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Confirmation of monarch butterfly breeding in winter shows hope for western population

Posted by Scott Weybright | November 1, 2021
A new study led by Washington State University has found western monarch butterflies can successfully breed and maintain populations on ornamental milkweeds during winter at urban sites in the South Bay of San Francisco.

Butterfly sits almost upside down close to the ground near green leaves.
A female Monarch lays eggs in the San Francisco bay area in February, 2021.

Western monarch breeding traditionally happens in the Pacific Northwest after the butterflies migrate there from California in the spring. Prior to this paper, published in the journal Insects, significant winter breeding had not been confirmed in the San Francisco Bay area.

The population of western monarchs has dropped from around 300,000 counted three years ago to fewer than 2,000 in the 2020-21 overwintering season. It’s hoped the adaptation to winter breeding in California will give the struggling insects a better chance at survival.

“We found that monarchs can breed well in the South Bay area during winter supported by availability of non-native milkweeds that stay green,” said David James, an associate professor in Washington State University’s Department of Entomology and lead author of the paper. “We also showed some evidence that the population may join the migrant population in spring heading north into the PNW.  So, this shift in strategy by monarchs may not be a population ‘sink’ as some scientists have feared.”

The paper is confirmation of a commentary article James wrote earlier in 2021 that was based on multiple observations.

The new paper relied on citizen scientists living in the cities like Palo Alto and Mountain View, California for data collections. The citizen scientists include paper co-authors Maria Schaefer, Karen Kimmer Easton, and Annie Carl.

Additional observations in the Pacific Northwest this summer showed that monarch populations were much higher than anticipated.

“Current estimates suggest there are already five times as many as there were last year,” James said. “This unexpected rebound in population size may have something to do with the winter breeding population contributing to the overall population.”

A previous article written about the commentary that James published in May includes more detail about the importance of these findings.