An annual census of western monarch butterflies overwintering along California’s coast has found the insects’ numbers are at their lowest point in five years.
Volunteers with the Xerces Society’s Western Monarch Thanksgiving Count visited more sites last fall than ever before, yet they tallied fewer than 200,000 monarchs, the organization said.
“Two decades ago, more than 1.2 million monarchs were recorded from far fewer coastal sites, ” Sarina Jepsen, Xerces’ endangered species program director, said in a press release.
The western count began in 1997.
In contrast, the nation’s longest-running monarch butterfly count based in Cape May found the East Coast population had rebounded in 2017. Butterflies that migrate through Cape May spend the winter in the mountains of Mexico.
The average hourly count of migrating monarch butterflies through Cape May Point from Sept. 1 to early November increased to about 95 last year from about 15 the year before, according to the Cape May Bird Observatory’s Monarch Monitoring Project.
Yet the numbers were still just about half that of 2012, when an average of about 183 were spotted per hour.
Monarchs that spend the winter in forests along California’s central coast are born on milkweed plants throughout western states such as Arizona, Utah, Idaho and Washington.
The western population of monarchs has undergone a long-term decline similar to that of eastern and central U.S. populations. Declines have been blamed on a lack of milkweed, the insects’ only food source for its larvae. Changing agricultural practices have resulted in less milkweed growing around farm fields.
Xerces received many reports of late-season breeding, the organization said, and later arrivals by monarchs at overwintering sites. That may partly explain the low counts, the organization said.
Another factor may be California’s unseasonably warm temperatures, wildfires, smoke and mudslides this past fall.
The count occurs during a three-week period around Thanksgiving. Biologists, land managers and citizen scientists visit overwintering sites every year to monitor the butterflies, according to Xerces.
Nearly 150 volunteers covered a record 262 sites this year, the organization said. Many also took part in the second annual New Year’s Count, the results of which are still being tallied and will be reported later this month.
“Counts at some of the state’s largest sites were dramatically lower,” said Emma Pelton, conservation biologist with the Xerces Society. “Pismo Beach State Park was down by 38 percent, a private site in Big Sur was down by 50 percent, and the Monarch Butterfly Sanctuary in Pacific Grove was down 57 percent, from 17,100 to just 7,350 butterflies.”