Monarch (Danaus plexippus) is a large butterfly, with a wingspan of 3.875 to 4.875 inches. It can be seen all year round in our local area, with numbers peaking in October-November. There is relatively little variation in the numbers from December through August, but they jump a little in September. Host plants are primarily various species of Milkweed (Asclepias), but also Balloonplant (Gomphocarpus physocarpus).

The largest Monarch populations are found in North America (U.S., Canada, Mexico), but they are also seen in some other parts of the world: Central and South America, Hawai’i, the Caribbean islands, New Zealand, eastern Australia, south Pacific islands, and western Europe/Morocco. However, only the North American population engages in a significant migration. Some other populations exhibit minor migrations, while many do not migrate at all. 

In December 2024, the U.S. Fish & Wildlife Service proposed listing the Monarch as a threatened species under the federal Endangered Species Act.  If the proposal is approved, they expect to add it to the threatened species list by the end of 2025. As part of this move, 4,395 acres in coastal California would also be listed as critical habitat for Monarchs. The first step in the listing process was a period of public comment. The initial 3-month period was extended through May 19, 2025.

There are really two different Monarch populations in our area. A migrating population flies north in the spring in 3 or 4 generations, eventually reaching Canada. The last generation then migrates south in the fall, and the butterflies overwinter in large clusters in California at sites along the central coast (notably Pacific Grove, and two sites in Santa Cruz) and the southern coast (notably Pismo Beach). Most years, there are small over-wintering clusters in Berkeley, the largest clusters being found on Albany Hill (up to 500 or 600 butterflies in recent years). Peak numbers at Pacific Grove were 7,604 in 2023; 15,960 in 2022, and 13,708 in 2021. Natural Bridges State Beach in Santa Cruz had 8,000 in 2023 and 7,500 in 2022. Pismo Beach, the largest overwintering site in California that is open to the public, had 16,347 in 2023, 24,128 in 2022, and 20,871 in 2021.

Then there is a permanent urban population, anchored by the milkweed that so many people now grow in their yards to help arrest the precipitous decline in the Monarch population over the last few decades. These butterflies are what we continue to see locally after the migrating population has headed north. A 2021 study estimated the number of Monarchs in urban gardens in northern and central California at roughly 12,000. It is not currently known how much the numbers may fluctuate from year to year.

Monarchs east of the Rockies migrate south to Central Mexico, overwintering in massive clusters in Oyamel Fir forests in 12 mountain areas in the states of México and Michoacán.

Periodically, the population of overwintering western Monarchs drops precipitously. The year from hell was 2020, when the Xerces Society for Invertebrate Conservation counted only 1,901 for the entire west coast, from Mendocino County to Baja California. Numbers bounced back handsomely the next three years (more than 230,00 each year), but 2024 evidenced a huge decline again, with a peak population of just 9,119 Monarchs.

Lighthouse Field in Santa Cruz had the largest overwintering population in 2024 (which was unusual). They counted 1,178 Monarchs on November 1st, and 1,307 on November 29th. This compares to 8,511 on December 2, 2023.

The other three main sites showed much steeper declines in 2024 compared to the previous year. Pacific Grove had only 202 on December 6th, compared to 5,228 on December 8, 2023. Natural Bridges State Beach in Santa Cruz had only 50 on December 8th, versus 8,000 on December 14, 2023. Pismo Beach had 197 on December 4, 2024, compared to 16,044 on November 27, 2023.

Locally, 2024 numbers were down sharply at Albany Hill.  There were only 8 or 10 on December 6th compared to roughly 300 at the same time the previous year. The largest (and only significant) local cluster in 2024 was in the Gill Tract Community Garden in Albany, with roughly 200 Monarchs on December 1st..