Monarch butterfly (Danaus plexippus) life-stage risks from foliar and seed-treatment insecticides

Authors: KRISHNAN, NIRANJANA - Iowa State University; ZHANG, YANG - Institute Of Plant Protection - China; AUST, MELANIE - Conservation Corp Minnesota And Iowa; Hellmich Ii, Richard; COATS, JOEL - Iowa State University; BRADBURY, STEVEN - Iowa State University

Submitted to: Environmental Toxicology and Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/11/2021
Publication Date: 2/16/2021
Citation: Krishnan, N., Zhang, Y., Aust, M.E., Hellmich II, R.L., Coats, J.R., Bradbury, S.P. 2021. Monarch butterfly (Danaus plexippus) life-stage risks from foliar and seed-treatment insecticides. Environmental Toxicology and Chemistry. 40(6):1761-1777. https://doi.org/10.1002/etc.5016.
DOI: https://doi.org/10.1002/etc.5016

Interpretive Summary: Efforts to conserve monarch butterfly populations include establishing habitats with milkweed and nectar plants in agricultural landscapes. 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, egg to adult. Chronic and acute dietary toxicity studies were undertaken with larvae and adults and acute topical bioassays were conducted with eggs, pupae, and adults using six representative insecticides: beta-cyfluthrin (pyrethroid), chlorantraniliprole (anthranilic diamide), chlorpyrifos (organophosphate), and imidacloprid, clothianidin, and thiamethoxam (neonicotinoids). 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. Studies are underway to estimate landscape-scale risks because many factors contribute to this, including frequency of insecticide use and wind patterns. Information in this paper will be useful to scientists and growers interested in establishing habitats to help the monarch butterfly.

Technical 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 North Central United States (U.S.). A variety of seed-treatment and foliar insecticides are used to manage early- and late-season pests in these landscapes. To inform habitat conservation practices there is a need to assess risks of these insecticides to monarch butterfly life stages. Chronic and acute dietary toxicity studies were undertaken with larvae and adults and acute topical bioassays were conducted with eggs, pupae, and adults using six representative insecticides: beta-cyfluthrin (pyrethroid), chlorantraniliprole (anthranilic diamide), chlorpyrifos (organophosphate), and imidacloprid, clothianidin, and thiamethoxam (neonicotinoids). Chronic dietary LC50 values for monarch larvae ranged from 1.6 x 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 non-migratory monarchs, considering these results within a landscape-scale context suggests 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 North Central U.S.