Documenting milkweed (Asclepias spp.) distribution to identify available breeding habitat for the monarch butterfly

Monarch butterflies face many threats. In 2008, the tri-national Commission for Environmental Cooperation published the North American Monarch Conservation Plan, which identified several factors that have contributed to the steady decline of monarchs. One important factor is the loss of their breeding habitat due to the ongoing decline of native milkweeds (Asclepias spp.), their required larval host plants.

During the spring, summer, and fall, western monarchs breed in the states west of the Rocky Mountains, including California, Nevada, Oregon, and Washington. The last generation of butterflies that is produced in the fall then returns to the California coast to spend the winter. The overwintering sites have been well studied, yet, very little is known about where and when monarchs breed in each of the western states. The Xerces Society and the Monarch Joint Venture are working to change this situation, and we could use your help!

We have prepared a short web-based survey to gather information about the location of milkweed stands in the western states that potentially serve as important monarch breeding areas. Sites that are identified may then be monitored or prioritized for future conservation activities. If you know where milkweed grows, we’d appreciate you completing this brief survey.

To document the distribution of available monarch breeding habitat, it is not necessary to distinguish one milkweed species from another. It is still possible to complete the survey even if you do not know which milkweed species you have seen. To help you identify milkweeds in your region, we have created guides to the native milkweeds of Oregon, Washington, California, and Nevada. You can access these guides using the expandable menu below.

The survey will be available online through 2015. If you regularly conduct fieldwork as part of your job or frequently spend time outdoors for recreational purposes, please keep an eye out for milkweed plants over the coming year and then complete the survey at a later date, when you have some sightings to report.

Report Milkweed Occurrences


milkweed identification guides

Guide to California’s most common milkweeds
Map of documented milkweed occurrences in California. US Forest Service lands are also shown.

Guide to Nevada’s most common milkweeds

Guide to Oregon’s four native milkweed species
Map of documented milkweed occurrences in Oregon. Bureau of Land Management and US Forest Service lands are also shown.

Guide to Washington’s three native milkweed species
Map of documented milkweed occurrences in Washington. Bureau of Land Management and US Forest Service lands are also shown.

more information about monarchs or milkweed

University of Minnesota’s Monarch Larva Monitoring Project has images of monarch caterpillars at numerous stages of development.

Journey North has photos of adult monarch butterflies.

Monarch Watch, a nonprofit educational outreach program based at the University of Kansas, has compiled profiles of milkweed species.

Read more about the North American monarch migration here.

Read about how Xerces is working to restore monarch breeding habitat by increasing the availability of native milkweed seed.




Click the image below to view our in-progress Western Milkweed Map, which is based on responses to our online milkweed survey in addition to the published literature, unpublished reports, online herbaria, and knowledgeable researchers and citizen scientists.

Western Milkweed Map




to protect invertebrates!


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Program Features
Program Highlights
  • • The Xerces Society has awarded two $3,750 Joan M. DeWind awards for research into lepidoptera conservation
  • Butterfly-a-thon pledges raise $30 per species that Bob Pyle observes for butterfly conservation work
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The Xerces Society • 628 NE Broadway Ste 200, Portland OR 97232 USA • tel 855.232.6639 • fax 503.233.6794 •
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