Abstract
Conservation of North America's eastern monarch butterfly (Danaus plexippus) population would require establishment of milkweed (Asclepias spp.) and nectar plants in the agricultural landscapes of the north central United States. A variety of seed-treatment and foliar insecticides are used to manage early- and late-season pests in these landscapes. Thus, there is a need to assess risks of these insecticides to monarch butterfly life stages to inform habitat conservation practices. Chronic and acute dietary toxicity studies were undertaken with larvae and adults, and acute topical bioassays were conducted with eggs, pupae, and adults using 6 representative insecticides: beta-cyfluthrin (pyrethroid), chlorantraniliprole (anthranilic diamide), chlorpyrifos (organophosphate), imidacloprid, clothianidin, and thiamethoxam (neonicotinoids). Chronic dietary median lethal concentration values for monarch larvae ranged from 1.6 × 10-3 (chlorantraniliprole) to 5.3 (chlorpyrifos) μg/g milkweed leaf, with the neonicotinoids producing high rates of arrested pupal ecdysis. Chlorantraniliprole and beta-cyfluthrin were generally the most toxic insecticides to all life stages, and thiamethoxam and chlorpyrifos were generally the least toxic. The toxicity results were compared to insecticide exposure estimates derived from a spray drift model and/or milkweed residue data reported in the literature. Aerial applications of foliar insecticides are expected to cause high downwind mortality in larvae and eggs, with lower mortality predicted for adults and pupae. Neonicotinoid seed treatments are expected to cause little to no downslope mortality and/or sublethal effects in larvae and adults. Given the vagile behavior of nonmigratory monarchs, considering these results within a landscape-scale context suggests that adult recruitment will not be negatively impacted if new habitat is established in close proximity of maize and soybean fields in the agricultural landscapes of the north central United States. Environ Toxicol Chem 2021;40:1761-1777. © 2021 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.
Highlights
Decline of North America's monarch butterfly (Danaus plexippus) populations, which was recently designated as a candidate species for listing under the US Endangered Species Act (US Fish and Wildlife Service 2020), has spurred collaborative conservation efforts that link federal and state agencies with a diversity of nongovernmental organizations and the Environmental Toxicology and Chemistry, 2021;00:1–17—N
We provide data to more rigorously test the hypothesis that the conservation benefits of establishing milkweed habitat close to maize and soybean fields outweigh the risk of insecticide exposure from foliar and seed‐treatment applications
Imidacloprid, thiamethoxam, and clothianidin constitute nearly 85% of total neonicotinoid sales (Bass et al 2015) and are extensively used to treat maize and soybean seeds (Tooker et al 2017). We conclude that these seed‐treatment uses pose little risk to monarch larvae and adults, consistent with the findings of Krischik et al (2015), Bargar et al (2020), and Olaya‐ Arenas et al (2020)
Summary
Decline of North America's monarch butterfly (Danaus plexippus) populations, which was recently designated as a candidate species for listing under the US Endangered Species Act (US Fish and Wildlife Service 2020), has spurred collaborative conservation efforts that link federal and state agencies with a diversity of nongovernmental organizations and the Environmental Toxicology and Chemistry, 2021;00:1–17—N. Insecticide exposure to monarch habitat in close proximity to row crop fields in the north central states has been reported in modeling (Krishnan et al 2020) and monitoring (Olaya‐Arenas and Kaplan 2019) studies. Potential risks of these exposures led the US Fish and Wildlife Service (2020) to identify insecticides as a factor threatening recovery of monarch populations
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