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While sitting under an oak tree, you might look up to see strings of silk hanging from the tree canopy. Look closer and you’ll probably see dozens of fuzzy caterpillars covered in gray, red and yellow tufts.
This spring, western tussock moth caterpillars can be spotted throughout Menlo Park, Atherton, Palo Alto, Mountain View and Los Altos, infesting local coast live oak trees.
This year’s stretch of unseasonably warm weather means Midpeninsula cities and towns are experiencing an outbreak of the caterpillars. They’re named for their tufts of prickly hairs, also known as setae, which can cause allergic reactions leading to rashes, hives and difficulty breathing for people with asthma.
The caterpillars hatched earlier than usual this year, said Jeoff Dunster, plant health care tech advisor for Urban Tree Management. An infestation on this scale hasn’t occurred in years, he added.
“We haven’t seen an outbreak like this in over six years,” said Jillian Keller, Menlo Park’s city arborist. “Usually, these outbreaks start in May. We believe that this year’s early outbreak was caused in part by unusually warm, dry weather.”
Live oak trees, which are commonly found on the Peninsula between Hillsborough and Mountain View, serve as the primary host for western tussock moths. The larvae and caterpillars also feed on shrubs and smaller plants as they fall from the trees and get carried by the wind. They can be spotted munching on Japanese maple trees, roses, lilacs and manzanitas.
When an infestation of caterpillars feeds on plants and defoliates their leaves, it can stress smaller plants, which use their energy to regrow to survive, Dunster said.
Keller added that if the infestation is severe enough, it can cause significant crown thinning in oak trees but healthy trees will survive and grow back their leaves within a few months.
The San Mateo County Parks Department issued a notice on March 30 saying that Flood Park in Menlo Park was experiencing an infestation of tussock moth caterpillars which “eat large amounts of foliage and cause a general nuisance.”
Parks officials explained that the caterpillars will be noticeable throughout April and early May but will have transformed into moths by July.Â
While the infestation is temporary, Dunster explained that even after the caterpillars are gone, the hairs they shed can cause physical reactions for weeks or months.
“Once they’re gone, they may not be completely gone,” he said.
Dunster recommends people remove caterpillar hairs by hosing down their yards, outdoor furniture and any surfaces that they might come into contact with around the end of May and early June. It’s a good sanitation practice, he added.
In late March, the city of Menlo Park announced that its Public Works Department would be using non-chemical management methods to treat its parks and highly trafficked areas. Menlo Park officials said the city started treating its trees the week of March 23 after it saw a large amount of caterpillars at Burgess Park.
Keller said the city is using high-pressure water to spray the trees, removing the caterpillars and their cocoons. She added that the recent rain also helped slow down the caterpillars.
“These methods are effective in reducing impacts while protecting beneficial insects, wildlife and the broader environment,” said city officials in the announcement.
“The worst of the infestation is over, and our use of high pressure water to spray infected trees has greatly reduced the number of caterpillars in most instances,” said Keller.
Urban Tree Management technicians have seen caterpillars cover the external walls of houses, fences and backyards.
Dunster said most of his clients seek help because they are struggling with allergic reactions to the pest. He’s met adults and children covered in rashes, experiencing difficulties sleeping from discomfort, he said.
Companies like Urban Tree are aware of the community’s wariness around insecticides and the secondary impact they may have on other insects, storm water and the environment but Dunster said spraying is sometimes necessary to protect public health.
“This pest wasn’t really a problem until people stopped spraying, and we had regular spray programs that went on for years to manage this particular pest,” said Dunster.
Tree management companies use tree-trunk injections that can help prevent infestations. Although this method can cause some damage to trees, Dunster said it’s a more widely accepted approach than aerial spraying.
