In California, the Number of Monarch Butterflies Has Dropped by 30 Percent

Every fall, monarch butterflies from west of the Rocky Mountains start arriving in California to wait out the winter.

The orange and black insects are closely monitored, because the number of western monarchs that come to California each year has dropped precipitously since the 1980s, when it was common to see millions annually.

This past winter, scientists and volunteers went to more than 250 overwintering sites in the state and counted around 233,000 butterflies, a 30 percent drop from the previous winter, according to a report released this week by the Xerces Society for Invertebrate Conservation.

The decline was probably caused by the severe storms that hit California in the winter of 2022, which may have been too intense for the insects to survive, according to Isis Howard, who coordinates the count for the Xerces Society. That caused the breeding season last year to begin with fewer butterflies, reducing the population that would return in the fall.

Monarchs were classified as endangered in 2022. A particularly steep downturn began in the winter of 2018, when about 30,000 monarch butterflies wintered in California, according to Emma Pelton, a monarch conservation biologist with the Xerces Society. Two years later, only 2,000 were counted across the state, and some of the groves that usually attract the most monarchs were devoid of them.

“In 2020, the bottom fell out,” Pelton told me. The moment prompted many “existential conversations in the monarch world” about whether the species would ever recover, she said. But in a “somewhat miraculous” turnaround, she added, the monarch population bounced back to around 200,000 in 2021, a figure similar to this week’s count.