Social butterflies: Western monarchs
Insects’ numbers back up on California coast
SAN FRANCISCO — The population of western monarch butterflies wintering along the California coast has rebounded for a second year in a row after a precipitous drop in 2020, but the population of orange-andblack insects is still well below what it used to be, researchers announced Tuesday.
Volunteers who visited sites in California and Arizona around Thanksgiving tallied more than 330,000 butterflies, the highest number of these insects counted in the last six years. It was a promising rebound after the annual winter count in 2020 recorded fewer than 2,000 butterflies. In 2021, the number recorded was 247,000.
“I think we can all celebrate and this is really exciting,” said Emma
Pelton, a conservation biologist at the Xerces Society, a nonprofit environmental organization that focuses on the conservation of invertebrates. “We were all so relieved last year when we had about 250,000 butterflies, and to see that number tick up even modestly this year it’s really a good sign that we’ve got a second chance.”
Pelton said it’s not clear why the population has rebounded but one explanation could be that eastern monarch butterflies, which tend to spend the winter in Mexico, could be mixing with their western counterparts.
The population is still far below what it was in the 1980s, when monarchs numbered in the millions.
Scientists say the butterflies are at critically low levels in western states because of destruction to their milkweed habitat along their migratory route as housing expands and use of pesticides and herbicides increases.
Climate change also is a main driver of the monarch’s threatened extinction, disrupting an annual 3,000-mile migration synced to springtime and blooming wildflowers.
Western monarch butterflies head south from the Pacific Northwest to California each winter, returning to the same places and even the same trees, where they cluster to keep warm. The monarchs breed multiple generations for thousands of miles before reaching California where they generally arrive at the beginning of November. Once warmer weather arrives in March, they spread east of California.
On the eastern side of the Rocky Mountains, another monarch population travels from southern Canada and the northeastern United States across thousands of miles to spend the winter in central Mexico. Scientists estimate the monarch population in the eastern U.S. has fallen about 80 percent since the mid-1990s, but the drop-off in the western U.S. has been steeper — more than 95 percent, earlier studies show.