollinator that may encounter pesticide residues in pollen, nectar and leaf tissue. We followed label precautions and applied sulfoxaflor to small alfalfa plots at four concentrations: the manufacturer’s maximum and minimum recommended doses for controlling lygus bugs, the dose for controlling aphids, and controls. Bees were allowed to forage the day after treatment and plots were monitored for pesticide residues and bee survival and development.

Results/Conclusions: Sulfoxaflor residues declined sharply but remained detectable in both leaves and larval provisions for two-three weeks following application. All adult leafcutter bees died within two days of application at the two highest rates. Larval survival was inconclusive, as there were no larvae to observe for several days post application in the highest treatments due to adult mortality. After the initial week following application, there was no difference in larval survival across treatments. Sulfoxaflor is highly toxic to bees before residues dry, and the label recommendation to spray at night presupposes the product will dry prior to foraging. In humid environments, such as Virginia, where this study took place, high toxicity may remain if the product fails to dry quickly. This has implications for both managed and wild pollinators. Farmers would need to better protect managed bees by postponing their release. Wild bees would face substantial risk when visiting the crop the day after application. Better understanding of the conditions causing bee death in this study, and generating the first data on larval bee performance shortly after pesticide application, are still needed to establish the conditions under which this pesticide may be used safely.