Another factor, she said: Some homeowners, eager to attract monarchs, have planted tropical milkweed. Although the butterflies will feed on them, these plants tend to spread disease, because they don’t drop their leaves, Dr. Schultz said, which may be contributing to the declining monarch population. Native milkweed supports the population without this risk, she said.
Climate change also plays a role in the challenges facing monarchs and other butterflies, said Chip Taylor, a professor emeritus at the University of Kansas, who also directs Monarch Watch, a network of students, teachers, volunteers and researchers. Temperatures in the Western monarch’s overwintering sites along the coast now average 2 degrees higher in January and February than they did just two decades ago — the highest rate of increase outside Alaska, he said.
Western monarchs are quite similar to their Eastern cousins, just a bit smaller and darker, Dr. Crone said. But they have a distinct migratory pattern.
While the Eastern monarchs migrate from Mexico to as far away as New England and Southern Canada, the Western ones mostly remain in Southern California or migrate from the mid-coast up as far as British Columbia and as far east as the Rockies, Dr. Schultz said.
In the last couple of years, she said, the range of their breeding grounds has been contracting. The butterflies are going inland as far as Nevada, but they’re not making it as far as Washington State anymore.
Like other insects, butterflies often have good years and bad. “Butterfly populations are bouncy,” Dr. Schultz said. “While we think the situation right now is very concerning, we do think there’s a lot of potential to turn it around.”