A plague of caterpillars hits Mountain View

It’s hard to get around town without running into them. Hordes of hairy caterpillars, freshly hatched after a warm February, can be found all over Mountain View and neighboring cities dangling down tree branches and wriggling on cars, playgrounds and garbage bins, leaving behind trails of stringy silk.

And while the hairy, crawling critters tend to make their debut around this time every year, Mountain View parks staff acknowledge that there’s an usually high number of caterpillars this season, and is taking steps to clear out the pests that have completely overtaken parks, play structures and other public spaces.

The culprit appears to be the western tussock moth caterpillar, which emerge from eggs as black-haired caterpillars around March and April, according to the University of California’s Integrated Pest Management program. Mature larvae tend to stick around until mid-spring, when they spin cocoons, emerge, and can continue to pester residents as moths.

Although the city of Mountain View treated trees in the city with a “systemic” insecticide to control the population of caterpillars, hot weather in February diminished its effectiveness. The result is that tree-heavy regions of the city — including residential yards and some city parks — have become inundated with a plague of wriggling caterpillars. Residents on the social media website Nextdoor are reporting unusually high numbers of caterpillars at Eagle Park as well as Castro and Mistral elementary schools, and complain that they are putting a damper on outdoor activities.

“Unfortunately our very, very warm February caused an early hatch before the insecticide had been fully absorbed by the trees,” Bruce Hurlburt, the city’s parks and open space manager, told residents in an email.

Hurlburt told the Voice in an email that this year’s infestation is worse than prior years, and that the city is trying to address the nuisance. While they don’t pose a health risk, the long “hairs” on the backs of caterpillars can irritate skin and cause a rash when handled. More problematic is the fact that a critical mass of moth caterpillars can make quick work of the leaves on trees, and can completely defoliate the canopy in a matter of days.

Parks staffers are vowing to take action — at least at city parks — by blasting play structures with pressurized water and sweeping away caterpillars at locales including Eagle, Pioneer and Rengstorff parks. In the most affected areas, staff will be using an insecticide known as Evergreen Pyrethrum Concentrate to bring down caterpillar populations while minimizing the effect on other insects.

Hurlburt said city staff will be treating areas with the highest concentrations of caterpillars, and will be administering the insecticide into trees through a power sprayer. The spraying will be done early in the morning, and treated areas should be safe to enter about an hour afterward, he said. Residents struggling with their own small-scale invasion can spray off caterpillars with a hose, or, if it’s truly problematic, call a pest control company.

Other agencies have sought less chemical-oriented strategies. During a particularly bad caterpillar outbreak in 2007, Stanford University sought an alternative to pesticides by setting the stage for a small-scale war, unleashing spined solider bugs and nematodes as a way to bring down the population of tussock moth caterpillars. Other Stanford documents show the university has sought to use power washing on trees in lieu of insecticides.