If you’re seeing fuzzy caterpillars on your trees or dealing with an unexpected itchy rash after spending time outdoors, Western tussock moths may be the cause.

This seasonal pest shows up across the Peninsula and South Bay and can affect both your landscape and your day-to-day comfort if populations build.

What to Look For

Egg Masses

Western tussock moth activity often begins with small, clustered egg masses attached to bark, leaves, or nearby surfaces.

Adult Moths

Male moths are winged and mobile. Females remain on the tree and stay close to where they emerged.

Caterpillars (Larvae)

This is the stage most people notice. The caterpillars are fuzzy and highly visible.
They move between plants by crawling or drifting on silk threads carried by the wind.

Tree Activity

You may see groups of caterpillars along trunks and branches, especially in dense canopies like Coast Live Oak.

Skin Irritation

⚠️ The fine hairs on these caterpillars can cause an itchy, uncomfortable rash.
Even indirect contact can lead to irritation that lingers for several days.

Common Host Plants

Western tussock moth is most often associated with Coast Live Oak, but it does not stop there.

You may also see activity on:

• Ceanothus
• Manzanita
• Buckwheat
• Other oak species
• Roses
• Prunus (ornamental and fruiting trees)

In many cases, understory plants take the brunt of the feeding. During heavier outbreaks, caterpillars may spread well beyond trees and show up on fences, structures, and outdoor surfaces.

When and Where It Shows Up

• Peak timing: March through April, typically around oak bud break
• Common areas: Atherton, Menlo Park, Palo Alto, Los Altos, Mountain View
• Movement: Spread happens through crawling and wind dispersal, not flight

Even after caterpillars are no longer visible, their hairs can remain in the environment and continue to cause irritation.

Why It Matters for Your Property

While Western tussock moth is seasonal, it can create real issues when populations increase:

• Defoliation of ornamental and understory plants
• Irritating skin reactions for people and pets
• Residual exposure from shed hairs left behind on surfaces

For properties with mature oaks or dense plantings, early awareness helps reduce both tree stress and human discomfort.

Treatment and Management Options

Managing Western tussock moths is usually about timing and selecting the right approach for your property.

🌿 Foliar Treatments

• Typically the most cost-effective option
• Applied during early seasonal activity
• May require repeat applications depending on conditions
• Needs careful application to avoid drift

🌳 Trunk Injection

• More targeted and contained within the tree
• Can provide longer-lasting results
• Higher upfront investment, but fewer repeat treatments

For properties where minimizing exposure, drift, or repeated applications is important, trunk injection is often the preferred option.

Practical Takeaway

Western tussock moth may look harmless at first, but it is a pest that’s best addressed early.

If you are seeing caterpillars, noticing plant damage, or dealing with unexplained skin irritation, it’s worth having the situation evaluated before activity peaks.

Maguire Tree Care has been serving the Peninsula and South Bay since 1999 with a focus on science-based, preservation-first tree care. Our arborists can help identify pest activity and recommend targeted treatment options that support long-term tree health.

FAQ

What causes the rash from Western tussock moths?

The irritation comes from tiny hairs on the caterpillars that can embed in the skin and cause an itchy reaction.

Are oak trees at risk?

Coast Live Oak is a primary host, but the pest often affects nearby understory plants as well.

When should I treat for Western tussock moths?

Early spring, around March and April, is typically the most effective time to address activity.

Can the irritation continue after the caterpillars are gone?

Yes. Shed hairs can remain on surfaces and continue to cause skin reactions.

Schedule an Arborist Inspection

If you’re noticing signs of Western tussock moths or want help identifying what’s affecting your trees, a professional inspection is the best next step.

Call Maguire Tree Care at 650-719-2253 or request an assessment at maguiretreecare.com/contact 

We’ll help you evaluate the situation and recommend a practical, property-specific plan.