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).
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 Santa Cruz and Pacific Grove) 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.
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.
