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).
In December 2024, the U.S. Fish & Wildlife Service proposed listing the Monarch as a threatened species under the federal Endangered Species Act. They expect to add the butterfly to the threatened species list by the end of 2025 following an extensive period of public comment. As part of this move, 4,395 acres in coastal California would also be listed as critical habitat for Monarchs.
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.
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 Xerces Society counted less than 2,000 in 2020 (for the west coast as a whole, from Mendocino County to Baja California). Numbers bounced back handsomely the next three years (more than 200,00 each year), but 2024 is evidencing a significant decline again.
This year, Lighthouse Field in Santa Cruz has the largest overwintering population (which is unusual). They had 1,178 on November 1st, and 1,307 on November 29th. This compares to 8,511 on December 2nd last year.
The other three main sites are all showing much steeper declines from last year’s numbers
Pacific Grove has only 202 as of December 6th. There were 94 on November 16th. Last year on December 8th, there were 5,228.
Natural Bridges State Beach in Santa Cruz has only 50 as of December 8th, versus 175 on November 28th, which was a sharp decline from 2,150 in late October. Last year, on December 14th, they had 8,000. For additional context, this year’s peak of 2,150 does exceed 2018's peak of 1,120, and 2019's peak of 1,997.
Pismo Beach has roughly 200, as of December 5th. There were 554 on November 20th (before the atmospheric river storm). That compares to 16,044 on November 27 last year.
Locally, numbers are down sharply at Albany Hill. There were only 8 or 10 on December 6th; at the same spot, there were 300 at this time last year. The largest (and only significant) local cluster seems to be in the Gill Tract Community Garden in Albany. There were roughly 200 Monarchs there on December 1st.
(These numbers will be updated if they change significantly over the rest of the winter.)