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Hundreds of mysteriously eaten monarchs found in sanctuary

- Scientists trying to figure out what’s killing P.G.’s favorite insect


Published: Dec. 30, 2011

A VORACIOUS predator has been gobbling up hundreds of monarchs at the Pacific Grove butterfly sanctuary. But despite the carnage, experts don’t believe it’s cause for panic just yet.

In the past few weeks, about 300 dead monarchs have been found in and around the sanctuary, many with their abdomens missing. Though scientists are not sure exactly what is responsible for the deaths, wasps seem to be topping the list.

“This is consistent with wasp predation,” according to Francis Villablanca, the science advisor with Cal Poly’s Monarch Alert Program, “but it is difficult to determine if wasps are solely responsible.”
Stuart Weiss, a biologist working for the city on its butterfly sanctuary maintenance plan, said other critters who frequent the forest could also be devouring the butterflies, eating only the meaty bits while leaving behind the wings.

“Some birds learn to avoid the distasteful parts, as do some rodents,” he said.

In any case, Weiss said there’s no need to worry too much at this point.

“Such predation is a normal feature of overwintering monarchs,” according to Weiss.

Esther Trosow, who heads the Pacific Grove Monarch Conservancy, agrees a certain amount of predation happens. Trosow also theorizes that the monarchs’ chosen location this year — in cypress and pine trees above an asphalt driveway at motel adjacent to the sanctuary — are making the dead insects appear more obvious to visitors than if they fell on the forest floor.

“If some fell in those locations,” Trosow said, “it would be less noticeable to fewer people, as they would fall on leaf litter or other ground cover and be away from the public,” which is prohibited from getting close to monarchs when they cluster in eucalyptus trees.

Overall, according to officials, only 2.5 percent of the population of monarchs in the city — estimated now to be nearly 9,000 — has been killed by predators.

Monarch docents with the Pacific Grove Museum of Natural History are helping track the numbers and the genders of dead butterflies and are collecting them so they can be studied.
City officials also said they will also continue to monitor the dead monarchs.

Experts and volunteers with Monarch Alert, who track the numbers of the insects at the sanctuary, have also noticed a general decline in the number of butterflies, which is being attributed to the normal reduction of butterflies in Pacific Grove and other areas of California during this time of year.

“This year’s decrease is not out of the ordinary and is consistent with the decrease found in other overwintering sites in San Luis Obispo and Monterey counties,” said P.G. city manager Tom Frutchey.
There are about 2,000 fewer monarchs in the city than there were at Thanksgiving. But there are still thousands of butterflies clustering on trees in the sanctuary.

The deaths haven’t seemed to deter monarch enthusiasts, who in droves last weekend visited the butterfly sanctuary on Ridge Road off of Lighthouse Avenue. The sanctuary is open from sunrise to sunset, and admission and parking are free. The best time to visit is between noon and 3 p.m., when the butterflies are most active and when docents with binoculars are available to talk about the insects.