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

The decline of monarch butterflies in both the eastern and western United
States has garnered widespread public interest. Planting milkweeds, their
larval host plants, has been promoted as one action individuals can take,
but little is known with respect to potential pesticide contamination of
store-bought milkweeds. In this study, we collected leaf samples from 235
milkweed plants purchased at 33 retail nurseries across the US to screen
for pesticides. Across all samples, we detected 61 different pesticides
with an average of 12.2 (±5.0) compounds per plant. While only 9 of these
compounds have been experimentally tested on monarch caterpillars,
38 % of samples contained a pesticide above a concentration shown
to have a sub-lethal effect for monarchs. We detected only a modest
predictive ability of retailer size and milkweed species, and plants with
labels advertising their value for wildlife did not 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 exposed
to store-bought plants.