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Milkweed, the only food source for monarch caterpillars, was ubiquitously contaminated.


Expert Contacts:

Matt Forister, Trevor J. McMinn Professor of Biology, University of Nevada Reno
(775) 784-6770  |  [email protected]

Sarah Hoyle, Pesticide Program Specialist, Xerces Society for Invertebrate Conservation
(914) 419-0104  |  [email protected]

PORTLAND, Ore.; Monday, June 8, 2020---A study released today in Frontiers in Ecology and Evolution found worrying levels of pesticides in milkweed plants growing in California’s Central Valley. Pesticides were found everywhere that samples were gathered, raising alarms for remaining western monarchs, a population already at a precariously small size.

More than 200 milkweed samples were gathered from urban, agricultural and natural sites as well as from retail nurseries, and tested for pesticides. A total of 64 different pesticides were found, with an average of nine per sample and as many as 25. Samples from all of the locations studied were contaminated, sometimes at levels harmful to monarchs and other insects.

“We expected to find pesticides in these plants,” said Matt Forister, a professor at the University of Nevada Reno, and coauthor of the paper, “but the ubiquity and diversity of pesticides in these milkweeds was a surprise, at least to me.”

Milkweed was chosen as the focus of this study because it is the only food source for monarch caterpillars in the West, and thus critical for their survival. Pesticides were found at levels known to be lethal to monarchs in 32% of the samples.

The current study’s findings paint a harsh picture for western monarchs. Agricultural and retail samples generally had more residues than natural and urban areas, but no area was entirely free from contamination—not even wildlife refuges. Some pesticides were present across all landscapes, with five appearing in more than 80% of samples. Two insecticides, chlorantraniliprole and methoxyfenozide, were found in 91% and 96% of samples, respectively.

“While this research sheds light on how pesticides may impact western monarchs, it is important to note that many other butterflies in the region show even steeper population declines and pesticides are likely one driver of this large scale phenomenon,” said Christopher Halsch, a Ph.D. student at the University of Nevada Reno, and coauthor of the paper.

Western monarchs are celebrated throughout the western states and especially along the California coast where large congregations overwinter in groves of trees. Sadly, over the last few decades their overwintering numbers have plummeted to less than 1% of the population size in the 1980s.

Declines in the population of western monarch butterflies have been linked to various causes, including habitat loss and degradation, pesticide use and climate change. While pesticide use has been associated with declines, studies have not previously attempted to quantify the residues that butterflies can encounter on the western landscape.

“We can all play a role in restoring habitat for monarchs,” said Sarah Hoyle, Pesticide Program Specialist at the Xerces Society for Invertebrate Conservation and coauthor of the paper. “But it is imperative that farmers, land managers and gardeners protect this habitat from pesticides if we hope to recover populations of this iconic animal.”

Areas of inland California, including the Central Valley, offer important monarch breeding habitat throughout the spring and summer. This area supports the first spring generation which will continue the migration inland to eventually populate all western states and even southern British Columbia.

This study was only a first look at the possible risks pesticides pose to western monarchs, but the findings indicate the troubling reality that key breeding grounds for western monarchs are contaminated with pesticides at harmful levels. While this research focused on threats to monarchs, other pollinators and beneficial insects are also at risk from pesticide contamination alongside the butterflies.


For More Information

Read the study published in Frontiers in Ecology and Evolution (open access)
[Citation: Halsch CA, Code A, Hoyle SM, Fordyce JA, Baert N and Forister ML (2020) Pesticide Contamination of Milkweeds Across the Agricultural, Urban, and Open Spaces of Low-Elevation Northern California. Front. Ecol. Evol. 8:162. doi: 10.3389/fevo.2020.00162]

Learn more about how to help western monarchs:

Help us understand the perils facing monarchs in western states with the Western Monarch Milkweed Mapper project:



The Xerces Society is a nonprofit organization that protects the natural world by conserving invertebrates and their habitat. Established in 1971, the Society is a trusted source for science-based information and advice and plays a leading role in promoting the conservation of pollinators and many other invertebrates. We collaborate with people and institutions at all levels and our work to protect bees, butterflies and other pollinators encompasses all landscapes. Our team draws together experts from the fields of habitat restoration, entomology, plant ecology, education, farming and conservation biology with a single passion: Protecting the life that sustains us.

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