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Bay Area monarch butterfly population shows rebound

Millions of monarch butterflies arrive each year in Mexico after travelling, in some cases, thousands of miles from the United States and Canada.
Millions of monarch butterflies arrive each year in Mexico after travelling, in some cases, thousands of miles from the United States and Canada.

Migrating western monarch butterflies used to be found in the millions on the coast of California. In the last two years, their population plummeted to just 2000. This raised fears that California’s monarchs might be unable to recover.

But early signs say they’re rebounding.

While the numbers are still small in comparison to past years, so far, about 10,000 hibernating monarch butterflies have been counted on the coast this fall. That is five times more than the count at Thanksgiving last year, with more on the way.

An article in the journal, "Insects", suggests that may be due to a change in the butterflies’ behavior last winter.

Though they typically breed on the coast, during last winter’s heat wave, monarchs mated and laid eggs inland, in the South Bay. They came to gardens at Mountain View’s Google campus and Palo Alto neighborhoods. This could suggest that butterflies can change as environmental conditions change.

According to the non-profit Western Monarch Advocates, urban gardens and residential areas could be critical. For example, Google started an ecology program in 2014, planting milkweeds and other nectar plants, according to its program manager. The company plans to conduct its own monarch monitoring effort in 2022 to assess the abundance and distribution of monarchs on its campus and understand how they are using the urban habitat.

An entomologist from Washington State University is leading a research project about the Bay Area monarchs. Even though we aren’t having a heatwave like last year, the team is already seeing new eggs this fall, suggesting last year’s rebound wasn’t a one-off, and that this could be the beginning of a new pattern of winter monarch breeding in urban parts of the Bay Area.