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| Welcome to our tree column, “Ask your local arborist.” Each month, arborists from the Davey Tree Expert Company in Menlo Park discuss a tree-related topic from general care and planting tips to how trees can impact your property. If you have a tree-related question or an idea for a topic to discuss, email lifestyle@embarcaderopublishing.com or post in the comments section at the end of this column. |
Leaf-feeding insects are a part of life in the Peninsula’s rich horticultural landscape. The region’s mild Mediterranean climate and diverse variety of native and ornamental trees create an ideal environment for many beneficial insects to thrive, but the landscape also is ideal for destructive leaf-feeding pests, as well. Common culprits in this area include aphids, oak moth caterpillars, oak silkworms and thrips.
While occasional feeding is normal, heavy infestations can harm tree health and aesthetics. These pests can weaken trees, reduce vigor and leave unsightly damage if not properly managed.
Regular tree inspections, proactive care, natural predators and targeted treatments are the best strategies for keeping these pests in check.
Here are some tips on how to combat some of these destructive pests commonly found in the region.
Aphids
Aphids are tiny, soft-bodied insects that gather on new growth, feeding on sap and producing sticky honeydew. This not only attracts ants but encourages black sooty mold that can stain leaves and structures. Trees like maples, sycamores and crape myrtles are frequently affected.
Homeowner fix: Spray a strong jet of water to dislodge aphids or apply insecticidal soap or neem oil before they become a major infestation. Encourage natural predators, like ladybugs, by avoiding broad-spectrum insecticides.
When to call in the pros: If trees are repeatedly infested or showing signs of stress (like yellowing or curled leaves), a certified arborist can apply systemic treatments that target aphids more deeply and help restore tree vitality.
Oak moth caterpillars
The California oak moth (Phryganidia californica) targets coast live oaks, defoliating trees in spring and summer. Caterpillars can strip entire canopies in severe outbreaks.
Homeowner fix: If noticed early, caterpillars can be controlled using Bacillus Thuringiensis (BT), a biological insecticide that targets larvae without harming beneficial insects.
When to call the pros: Extensive defoliation weakens trees and may require nutrient support, deep root fertilization or trunk-injected treatments to prevent future outbreaks. Arborists also can assess long-term oak health and apply dormant-season treatments to disrupt the moth’s lifecycle.
Oak silkworm
The oak silkworm (Anisota Senatoria) is less common but equally damaging. These large caterpillars feed aggressively and can cause noticeable canopy thinning.
Homeowner fix: Hand removal may work on small trees or isolated branches.
When to call the pros: For high or inaccessible branches or widespread infestations, arborists can provide targeted foliar sprays or soil treatments to minimize damage while protecting beneficial insects and pollinators.
Thrips
Thrips are minuscule insects that leave silvery streaks, or stippling, on leaves and flowers. They’re particularly problematic on ornamental trees and shrubs like roses, magnolias and Japanese maples.
Homeowner fix: Use blue or yellow sticky traps, prune and dispose of affected growth, and spray neem oil or insecticidal soap early in the infestation.
When to call the pros: Severe or recurring thrips damage can be mitigated with professional integrated pest management (IPM). Arborists can also inspect for underlying stressors—such as poor soil or irrigation issues—that make plants more susceptible.
George Reno is a local arborist at The Davey Tree Expert Company in Menlo Park, which serves communities on the Peninsula, including Menlo Park, Atherton, Palo Alto and Redwood City. Reno can be reached at 866-923-5658.
Joan Lane, trailblazing volunteer and Stanford’s go-to for ‘sticky problems,' dies at 97March 25, 2026 3:23 pm