A new study published in Environmental Toxicology and Chemistry found nearly ubiquitous pesticide contamination on butterfly host plants across two U.S. cities, Albuquerque, New Mexico, and Sacramento, California. In 20% of the plants tested, pesticide residues were at levels known to kill or otherwise harm butterflies and moths.
Scientists from the Xerces Society for Invertebrate Conservation and the University of Nevada, Reno, sampled 336 individual plants across the two cities. They detected pesticide residue in 314 of these plants, with an average of three compounds per plant and a maximum of 18 pesticide compounds on a single plant. In Sacramento, the fungicide azoxystrobin was found in 84% of samples, and the insecticide methoxyfenozide was present in 78% of samples. This is similar to previous research in California’s Central Valley, which found methoxyfenozide in 96% of sampled milkweed plants. The herbicide atrazine was found in 70% of samples in Albuquerque.
In 71 samples, concentrations of the insecticide chlorantraniliprole or the fungicide azoxystrobin exceeded the levels known to kill or cause other negative impacts to butterflies.
“Pesticides are one of the leading causes of butterfly declines,” said study co-author Aaron Anderson of the Xerces Society. “We know that in addition to killing butterflies or their caterpillars outright, pesticide exposure can cause subtle changes that could severely harm them. For example, one of the fungicides we detected causes decreased wing size in monarch butterflies, which could hurt their ability to migrate long distances.”
Anderson emphasized that the specific impacts most of the detected pesticides in this study have on native butterfly species are still unstudied, so these findings likely underestimate the true, cumulative risk contaminated plants pose to butterflies in these cities.
A recent study published in Science found that butterflies across the U.S. have suffered a startling decline, with a 22% decrease in abundance between 2000 and 2020. To recover these species’ populations, butterflies require host plants for caterpillars to feed, flowers for adults to visit for nectar, and protected overwintering sites. But this habitat must be protected from pesticide contamination. Previous research has found that pesticide use is a leading cause of butterfly declines, but little research has been conducted on pesticide contamination in urban invertebrate habitat.
“Urban communities do have potential to be a refuge for butterflies, birds and other wildlife,” said Aimee Code, the Xerces Society’s pesticide reduction program director. “To attain that goal, we have to address pesticide contamination, which this study shows is very widespread. Fortunately, many home gardeners, farmers and local governments are already making progress.”
Not all the pesticides detected in this study were from applications within city limits. For example, the herbicide thiobencarb was detected in Sacramento, which can only legally be applied to rice in California. The researchers say this illustrates that pesticide drift and runoff can contribute to contamination, even in locations far from the application site.
Like canaries in a coal mine, butterflies indicate the health of the environment. This study helps us understand the role pesticides play in butterfly declines and will help hone recovery efforts.
