Monarch butterflies travel from Mexico to the Midwestern U.S. every year where prairie habitat provides wildflowers with nectar for adults and milkweeds, the larval host plant. In Indiana, prairies make up only a fraction of their original range due to conversion to agriculture. Milkweeds around agricultural areas have decreased after the increased use of glyphosate resistant crops, potentially causing a decline in monarch butterflies in the Midwest. Some milkweeds survive glyphosate, but may be exposed to other pesticides that can negatively affect monarchs in their early life stages. We sampled nine sites, five in 2015 and four in 2016, collecting leaf tissue from 262 plants in 2015 and 240 plants in 2016 during June, July and August. We selected plants along a distance gradient (0 to 2,000+ m) from corn fields to evaluate the effect of linear distance of pesticides residues. We targeted 65 agrochemicals used in corn and soybeans and observed 12 compounds mainly applied to corn and soybean.
Results/Conclusions
The variation of pesticide residues in milkweed plants was mainly explained by time of year (i.e., month; 59% of the variation in 2015 and 67% in 2016), whereas distance explained a smaller portion of the variation (21 and 22% for both years). Herbicides were more abundant in June and decreased through the season, while fungicides showed the opposite pattern. Monarch caterpillars for both years were more abundant late in the season, which interestingly is when milkweeds had the lowest nutritive quality (i.e., % nitrogen) and were more exposed to fungicides. These results reflect the importance of managing temporal variation in pesticides and nutritional factors to protect natural areas and their associated species around agricultural land.