If you’ve noticed fuzzy caterpillars on your trees—or worse, an itchy rash after being outside—you might be dealing with the Western Tussock Moth. This common Bay Area pest is more than just a curiosity… it can impact your landscape and your comfort.
What Does It Look Like?
Egg Mass / Early Stage
Western tussock moths start as tightly clustered egg masses, often found on bark or leaves.
Adult Moth
The adult male moth is winged and mobile, while females stay put on the tree—waiting to mate.
Larvae (Caterpillar Stage)
These fuzzy caterpillars are the most recognizable—and problematic—stage. They feed on leaves and can spread by crawling or “ballooning” on silk threads carried by wind.
Tree Infestation
Heavy populations can cluster on trunks and branches, especially in dense oak canopies.
Skin Reaction
Important: The tiny hairs (urticating setae) from these caterpillars can cause painful, itchy rashes that last for days—and sometimes disrupt sleep or daily comfort.
Host Plants
While Coast Live Oak is the primary host, Western tussock moths also affect:
- Ceanothus varieties
- Manzanita (Arctostaphylos)
- Buckwheat (Eriogonum)
- Oaks (multiple species)
- Roses
- Prunus (ornamental and fruiting)
In many cases, understory plants are hit harder than the canopy itself- At high pest populations it will eat anything and lay eggs on buildings, gutters, fences, pots, mailboxes.. almost anywhere.
Where & When to Watch
- Peak activity: March–April (around oak bud break)
- Hotspots: Atherton, Menlo Park, Palo Alto, Los Altos, Mountain View
- Spread: Not by flight—larvae move by crawling or wind dispersal
Even after caterpillars are gone, their irritating hairs can linger for weeks or months.
Why It Matters
- Defoliation of ornamental and understory plants
- Irritating rashes (especially risky for children and pets)
- Long-lasting exposure from shed hairs and cocoons
Control Options
Foliar Spraying
- Most cost-effective
- Needs annual spring application
- Risk of drift into neighboring areas
Trunk Injection
- More targeted and environmentally controlled
- Lasts about one year
- Higher upfront cost (but lower long-term hassle)
Pro tip: If clients are concerned about safety, liability, or repeat treatments, trunk injection is often the better investment.
Final Takeaway
The Western Tussock Moth may look harmless—even beautiful—but it’s a pest that deserves attention. Early identification and the right treatment approach can protect both your landscape and your comfort.
If you’re seeing activity or experiencing irritation, it’s best to act early before populations peak.
Urban Tree Management has been serving the Mid-Peninsula since 1995, helping property owners identify pest issues early and apply targeted, science-based treatments that prioritize long-term tree health.
If you’re unsure what you’re seeing or want guidance on the best approach, scheduling a professional arborist inspection is the most reliable next step. Contact us today!
