About


Western Monarchs

The monarch butterfly is well known for its long-distance seasonal migration and its spectacular winter gatherings. In the western United States, monarchs usually migrate to the California coast, which is the only place in the country that regularly hosts the awe-inspiring sight of thousands of monarchs gathered for winter. These days, most of California’s monarchs cluster in groves of nonnative eucalyptus, although they also use native trees such as Monterey pine, Monterey cypress, and redwood.

Each spring, monarchs disperse across California and several western states, searching for milkweed plants on which to lay their eggs. Milkweed is the only plant monarch caterpillars can eat to grow and develop into adults. Several generations are produced throughout the spring, summer, and fall, with each generation spreading further across the landscape. The last generation then migrates all the way back to the overwintering grounds in the fall. Remarkably, monarchs return to the same groves of trees as their ancestors.

See our North American monarch migration map here.

What is the Western Monarch Count?

The Western Monarch Count is an annual effort of volunteer citizen scientists to collect data on the status of monarch populations along the California coast during the overwintering season, which occurs from approximately October through March. The height of this volunteer effort occurs during the Western Monarch Thanksgiving Count, which runs for three weeks surrounding the Thanksgiving holiday. Thanks to the extraordinary effort of a cadre of volunteers, we now have over 19 years of data demonstrating that monarchs have undergone a dramatic decline estimated at 74% in the western U.S. since the late 1990’s. This effort has been coordinated by Mia Monroe, Shawna Stevens, and Dennis Frey in the past, and is now coordinated by Mia and The Xerces Society for Invertebrate Conservation.

New! In January 2017, The Xerces Society and Western Monarch Thanksgiving Count volunteers expanded their efforts to monitor western monarchs for a second period each winter. This first New Year’s Count includes a smaller subset of the overwintering sites monitored during the Thanksgiving count period and we could use YOUR help to expanding the number of sites monitored around the New Year’s holiday. Contact wmtc@xerces.org or your regional coordinator to learn more.

How Can You Participate?

Are you interested in joining this effort? We are looking for volunteers in California who can commit to visiting the same overwintering sites year after year. Most of what we know about the population trends of western monarchs is a result of the incredible work done by volunteers like you. Roughly 100 overwintering sites from Mendocino to San Diego Counties are monitored each year, but we know of approximately 450 sites that monarchs have either used in the past or are currently using. With your help we can start monitoring more sites and get a better picture of the status of these sites! All the tools you need to participate can be found on this website. You can start by finding an overwintering site near you.

Don’t live near an overwintering site? There are other things you can do to help!
  1. Plant native milkweed. This can be in your backyard, at your workplace, or at your school. Find sources of local, native milkweed seed in your state using our Milkweed Seed Finder.
  2. Plant native flowers. Monarchs need nectar to provide energy to migrate, breed, and overwinter. Flowers can be planted anywhere, including overwintering sites.
  3. Support organic and GMO-free agriculture.
  4. Avoid using insecticides and herbicides. These may kill butterflies or caterpillars, or kill the plants that monarchs use for nectaring or breeding.
  5. Get involved in another citizen science project.
  6. Support the Xerces Society’s monarch conservation efforts.