Host plant effects on insecticide detoxification gene expression in monarch butterflies

The monarch butterfly (Danaus plexippus) population has declined over the past two decades and is proposed for listing under the Endangered Species Act. Insecticide exposure has been cited as one of the multiple stressors contributing to the decline. Fall migrating adults may come into contact with late-season applications of pyrethroids in soybeans in the US Midwest, which has been shown to cause lethal effects if spray drift is deposited on the host plant, milkweed (Asclepias sp.), around field margins. However, milkweed itself also produces insecticidal defense compounds known as cardenolides. The concentration of these endogenous plant toxins varies based on milkweed species and may affect detoxification processes in monarch caterpillars. Here, we present findings from controlled laboratory experiments detailing the toxicology of bifenthrin to monarch caterpillars fed on different species of milkweed. The differential expression of insecticide detoxification genes, including esterases, cytochrome P450 monooxygenases, glutathione S-transferases and ABC transporter, and the cardenolide target Na⁺/K⁺-ATPase will be reported across bifenthrin and cardenolide treatment combinations.