Assessing the toxicity and risk of newer conventional and biological pesticides on monarch butterflies

The eastern North American monarch butterfly (Danaus plexippus) population has significantly declined. To reach monarch conservation goals, it is critical to establish milkweed (Asclepias species) habitat in agricultural landscapes, where pesticides are often employed. Thus, it is imperative to assess the risk of pesticide exposures to monarch butterflies to help inform pesticide regulatory decisions, integrated pest management programs, and conservation strategies. While the risk of older conventional pesticides has been well elucidated, toxicity data for newer conventional and organic pesticides are lacking, precluding their evidence-based risk assessments. To address these data gaps, we undertook toxicity studies—acute topical and dietary studies with second-, third-, and fifth- instar monarch larvae—with cyantraniliprole (diamide), sulfoxaflor (sulfoximine), broflanilide (meta diamide), afidopyropen (pyropene), Pseudomonas chlororaphis, Bacillus amyloliquefaciens, and copper (II) hydroxide. We found that cyantraniliprole and broflanilide induced significant mortality at low doses, while sulfoxaflor and afidopyropen induced significant mortality only at high doses. In contrast, the biofungicides Pseudomonas and Bacillus caused no observable effect following high dose exposure. Copper (II) hydroxide was intermediately toxic. We compared the toxicity data with foliar exposure estimates derived from AgDRIFT, a spray drift model, to assess mortality risks to monarchs 100 ft downwind of treated crop fields. Additionally, to assess the risk of a newer seed-treatment insecticide, we are analyzing chlorantraniliprole (diamide) residue concentrations in milkweed planted downslope to chlorantraniliprole seed-treated corn fields. These dietary exposure estimates would be compared with corresponding toxicity data to estimate monarch butterfly risks in landscapes dominated by diamide-treated seeds.