Migratory monarch nectaring on Mexican sunflower, Tithonia rotundifola in a Vacaville garden. (Photo by Kathy Keatley Garvey)
Since Saturday, Sept. 27, we've recorded almost daily sightings of migrating monarchs passing through our pollinator garden in Vacaville on their way to their overwintering sites along coastal California.
The monarchs are stopping by for some flight fluel--nectar from our stand of Mexican sunflowers, Tithonia rotundifola, that reach 12 feet high.
Sometimes the monarchs sip nectar from our milkweed.
"With the monarch migration nearing the end point, butterflies should be accumulating in California ready to form overwintering colonies at their traditional locations along the coast. Temperatures in the Bay area have been typical for September, and most migrants should move to the coast rather than establish breeding populations. By the end of this month, overwintering colonies should be at least 50% formed at sites like Lighthouse Field, Natural Bridges, Pacific Grove and Pismo Beach. Analysis of monarch sightings in the PNW since mid-August provides some optimism that the size of the overwintering population this year will be larger, IF what we are seeing is also occurring in Utah, Nevada, Arizona and California. Briefly, in 2023, 146 monarchs were sighted in the PNW between August 15 and September 30 which was followed by an overwintering population of more than 233,000. In 2024, only 71 monarchs were sighted, followed by the second lowest recorded overwintering population of just over 9,000 individuals. This year during August 15-September 30, there have been 105 sightings, primarily in Oregon and Idaho, almost a 48% increase over 2024. If we translate this to the overwintering numbers, we might expect an increase this year to around 13,000 butterflies. This could be larger or smaller depending on how monarchs are faring in other western states. A quick look at the iNaturalist numbers for UT, NV, AZ and CA indicates there have been almost double the number of sightings during August 15-September 30 this year than during the same period last year! Based on all these data, I would expect a modest increase in the overwintering population to around 15,000."
What's the status now of the overwintering monarchs in California?
On Oct. 15, James emailed us this update: "Well, the community scientists in Pacific Grove tell me there are no monarchs overwintering there yet. Slightly better news in Santa Cruz where there are approximately 100 at Lighthouse Field and maybe 50 at Natural Bridges. Similar to this time last year. So, I have no reason to change my predictions from what I wrote recently. What happens in the next 2 weeks, however, will tell the story."
