Milkweed plants bought at nurseries may expose monarch caterpillars to harmful pesticide residues

Abstract

The decline of monarch butterflies in both the eastern and western United States has garnered widespread public interest. Planting milkweed has been promoted as one action that individuals can take, but very little is known with respect to potential pesticide contamination of store-bought milkweeds during the process of production and transport to market. In this study, we collected 235 milkweed leaf samples from 33 retail nurseries across the US to screen for pesticides. Across all samples, we detected 61 different pesticides with an average of 12.2 compounds per leaf. While most of the compounds have unknown effects on caterpillars, 48 samples contained a pesticide at a concentration that exceeds a published lepidopteran LD50 and 89 contained a pesticide above a concentration shown to have a sub-lethal effect for a monarch. We detected only modest effects of store size and milkweed species; however, plants with labels advertising their value for wildlife were more likely to contain pesticides at concentrations known to have negative effects on monarchs. These results demonstrate the extensiveness of pesticide exposure within nursery milkweeds and the potential impacts on monarch caterpillars and other insects consuming store-bought plants.