Documenting milkweed (Asclepias spp.) and monarch distribution across the western U.S.

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 (those that overwinter along the California coast) breed in the states west of the Rocky Mountains. The overwintering sites have been well studied, yet very little is known about where and when monarchs breed in the west. 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 monarchs and milkweed stands in the eleven states west of the Rocky Mountains: Arizona, California, Idaho, Nevada, Oregon, Utah, Washington, New Mexico, Colorado, Wyoming, and Montana. If you know where milkweed grows or have observed breeding or migrating monarchs, we’d appreciate you completing this brief survey. It is not necessary to distinguish one milkweed species from another in order to complete the survey; however, we have created guides to the native milkweeds of Oregon, Washington, California, and Nevada to help you identify milkweeds in your region. You can access these guides using the expandable menu below.

The data collected from this survey is being used to help the many agencies and organizations engaged in monarch conservation to best target habitat enhancement and restoration projects in the west. In fact, the Xerces Society has recently partnered with the four western regions of the U.S. Fish & Wildlife Service (USFWS) to model habitat suitability for the western monarch population, using the milkweed occurrence data that we have collected through this survey. If you regularly conduct surveys for your job and have more detailed information to share with the USFWS, please consider using this Excel spreadsheet.

Report Milkweed and Monarch 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.

Milkweeds and Monarchs in the Western U.S. – This guide outlines how land managers can join existing efforts to help western monarchs by identifying and reporting milkweed stands and monarch breeding occurrences on their lands.




Click the image below to view our in-progress Western Milkweed and Monarch Breeding 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.

Milkweed and Monarch Map

Click here for a full list of data providers and references.




Sign up for our newsletter to receive up to date information about our programs and events.

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
Additional Information