Why are Western Monarch butterflies numbers rising?

By Richard Rachman

Western monarch numbers are rising all across the west coast, from just a few thousand last year to perhaps over 100,000 this year. Why? It’s a mystery, but insects have been known to expand and contract in population year to year. Overall, this may not be cause for too much excitement yet as overall the western monarch butterfly is in trouble. We still have a lot of work to do.

Xerces Society has been monitoring overwintering monarchs for years. Eastern and western monarchs live on different sides of the Rocky Mountains, but both populations migrate during the winter to warmer areas to wait out the winter, flying back inland and north in the Spring to mate on their larval host plant, milkweed. The best time to see how these migratory butterflies are doing is to count them when they are overwintering and clustering in tight groups on the branches of trees.

Overwintering monarchs; photo courtesy of the Xerces Society

Volunteers all across the state have been counting monarchs on branches up and down the coast. There are hundreds of new and historic sites where monarchs return year after year to overwinter, some of the more famous sites being in San Luis Obispo and Santa Cruz County. Butterflies will cluster when the temperature is around 55F at sunrise, trying to warm up on the east side of large trees blocked from the wind. This is when volunteers with binoculars count and collect data for the community science effort. The butterflies then warm up and nectar feed on plants close by, before returning to a branch for the night.

How can we help overwintering monarchs?

Well if you live by the coast, you can take care of trees that monarchs may be using to rest and overwinter in, these include native trees like Torrey Pine and Monterey Cypress, as well as non-native trees like Bluegum Eucalyptus. You can also plant nectar plants around your yard or balcony so that overwintering monarchs and other wildlife can feed during the day. Native flowers and shrubs are best, as these are what insects have adapted to feed off of, also they require less water and often no fertilizer or pesticides. Use CalScape to find what plants and nurseries that have those plants are native to your neighborhood, or become a member of your local California Native Plant Society Chapter.

You can help monarchs, not just by planting native plants in your yard, but also by contributing to community science projects like the Thanksgiving and New Years County with Xerces Society. Train to become a monitor for monarch butterflies overwintering at a site near you. It’s a great holiday tradition, and the data is extremely useful to scientists and land managers working to conserve monarch butterfly habitats. Make sure to also take pictures of monarchs and milkweed plants and post them on iNaturalist in the Monarch Milkweed Mapper project. iNaturalist is a great way to not only contribute to community science but to learn more about the plants and animals in your community.

Richard is an LA County Volunteer Coordinator for the Xerces Society Thanksgiving and New Years Overwintering Monarch Count Botanist. SBPC is excited to participate in this new tradition at Wilderness Park this year.

Additional Resources:
https://www.westernmonarchcount.org/
https://phys.org/news/2021-11-monarch-butterflies-california.html
https://www.monarchmilkweedmapper.org/