Toxicology of Chemical Stress to Monarch Butterflies (\u3ci\u3eDanaus plexippus\u3c/i\u3e L.)

Monarch butterfly (Danaus plexippus L.) population declines have caught the attention of the country and prompted nationwide conservation initiatives. The United States Fish and Wildlife Service has identified insecticide exposure and loss of milkweed (Asclepias spp.) reproductive habitat as primary threats to the monarch. In the Midwestern US, milkweed largely occurs around cropland borders where there may be a spatial and temporal overlap of monarch larvae, insecticide usage, and fertilizer applications. In this study, the acute toxicity and sub-lethal effects on growth and diet consumption of two commonly used pyrethroid insecticides, bifenthrin and beta-cyfluthrin, were characterized in fifth instar monarch larvae. While beta-cyfluthrin was more toxic than bifenthrin, foliar applications of formulated products, Baythroid (beta-cyfluthrin) and Brigade 2-EC (bifenthrin), would result in sub-lethal and lethal effects at similar distances from a treated field edge according to the United States Environmental Protection Agency AgDrift model. As monarch larvae consume milkweed leaves they also ingest insecticidal cardenolides, which are antagonized by potassium. We examined the effects of ouabain, a hydrophilic cardenolide, and potassium chloride (KCl), a commonly used potassium fertilizer, in monarch caterpillars following chronic oral exposure. Once effect thresholds of ouabain and KCl were determined, bifenthrin toxicity was compared between different combinations of ouabain and KCl diets. Elevated concentrations of ouabain increased caterpillar growth and decreased development time whereas elevated concentrations of KCl decreased caterpillar growth and diet consumption. There was no difference in bifenthrin toxicity on different diet combinations. Milkweed species contain a variety of cardenolides that vary in concentration and polarity. Toxicity of bifenthrin and effects on detoxification enzymes were characterized in monarchs feeding on tropical milkweed (A. curassavica), a high cardenolide species, and swamp milkweed (A. incarnata), a low cardenolide species. Detoxification gene expression and enzyme activity significantly differed between milkweed species and between solvent control and bifenthrin treated caterpillars on each species. Understanding physiological differences in monarchs developing on different milkweed species is important for maximizing the benefits of habitat restorations among agricultural landscapes. Adviser: Troy D. Anderso