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Due to the additional aspects of the overwintering sites as well as the size of the state of California, updates are provided by three liaisons:  Terry Smith for Northern CA/Bay Area,, Diana Magor for Central CA, and Susie Vanderlip for Southern CA, Please contact them for additional information.


Collapse of Migration to the Coast and Expansion of Resident Population in California


This past November 2020 thru March 2021, California Monarch behavior has seen significant change.


Monarch butterflies that breed in North America west of the Rocky Mountains are known as western monarchs. In the late fall, most western monarchs have migrated to the California coast and spent the winter (late November to February) clustering in groves of pine, cypress, and eucalyptus trees, during which time they do not breed (reproductive diapause). As temperatures warm and daylight hours increase, they mate and migrate inland to lay eggs on emerging milkweeds. However, this 2020-2021 winter, the phenomenon of western monarch migration changed. A very meager number of monarchs migrated to the coast, and a significant number overwintered and continued to breed inland in northern and southern California.


To put it into perspective:
According to Xerces, 4.5 million monarchs overwintered on the California coast in the 1980’s. By 1997, there were about 1.2 million monarchs. As of 2015, that number had declined to 293,000. Two years ago, the decline fell to 30,000. And, according to Monarch Joint Venture, a total of only 1,642 monarchs were counted at Thanksgiving and 1042 at Christmas across over 200 coastal overwintering sites.

This past November 2020, a significant number of monarchs were found inland, and bred throughout the winter sustained by non-native tropical and balloon milkweed. A large “urban” or resident population grew dramatically across the state. Many gardeners from San Diego through the San Francisco Bay area observed this throughout the winter in volumes unseen before. Reports of dozens of eggs being laid on mostly non-native milkweed all winter led to reports of hundreds of monarchs eclosing (emerging) from chrysalides January thru March 2021.

A confluence of factors seems to have led to this situation. According to Dr. Chip Taylor, Founder and Director of Monarch Watch, the primary cause is climate change. California is heating up faster than nearly all other states in the US. The warmer fall and winter in Southern California make it possible for monarchs to stay inland rather than migrate to the coast. This is true of Northern California in mild years like 2020-2021 as well. The availability of milkweed in the fall, winter, and early Spring due to the introduction of tropical milkweed from central and south America provides the host environment for monarchs to continue to breed and reproduce inland.

Many Californians and conservation groups are asking, what can individuals do to help the monarch population sustain, grow, and possibly recover the population of western monarchs. First and foremost, it is recommended that as much monarch butterfly habitat as possible be restored and developed. Everyone is encouraged to plant organically grown milkweed away from overwintering sites, and nectar plants for monarch butterflies throughout California. Native milkweed is highly encouraged. However, with the wide availability of Tropical (Asclepias curassavica) in large retail stores, a great deal of tropical has been planted. It is recommended that non-native milkweed (tropical and balloon) be pruned in mid-October and at other times during the year. Pruning includes cutting the stems to 6” length and stripping off all the leaves. This is necessary to reduce the microscopic parasite O.e. that accumulates on non-dormant milkweeds which weakens and/or kills monarchs.


Several types of California native milkweed can be planted, with plants available April through October.  To identify the native milkweed for your area, use the Calscape website ( )  and search by zip code. Beware of milkweed purchased from large retail stores such as Home Depot, Lowes, and most Armstrong’s Gardens since it is non-native (tropical) milkweed regardless of flower color and often treated with pesticides, especially neonicotinoids, although recently these stores have responded to customer requests and  been selling untreated plants.


Of even more importance for the survival of California monarchs is the reduction of pesticides and herbicides everywhere in our ecosystems. Gardeners are encouraged to eliminate all pesticide use. Many monarch caterpillars are killed or weakened by ingesting plants that have been grown with pesticides or in gardens where they’re used. Neonicotinoids used by growers to bring plants to large retail stores are serious killers of pollinators from bees to butterflies. (Pesticides | The Monarch Joint Venture). The weed killing herbicide glyphosate (key ingredient in RoundUp and other herbicides) is another known toxin for milkweeds. Please participate in responsible pesticide use in our environment. Learn more from Pesticide Action Network (PAN) PesticideInfo | California Pesticide Use


Stay tuned for updates and recommendations for monarch butterfly gardeners and habitat creation across California.   For more information on California monarchs by region, see below.


Northern California | Bay Area & Surrounding

Reported by Terry Smith – Pollinator Posse Co-founder


Northern California, in particular the East and South Bay Areas, has continued to experience an unusual number of resident Monarchs throughout the winter and early spring.  Less than 150 Monarchs were counted at northern overwintering sites during the Xerces Thanksgiving count, yet Inland, adult Monarchs were sighted in significant numbers from December through March.  There were also unusually high numbers of caterpillars reported for the winter months, almost exclusively on non-native milkweed due to the dormancy of most native milkweed.  The numbers of caterpillars and adults declined through January and February, picking up again in late March. Northern California experienced a mild winter in general with low rainfall. Caterpillars and chrysalides that were observed took a much longer time to reach maturity, as would be expected given the cooler temperatures, and outdoor survival rates were reported as very low.  By mid-March there was a significant increase in the number of adults sighted - flying, mating, and egg laying.  With the native milkweed just beginning to appear, large numbers of eggs have been reported on tiny plants just breaking ground – up to 15 eggs on a single small sprig.  By the end of March, narrowleaf milkweed was sprouting and showy was beginning to appear.  You can find more detailed information based on data collected at the Pollinator Posse website under the resources tab – just look for Citizen Science Projects.

Monarch habitat has been established in a large number of  both home and public gardens in Northern California, and several projects are currently underway.  Xerces, Monarch Joint Venture and RCD’s in the area are working to establish habitat on farmland and work is also being restoring some traditional overwintering sites.  At last count 4 mayors have signed on to the World Wildlife Fund Mayor’s Monarch Pledge and more are considering it.   For more information and details of projects throughout Northern California link here.

Central California

Reported by Diana Magor -  Citizen Scientist

The Fall 2020-Winter 2021 “migration” was, to say the least, unusual.

The few surviving Western monarchs seemed to choose to overwinter either on the coast, or inland. Only 1,642 (recently revised) were counted by observers along the California coast at the New Years Count The largest aggregation in the state (550) was observed in November at Natural Bridges State Beach in Santa Cruz. About 100 monarchs were seen at Lighthouse Field. Many of the migrating monarchs may have remained inland (an unofficial estimate by Elizabeth Crone for inland monarchs was 12,000). See reports on N. Cal and S. Cal.

Overwintering behavior. There appeared to be differences in behavior between the monarchs that migrated to the coast (approximately 550 clustered in shade at Natural Bridges and were mostly inactive for 2-3 months, which helps extend their fat reserves to survive cold winters and migrate to their breeding grounds in the Spring).  The overwintering clusters at Natural Bridges broke up by 1-25-21 with only about 20 of the original 550 remaining.

Many Santa Cruzans saw gravid females ovipositing on new milkweed shoots in January and February. Even when there was no active milkweed in the garden, male monarchs patrolled regularly apparently searching for females.

A major improvement to the native milkweed availability would be to extend the growing season for natives, now dying back early in the late summer/fall due to drought and removal of watersheds, by reintroducing beavers to revive riverscapes & extend watersheds. See Reviving Riverscapes, Science News March 27, 2021.

In Carmel Valley, Scott Hennessy has been raising a form of Asclepias fascicularis that breaks ground earlier than the standard form, stays greener well into Fall, apparently responding extremely well to occasional watering, and has broader leaves. Connie Masotti has seen a few monarch caterpillars on Scott’s test sites at Palo Coronado recently, but found no monarchs at the Pacific Grove site in the ‘20-21 winter.

2 Favorite Monarch nectar plants for home gardens covering all seasons. 

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Monarch nectaring on yellow evergreen Mexican Sunflower bush, Tithonia diversifolia, Blooms July to April in Central and S. California.

Southern California

Susie Vanderlip – Monarch Conservation Specialist, Monarch Watch


December thru March 2021: Resident monarchs were seen breeding in gardens across the southland, from Ventura to San Diego and inland to Riverside cities including Redlands, Palm Springs, and La Quinta.

Southern California cities where people reported observing monarchs breed, oviposit, caterpillars eating, chrysalides, and newborn monarchs from January thru March 2021 included:

  • Anaheim

  • Buena Park

  • Burbank

  • Chatsworth

  • Chino Hills

  • Costa Mesa

  • Cypress

  • Fountain Valley

  • Garden Grove

If you are in or interested in southern California monarch activity, you are encouraged to join the new private Facebook group dedicated to southern California monarch observations:
“Monarch Butterfly Guardians (southern California)”

Events and Projects Across the Southland


Many new creative monarch projects have been started and/or expanded by southern California agencies, associations, businesses, and individuals from December 2020 thru March 2021. It is heartening to see the level of interest in helping monarch butterflies in across southern California. Hopefully, one of these projects will spark interest and creativity with you, our readers! Some of these interesting and creative projects are from the following:

  • Resource Conservation District of Greater San Diego County

  • Ventura Resource Conservation District

  • Sepulveda Basin Wildlife Preserve

  • Front Porch Retirement Communities

  • City of West Hollywood

  • Girl Scouts of Orange County

  • Laguna Woods Village for 55+

  • California Associations Institute (CAI)

  • Discovery Network and Toyota Motors of North America

Read details on these and future projects at Monarch Projects Across the Southland

If you know of or part of a Monarch Butterfly habitat project in southern California, please email me at

Informative Resources

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