There are three widely used neonicotinoid seed coat formulations that protect young crop plants (e.g. corn and soybean) against pest herbivory (thiamethoxam, imidacloprid, and chlothianidin). These insecticides are systemic, meaning they can be absorbed and distributed throughout the plant. Once seeds germinate, the insecticides are translocated throughout the growing corn plant[6]. When the plant is consumed by herbivorous insects, the insect also consumes and is affected by the toxicity of the insecticide.
Not all of the neonicotinoid insecticide remains with the seed or growing corn plant. Owing to their high water solubility, a portion of the insecticide is leached away from the seed reaching groundwater and adjacent water bodies. Some remains in the crop soils and can be translocated from the crop area when dust is generated[9]. In addition, since the insecticide is systemic, it has been found in pollen, which can also be transported away from the crop area via the wind. When all of these mobilization processes occur, this group of insecticides can contaminate adjacent habitats and would then be available for uptake by other plants including milkweed. Since milkweed is the larval host plant for monarch butterflies, the larvae can be exposed and affected by the neonicotinoid insecticide.
Due to the potential for off-site transport, we previously conducted a study evaluating monarch utilization of milkweed planted in close proximity to thiamethoxam-treated corn seed. We sampled, analyzed, and found thiamethoxam residues in the milkweed leaves. Since the corn plants never went to tassel, only uptake of thiamethoxam residues from the soil was possible. We also investigated the effect of the contaminated milkweed plants on monarch utilization of those plants. Because it was an open system (e.g., wild monarchs laid eggs, predators could access the plants, fully grown larvae could escape the plants), the only effect we could confidently detect was reduced larval survival. Pecenka and Lundgren found similar results. They measured clothianidin, a thiamethoxam degradate, on milkweed leaves sampled from crop-margins and conducted toxicity tests with monarch larvae that consumed leaves dosed with clothianidin and imidacloprid. In their study, larval growth was negatively affected by concentrations similar to those measured on the leaves sampled from the crop margins, and to those concentrations found in leaves from our study. Clearly, contamination of habitat by neonicotinoid pesticides from corn crops can affect monarch butterflies.
Our hypothesis is that reduced larval growth leads to reduced pupation and adult butterfly emergence. If the presumption is correct, this would affect fitness and reproductive success. While larvae in our prior study developed to the point of pupation and adult emergence, the study design did not allow accurate quantification of pupation rate or successful eclosion in relation to the treatment. After learning from the first experiments, we expect data from the proposed study will allow a model to be developed between those endpoints and residue levels in milkweed leaves, larvae, and adult butterflies. In addition to using the neonictinoid thiamethoxam for the toxicity study, we will be conducting a second similar study using the neonictinoid clothianidin.
