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Monarch butterfly numbers plummet 86 percent in California

Claudia Boyd-Barrett. USA TODAY•January 7, 2019

CAMARILLO, Calif. — The number of monarch butterflies turning up at California’s overwintering sites has dropped by about 86 percent compared to only a year ago, according to the Xerces Society, which organizes a yearly count of the iconic creatures.

That’s bad news for a species whose numbers have already declined an estimated 97 percent since the 1980s.

Each year, monarchs in the western United States migrate from inland areas to California’s coastline to spend the winter, usually between September and February.

“It’s been the worst year we’ve ever seen,” said Emma Pelton, a conservation biologist with the Xerces Society who helps lead the annual Thanksgiving count. “We already know we’re dealing with a really small population, and now we have a really bad year and all of a sudden, we’re kind of in crisis mode where we have very, very few butterflies left.”

Results from the count so far show that the number of monarchs at 97 California overwintering sites has dropped from around 148,000 in 2017 to just over 20,400 this year. Counts for dozens of other sites are still being tabulated, but the outlook is troubling, Pelton said.

What’s causing the dramatic drop-off is somewhat of a mystery. Experts believe the decline is spurred by a confluence of unfortunate factors, including late rainy-season storms across California last March, the effects of the state’s years long drought and the seemingly relentless onslaught of wildfires that have burned acres upon acres of habitat and at times choked the air with toxic smoke.

The Thomas Fire last year burned almost 300,000 acres, including areas important for monarch breeding and migration, Pelton said. More recently, the Woolsey Fire damaged at least four monarch butterfly overwintering sites in the Malibu area, according to Lara Drizd, a wildlife biologist with the U.S. Fish and Wildlife Service in Ventura.

These events add to the burden of other threats to monarch butterflies, including pesticide use and habitat loss. There are also questions about whether hotter weather brought about by climate change is affecting monarchs’ breeding behavior, Pelton said.

“We don’t think it’s just fires or just those bad storms in March … but it’s a really small population that has already had a bad year last year and then it got hammered by more stressors,” Pelton said. “We’re seeing the population really collapse in a single year. Now we’re worried: Are they going to bounce back? We’re not sure.”

Western monarchs generally follow a different migration pattern than eastern monarchs, which fly from the East Coast down to Mexico to overwinter. One piece of good news is that the eastern monarch population seems to be doing well this year, although their numbers, too, have declined by an estimated 80 percent since the mid-1990s.