Integrating existing evidence and new toxicity data to assess pesticide risks for informing monarch conservation

The monarch butterfly (Danaus plexippus) population has significantly declined, prompting the USFWS to propose listing it as a threatened species under the Endangered Species Act. Habitat loss, climate change, and pesticide use are the major threats to monarch recovery. To reach monarch conservation goals, establishing milkweed (Asclepias species) habitat in agricultural landscapes – where pesticides are often applied – is critical. Therefore, it is imperative to assess the risk of newer classes of insecticides and bio-pesticides to monarch butterflies to help inform pesticide regulatory decisions, pest management practices, and conservation strategies. While the risk of older conventional pesticides has been well elucidated, toxicity data for newer synthetic and biopesticides are lacking, precluding their evidence-based risk assessments. To address these data gaps, we conducted acute topical and dietary toxicity bioassays on second-, third-, and fifth- instar monarch larvae with four synthetic insecticides – cyantraniliprole, sulfoxaflor, broflanilide, afidopyropen – along with two formulated biopesticides [Double Nickel® (Pseudomonas chlororaphis strain AFS009) and Howler® (Bacillus amyloliquefaciens strain D747)]. We found that the biopesticides caused minimal to no mortality. Sulfoxaflor and afidopyropen exhibited very low toxicity. However, cyantraniliprole showed moderate to high toxicity while broflanilide was the most toxic synthetic insecticide tested.  Using AgDRIFT, a spray drift model, we compared the toxicity data with foliar exposure estimates to assess mortality risks for monarchs 100 ft downwind of treated fields. Our results indicate low risk from exposure to biological fungicides but high risk from exposure to chemical pesticides.