Pyrethroid Exposure Reduces Growth and Development of Monarch Butterfly (Lepidoptera: Nymphalidae) Caterpillars

Insecticide exposure has been identified as a contributing stressor to the decline in the North American monarch butterfly Danaus plexippus L. (Lepidoptera: Nymphalidae) population. Monarch toxicity data are currently limited and available data focuses on lethal endpoints.This study examined the 72-h toxicity of two pyrethroid insecticides, bifenthrin and beta-cyfluthrin, and their effects on growth and diet consumption.The toxicity of bifenthrin to caterpillars was lower than beta-cyfluthrin after 72 h. Survival was the most sensitive endpoint for bifenthrin, but diet consumption and caterpillar growth were significantly reduced at sublethal levels of beta-cyfluthrin. Using AgDRIFT spray drift assessment, the aerial application of bifenthrin or beta-cyfluthrin is predicted to pose the greatest risk to fifth-instar caterpillars, with lethal insecticide deposition up to 28 m for bifenthrin and up to 23 m for beta-cyfluthrin from treated edges of fields. Low boom ground applications are predicted to reduce distances of lethal insecticide exposure to 2 m from the treated field edge for bifenthrin and beta-cyfluthrin. Growth and survival of fifth-instar monarch caterpillars developing within the margins of a treated field may be significantly impacted following foliar applications of bifenthrin or beta-cyfluthrin.These findings provide evidence that pyrethroid insecticides commonly used for soybean pest control are a potential risk to monarch caterpillars in agricultural landscapes.

Automated summary

The study indicates that pyrethroid insecticides pose a potential risk to monarch caterpillars, with bifenthrin being less toxic than beta-cyfluthrin, but causing significant impacts on diet consumption and caterpillar growth at sublethal levels.