Western Monarch Butterfly ‘Migration Is Collapsing,’ Scientists Say. You Can Help

Western monarch butterflies once arrived on California’s coast by the millions, clustering in eucalyptus, pine and cypress groves from Mendocino to Baja California. This winter, volunteers counted fewer than 13,000.

The 29th annual Western Monarch Count, conducted during the peak overwintering period in late November and early December, is the third lowest since monitoring began in 1997, following another poor winter and weak summer breeding. The three lowest tallies have occurred in the past six years — 1,901 monarchs in 2020, 9,119 in 2024 and 12,260 in 2025 — raising concerns about the long-term health of the western monarch migration.

In the 1980s, western monarchs regularly numbered in the low millions. Today’s numbers are not surprising but a sign of a migration under sustained stress.

“Western monarchs are in serious trouble. The migration is collapsing,” said Emma Pelton, a senior conservation biologist with the Xerces Society for Invertebrate Conservation, which carries out the annual count.

Long-term research shows western monarch populations have declined by about 10% per year since the 1980s, driven largely by habitat loss, pesticide use and climate change.

Milkweed — the only plant monarch caterpillars can eat — has disappeared from much of the West due to development and herbicides. Insecticides can poison monarchs directly or reduce the nectar plants that adults rely on. And climate change is intensifying heat waves, droughts and storms that disrupt the butterflies’ breeding and migration.

“When populations get this small, they become far more vulnerable to both short-term stress and long-term pressures,” said Isis Howard, a conservation biologist with the Xerces Society who coordinates the count. “With low overwintering numbers becoming more common, monarchs have fewer chances to rebound from extreme weather or pesticide exposure.”

The loss of overwintering habitat adds another layer of risk. Since tracking began, more than 60 known monarch groves along California’s coast have been destroyed, according to Sarina Jepsen, director of the Xerces Society’s endangered species program. In just the past year, at least three active sites were significantly damaged by inappropriate tree removal, often tied to development, Jepsen said.

“Without adequate places for monarchs to overwinter, we will lose the migration,” she said.

There were, however, a few bright spots. Among the 249 sites where this winter’s count found monarchs, Natural Bridges State Beach in Santa Cruz County hosted the largest overwintering cluster, with about 2,500 butterflies.

Researchers also deployed ultralight radio tags at some sites to better understand how monarchs move within and between groves, data that could help improve habitat management.

Howard said she wasn’t surprised by the low numbers, given weak signs from last summer’s breeding season.

“I’m always hoping for a bounce-back,” she said, “but I also know it’s going to take years of dedicated conservation action to see the long-term recovery we’re really looking for.”

That action, she added, can start close to home. Californians can help by avoiding pesticides, planting native milkweed and nectar-rich flowers, protecting overwintering groves and participating in community science projects like the Western Monarch Count or iNaturalist, which allows people to record and share their observations of plants and animals.

“There’s an insect apocalypse underway, and monarchs aren’t even the worst off,” Howard said. “But we can change course. We need to act now, and everyone has a role to play.”