Impact of fungicide-insecticide interactions on monarch (L. Danaus plexippus) caterpillars

The intensively cultivated Midwestern U.S. is a key breeding ground for the monarch butterfly (Danaus plexippus). Monarch caterpillars present on their host plant (Asclepias spp.) near agricultural fields are likely exposed to pesticide spray drift. In the honeybee, Apis mellifera, combined exposure to -azole fungicides with pyrethroids, anthranilic diamides, or neonicotinoids, led to decreased honeybee survival compared to individual insecticide exposures. The increased mortality caused by -azole fungicides and insecticides combination is hypothesized to occur through inhibition of cytochrome P450 enzymes by -azole fungicides. In monarchs, these synergistic interactions have not yet been studied. We explored the potential interaction between -azole fungicides and insecticides in monarchs by evaluating the effect of the -azole fungicide propiconazole with three insecticides, bifenthrin, chlorpyrifos, and thiamethoxam. Third instar monarchs were orally exposed to combinations of propiconazole with each insecticide in an artificial diet, and the experiment was repeated with three different generations. Daily morality over 96 hours, morality until pupation, pupation success, and pupal eclosion success was recorded. Exposure concentrations were based on AgDRIFT Tier I Ground spray drift deposition estimates using high spray label rates of formulated products at 0, 5, and 10 meters away from the treated field edge. The results from this research will determine the potential for synergistic toxicity with propiconazole for monarchs developing close to treated agricultural fields.