Abstract
The decline of monarch butterflies in both the eastern and western United States has garnered widespread public interest. Planting milkweed, the larval host plants, has been promoted as one action that individuals can take, but 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 only 9 of these compounds have been experimentally tested on monarch caterpillars, 89 samples contained a pesticide above a concentration shown to have a sub-lethal effect for a monarch. We detected only a modest predictive ability of retailer size and milkweed species; and plants with labels advertising their value for wildlife were not more likely to have fewer pesticides at concentrations known to have a negative effect on monarchs. These results demonstrate the extensiveness of pesticide exposure within nursery milkweeds and the potential impacts on monarchs and other insects consuming store-bought plants.
Highlights
Milkweed was collected from stores in the United States and screened for pesticides.
We detected multiple pesticides in every milkweed leaf sampled.
Over one third of samples contained a pesticide at a known harmful concentration.
Labels identifying plants as safe for wildlife are not reliable indicators of risk.
Competing Interest Statement
The authors have declared no competing interest.
