Milkweed plants purchased at retail nurseries across the United States were contaminated with pesticides harmful to monarch caterpillars that rely on milkweed, a study led by researchers at the University of Nevada, Reno found. Every plant sampled was contaminated, even those that were labeled friendly to wildlife.
Researchers gathered 235 milkweed leaf samples from retail nurseries across 15 states and tested them for pesticides. A total of 61 different pesticides were found, with an average of 12 per plant and as many as 28 per plant, according to the study in collaboration with the Xerces Society for Invertebrate Conservation and published Monday in the peer-reviewed science journal Biological Conservation.
Milkweed in nurseries are often purchased and planted by people hoping to support the monarch butterfly, which was recently listed on the IUCN Red List of Threatened Species™ as endangered. Western monarch caterpillars depend on milkweed for food. They are specifically adapted to the plant, which is toxic to other animals, and don’t have alternative sources of food.
“In a previous study in California that primarily looked at milkweed in agriculture and urban interfaces, we had looked at a small number of plants from retail nurseries, and found that they contained pesticides,” Matt Forister, a biology professor at the University who studies insect ecology and is a coauthor of the paper, said. “So we were prepared for this much larger sample of nursery plants to again uncover contamination, but it was surprising to see the great diversity of pesticides found in these plants. In many ways, they are as contaminated or even worse than plants growing on the edges of agricultural fields. That was a surprise, at least to me.”
While researchers couldn’t fully assess the toxic load carried by these plants, 38% of the samples had residue levels that could