Assessing field-scale risks of foliar insecticide applications to monarch butterfly (Danaus plexippus) larvae

Authors: KRISHNAN, NIRANJANA - Iowa State University; ZHANG, YANG - Institute Of Plant Protection - China; Bidne, Keith; Hellmich Ii, Richard; COATES, JOEL - Iowa State University; BRADBURY, STEVEN - Iowa State University

Submitted to: Environmental Toxicology and Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/15/2020
Publication Date: 1/21/2020
Citation: Krishnan, N., Zhang, Y., Bidne, K.G., Hellmich II, R.L., Coates, J.R., Bradbury, S.P. 2020. Assessing field-scale risks of foliar insecticide applications to monarch butterfly (Danaus plexippus) larvae. Environmental Toxicology and Chemistry. 39(4):923-941. https://doi.org/10.1002/etc.4672.
DOI: https://doi.org/10.1002/etc.4672

Interpretive Summary: The population of North America’s eastern monarch butterfly has declined over the past two decades due to many factors including loss of habitat. Establishment and maintenance of milkweed plants in agricultural landscapes of the North Central United States is needed to reverse this decline. Milkweeds in the genus Asclepias are the only host plants for larval monarch butterflies. The extent to which insecticide use influences productivity of monarchs when habitat with milkweed is established close to crop fields is unknown because monarch toxicity data are lacking. To assess the potential effects of foliar insecticide applications in maize and soybean pest management, the toxicity of five representative insecticides were evaluated: beta-cyfluthrin (pyrethroid), chlorantraniliprole (anthranilic diamide), chlorpyrifos (organophosphate), and imidacloprid and thiamethoxam (neonicotinoids). Monarch caterpillars develop through five stages, called instars, before they form a chrysalis (pupa). In this study first, second, third, and fifth instars were evaluated. For aerial and ground applications of insecticides, mortality rates were highest for beta-cyfluthrin and chlorantraniliprole and lowest for thiamethoxam. 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: Establishment and maintenance of milkweed plants (Asclepias spp.) in agricultural landscapes of the North Central United States is needed to reverse the decline of North America’s eastern monarch butterfly (Danaus plexippus) population. The extent to which insecticide use influences productivity of monarchs when habitat with milkweed is placed in close proximity to crop fields is unknown, in part because of the paucity of monarch toxicity data. With foliar insecticide applications, larvae can be exposed directly through the cuticle by spray drift or through the diet by consuming milkweed leaves with insecticide residues. To assess the potential effects of foliar insecticide applications in maize and soybean pest management, the toxicity of five representative active ingredients were determined: beta-cyfluthrin (pyrethroid), chlorantraniliprole (anthranilic diamide), chlorpyrifos (organophosphate), and imidacloprid and thiamethoxam (neonicotinoids). To estimate larval mortality rates downwind from treated fields, modeled insecticide exposures to larvae and milkweed leaves were compared to concentration-response curves obtained from acute topical and dietary bioassays with first-, second-, third-, and fifth-instar larvae. Sub-lethal effects also were examined and arrested pupal ecdysis was observed when fifth instars were exposed to neonicotinoids and chlorpyrifos. For aerial and ground applications, predicted mortality rates were highest for beta-cyfluthrin and chlorantraniliprole and lowest for thiamethoxam. To estimate landscape-scale risks, field-scale mortality rates must be considered in the context of spatial and temporal patterns of insecticide use. Mention of trade names or commercial products in this publication is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture.