Butterfly numbers plummet in West
The numbers are grim. This year when the western monarch butterfly made its annual wintertime stop in Pacific Grove and other parts of California, the number of fluttering blackandorange insects added up to only about 7% of last year’s population, which already was a fraction of its usual amount.
Probably the most beloved and recognized butterfly in the United States, the western monarch is essentially on the brink of extinction, said Katy Prudic, coauthor of a new report from the University of Arizona that found that the
monarchs, along with about 450 butterfly species in the Western United States, have decreased overall in population by 1.6% per year in the past four decades. The main culprit seems to be rising temperatures in the fall.
“So as you look across the entire West — this includes urban areas, suburban areas and wild areas — the biggest correlate with decline is fall warming,” said Prudic, an assistant professor at the University of Arizona School of Natural Resources and the Environment. “It looks like these declines are pretty consistent across each state.”
This year fewer than 2,000 western monarch butterflies were recorded in the Xerces Society for Invertebrate Conservation’s annual count of the insect when it overwinters in coastal areas from the Mexico border to Mendocino County. That compares to around 29,000 last year and several hundred thousand in 2017 — overall its population has dropped by 99.9% since the 1980s.
“We may be losing the spectacular migratory phenomenon of urban resident monarchs,” wrote Stephanie McKnight, conservation biologist in the endangered species program at Xerces Society, in a recent blog post announcing the latest count. “The West without migrating monarchs would be a very different place.”
The World Wildlife Fund also recently that the area of forest monarchs overwintering in Mexico had declined by 26% compared to last year.
Surprisingly, though, the monarch, or Danaus plexippus, is only in the middle of the pack when it comes to population decline among western butterflies. Others that have suffered declines include the cabbage white, or Pieris rapae, which thrives in otherwise degraded habitats, and the West Coast lady, or Vanessa annabella, a species with broad migration ranges. The monarch tends to get the most attention because it’s what Prudic calls “the panda butterfly” — and one of North America’s most beloved creatures.
Rising fall temperatures likely cause stress to butterflies because they are ectotherms that depend on external sources for body heat and can’t regulate their own temperature. When summer heat lasts into fall, the insects and their eggs can become dehydrated. When temperatures don’t drop, they also don’t get the signal to prepare for hibernation, a series of physiological changes that include slowing metabolism. So even if they can reproduce, they don’t make it to hibernation or migration in winter.
“They are so stressed they are dying. If they lay any eggs, their eggs are dying,” she said. “They can’t find enough food, and they’re dying from overheating.”
However, Prudic said there are many things people can do to help butterflies, including planting nectar plants that flower in fall, such as milkweed, sunflowers and pea plants, even in a window flower box, and avoiding using pesticides in the garden. She also recommends volunteering in habitat restoration or as a citizen scientist to collect data, and supporting wildlife conservation efforts politically.
“Moving forward we need to be thinking about urban spaces being healthy spaces,” she said, “for insects as well as people.”