The California oakworm is a native insect to North America in the order Lepidoptera restricted to the California region. The young caterpillars can be a nuisance in the landscape setting by skeletonizing leaves on oak trees and dropping green frass everywhere below. As adults the moths are weak fliers and will usually congregate around the host tree. Damage is most common on coast live oak, Quercus agrifolia, but all oaks are susceptible to feeding. Pest pressure can vary immensely from year to year.
Healthy trees can tolerate a single defoliation event; however, multiple defoliation events can cause dieback and decline even on healthy trees especially in stressful urban sites where feeding is coupled with stress from abiotic factors. Commercial insecticide treatments on individual high value shade trees are the only way to ensure predictable protection during high pest pressure.
Nuisance from frass droppings may also warrant treatment. Arbormectin can be applied to control larvae for a full season. Lepitect may also be used if active feeding is noticed and immediate action is required. For large trees and heavy oakworm populations re-apply Lepitect every 30-45 days throughout the leaf feeding period of oakworm. Consider Mectinite or Lepitect Infusible as tree injection treatments for trees that cannot be treated with foliar sprays or other systemic options.
Other Treatment Practices
- Cambistat can be used to help improve drought and disease tolerance, root development, and the overall health of the tree.
- Promote health and vigor with proper irrigation, mulching, proper pruning and prescription based fertilization practices.
Good control from local arborists with bifenthrin and permethrin sprays. Lepitect will control caterpillars for one month and is of particular interest due to chemical trespass with foliar sprays.
Signs of Damage
- Brownish grey appearance to canopy due to defoliation
- In severe years trees can be completely defoliated by May-June or July-Sept by other generation
- Green frass droppings litter the ground beneath
- Heavy infestation years adult moths can be seen in large masses flying around oak trees
Trees At Risk
- All oaks, especially coast live oak, Quercus agrifolia
- Adults are uniform tan to grey with prominent wing veins.
- Adults are ½” long and males have feathery antennae.
- Weak fliers.
- Eggs are laid in irregular rows of 2-60 on the underside of leaves.
- Can also be laid on grass, twigs, or brush.
- Eggs are initially white, but change to pinkish, brownish/grey with maturity.
- Larvae are brown with hairs and indistinct longitudinal lines.
- 2 color phases.
- Light phase (prominent dark lines on green/yellow background) common.
- Dark phase (opposite from light phase) less common.
- Pupae is white to yellow, smooth and shiny and can sometimes be found on the trunks of oaks.
- Young larvae feed on soft surface tissue causing a brown stippling.
- Larvae feed on leaves outward-in completely skeletonizing & defoliating trees during epidemics.
University of California
Always refer to product label for rates and approved uses. Some images courtesy of forestryimages.org or Wikimedia Commons. Use of the images does not imply endorsement of treatments.