PESTICIDE EXPOSURE RISK AND DEVELOPMENTAL CONSEQUENCES FOR MONARCH BUTTERFLIES IN AGRICULTURAL LANDSCAPES
Chapter 1. Monarch butterflies are undergoing a long-term population decline, which has led to a search for potential causes underlying this pattern. One poorly studied factor is exposure to non-target pesticides on their primary host-plant, the common milkweed Asclepias syriaca, during larval development. This species frequently grows near agricultural fields in the Midwestern U.S., but the spectrum of pesticides encountered by monarch caterpillars on milkweed leaves is unknown. Further, it is unclear whether pesticide exposure can be avoided by isolating restored milkweed patches at sites far from cropland. Over 2 years, we analyzed 1,543 milkweed leaves across seven sites in northwestern Indiana for the presence and concentration of a range of commonly used agricultural insecticides, fungicides, and herbicides. Additionally, we tested the ability of local (i.e., nearest linear distance to crop field) and landscape-level (i.e., % of corn/soybean in 1km radius) variables to predict the presence of pesticides on focal milkweeds. Overall, we detected 14 pesticides−4 insecticides, 4 herbicides, 6 fungicides—on milkweeds that varied widely in their prevalence and concentration. The neonicotinoid clothianidin, the only pesticide for which toxicity data are available in monarchs, was detected in 15–25% of plants in June with nearly 60% of milkweeds at some sites testing positive (mean conc. = 0.71 and 0.48 ng/g in 2015 and 2016, respectively); however, no samples from July or August contained clothianidin. The related neonicotinoid thiamethoxam and the pyrethroid deltamethrin were detected in most (>75%) samples throughout the season, but only in the second year of the study. For thiamethoxam, isolating milkweeds 50–100m from the nearest corn or soybean field tended to decrease the concentration and likelihood of detecting residues, whereas landscape composition surrounding milkweed sites had comparatively weak predictive power. These data suggest that monarch caterpillars frequently consume a diversity of pesticides in their diet; the lethal or sublethal impacts of this exposure remain to be tested.
Chapter 2. Hundreds of recent studies have voiced concern over the negative impacts of non-target pesticides on pollinator health. However, pesticide loads are highly variable across agricultural landscapes and it is unclear whether pollinators exhibit behavioral responses (e.g., aversion) that mediate their exposure risk under realistic foraging environments. We tested whether monarch butterfly (Danaus plexippus) adults and larvae base their oviposition and foraging decisions, respectively, on the presence and concentration of pesticide residues on their milkweed host-plant, Asclepias syriaca. Using a two-year dataset that quantified pesticides on milkweeds bordering corn or soybean fields, we simulated field-realistic levels for six of the most commonly detected pesticides—one insecticide, two herbicides, and three fungicides—either alone or in combination. These laboratory and greenhouse manipulations experimentally paired an untreated control with the pesticides at their mean or maximum concentrations. Butterflies placed fewer eggs on milkweeds treated with a cocktail containing all six pesticides, but only when applied at their maximum-detected concentration, resulting in ca. 30% less oviposition compared to the untreated control. Neonate (1stinstar) larvae also showed a preference for pesticide-free leaves in paired disc assays for most compounds tested, with feeding aversion observed at both mean and maximum concentrations. Later instars did not show a comparable behavior reaction to pesticide presence or concentration, but this could be partially due to the feeding-deterrent properties of the acetone solvent used. Our data provide evidence that monarchs are capable of adaptively adjusting their oviposition and foraging behaviors based on which pesticides are present on their host-plants. Yet, for gravid females, this impact was only observed at higher than average concentrations, meaning that in the field eggs are likely placed on milkweeds regardless of pesticide presence in most cases. Thus, it is unlikely that monarchs behaviorally regulate pesticide exposure risk by avoiding contaminated plants.Finally, in Chapter 3 we used a no-choice experiment to evaluate the effects of continuous exposure to field-realistic pesticide concentrations on monarch butterfly larval and pupal development time, pupal weight, adult longevity and survival. Most monarch life stages were relatively unaffected by continuous exposure to the six pesticides tested. A negative effect in wing development and length was observed when larvae were exposed to the fungicides pyraclostrobin and trifloxystrobin and the mix of all six pesticides tested. Larval stage had higher mortality than the pupal stage and instars 2 and 5 were relatively more vulnerable. The negative effect on wing span and wing development could negatively impact migration, reproduction in the short-term and population at long-term. Strategies to reduce contamination by pesticides of non-target plants and insects should be considered to protect diversity and maintain ecosystem integrity in the landscape.