Monarch Deaths A Natural Whodunit

Something is killing monarch butterflies wintering in Pacific Grove’s Monarch Grove Sanctuary.

Monarch butterflies cling to a pine tree branch in Pacific Grove's Monarch Grove Sanctuary on Friday. Scientists say the butterflies' recent deaths are a "normal process of nature."


Several hundred of the stunning black-and-orange butterflies were found over the past few weeks with abdomens missing, and folks visiting the sanctuary were growing alarmed.

“We were getting a number of calls,” said Lori Mannel, executive director of the Pacific Grove Museum of Natural History, which has docents at the sanctuary.

Was it natural — part of nature’s “red-in-tooth-and-claw” scheme — or something extraordinary threatening to deal a setback to the renovated sanctuary? Was an infestation of wasps wreaking havoc on the monarchs?

“There has been wasp predation in the past, but that hadn’t been seen for many years,” Mannel said.

Scientists who have worked with the museum and city to improve things for the migratory monarchs, who winter on the Central Coast from November to February, say there is nothing to be alarmed about.

“Based on what we’re observing, it’s not out of the normal process of nature,” Mannel said Friday, a day after the city and museum issued a joint press release headlined “Pacific Grove Monarch Numbers Still Healthy.”

“It’s not unusual in the larger scheme of things,” Mannel said.

Francis Villablanca, adviser for the Cal Poly San Luis Obispo Monarch Alert program, said in the press release: “This is consistent with wasp predation, but it is difficult to determine if wasps are solely responsible.”

Stuart Weiss, a scientist working with the city on the sanctuary’s maintenance plan, said in a prepared statement: “Such predation is a normal feature of overwintering monarchs. Some birds learn to avoid the distasteful parts, as do some rodents.”

Birds and rodents usually find the monarch anything but tasty, but they may munch some butterflies if they are hungry.

Cooler temperatures during recent nights and mornings may have made the monarchs, which are unable to fly unless it’s 55 degrees or warmer, more susceptible to predators, Mannel said.

“There is no cause for panic, but it’s definitely cause to watch closely,” she said.

Docents are keeping track of the numbers of dead monarchs and their gender, and collecting specimens for scientific study.

Current estimates put the fluid monarch population in the sanctuary at 8,800. During the peak, late-November period of their annual visitation, there were about 12,000 butterflies hanging in sanctuary clusters.

“This year has been a phenomenal year,” Mannel said.

Paths, plant life and drainage have been changed, and the only building in the sanctuary was torn down to make accommodations more monarch-friendly. Two years ago, the top monarch count in the sanctuary was about 1,000.

The best time to view the butterflies is between noon and 3 p.m. — the warmest part of the day, Mannel said.

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