Creeping, crawling critters: Parks smothered by caterpillar overpopulation

Western Tussock Moth Caterpillar
Megan V. Winslow/Town Crier
Adult western tussock moths are native to the Pacific states, but they can wreak havoc in the spring as caterpillars, above.

Creeping across benches. Crawling over playground equipment. Dangling from silk threads like tiny ninjas. The western tussock moth caterpillar class of 2018 is out in force and its members have infiltrated some local neighborhoods at unprecedented levels.

In Mountain View, infestation causing extreme tree defoliation and skin rashes has warranted insecticide applications at several parks, according to Jakob Trconic, the city’s park section manager.

“Literally, they were blanketing trash cans and benches to the point where it looked like the trash cans were moving,” Trconic said.

Adult western tussock moths are native to the Pacific states, but they can wreak havoc in the spring when the larval caterpillars fatten up for their pupal cocoon stage by feasting on deciduous tree foliage, including that of the oak, citrus and stone fruit varieties, according to Andrew Sutherland, University of California Integrated Pest Management adviser for the Bay Area.

“Certainly, they’ll get into our native oak trees and other native shrubs,” Sutherland said. “And when they’re hungry, they’ve got to keep eating, so oftentimes they’ll migrate to a new plant.”

Last year, the caterpillars devoured Los Altos resident Janet Wilson’s oak leaf hydrangeas in a single day.

“I just remember walking out and thinking, ‘Oh, my. Will they ever come back?’” Wilson said of her flowers.

Population control

The caterpillars spotted around Silicon Valley in recent days are full-grown larvae noted for their red spots, spiky bristles and plumed dorsal tufts along their inch-long bodies. Beware the fuzz: Those protruding hairs feature an irritant capable of causing allergic reaction.

Mounting complaints from residents with rashes combined with the ongoing destruction to local trees have prompted Mountain View city officials to advance beyond manually removing, sweeping and blasting the caterpillars with pressurized water to applying insecticides. Their chosen product is EverGreen Pyrethrum Concentrate, which Trconic characterized as the safest option capable of making the most impact.

“I don’t like putting chemicals on trees, but given these circumstances, I’m kind of in a no-win situation,” he said.

Trconic’s team began April 9 spraying the insecticide on infested trees and play structures in areas with the highest concentrations of caterpillars: Castro Park, Mariposa Park and Pioneer Park and at the Rengstorff Park barbecue area, within downtown’s public parking lot six and on the north side of the senior center. He expects they will selectively target parts of Eagle Park and Mercy-Bush Park this week.

After each application, workers remain on scene until playground equipment dries, and then they hose it down as an added precaution, according to Trconic. He said Pyrethrum is considered safe once it dries in the sunlight, and it breaks down completely within three days.

Los Altos’ tussock moth caterpillar population has yet to rise to the alarming levels witnessed in Mountain View, though the insects could be observed last week clinging to Shoup Park oak tree leaves and hanging from branches over Arastradero Road’s pedestrian pathways. Within weeks, these tiny infiltrators will be gone – whether through human intervention or their natural retreat into cocoons facilitating their transformation into moths a month or so later.

Sutherland doesn’t endorse insecticides – especially broad-spectrum ones like pyrethrums, which can kill beneficial insects, including the moth’s natural enemies. Instead, he suggested pressure-washing their cocoons and the egg cases.

“The cool thing is that since the female (moths) can’t fly, you’re not going to get a female from another area flying in and laying eggs,” he said. “If you can manage your regional or neighborhood population, you can control it pretty good.”

For more information on the western tussock moth, visit the UC Integrated Pest Management website at

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