There are several caterpillars that feed on oaks including the California oak worm and tussock moth.
Each of these caterpillars feed on oak leaves. Oak worms are smooth, small, yellow-green caterpillars with brown heads and dark stripes down their sides. They can range from 1/10 to 1 inch in length throughout their development. Tussock moth larvae are very distinctly hairy with three prominent creamcolored dots towards the head capsule.
In California, Oak Worm is most commonly found on coastal live oak in San Francisco, Monterey Bay, Santa Barbara, and other areas close to water sources. Tussock moth is common in San Francisco but can also be found along the Central Coast.
Healthy oaks affected by Oak Worm experience defoliation in the Spring and throughout the Summer from one or more of these pests. Damage may appear sporadically or throughout the entire canopy. Many leaves appear partially chewed (skeletonized) and will turn brown and die, while other leaves may be completely eaten. Oaks that are experiencing other stresses, such as drought, can decline from oak worm infestation at a quicker rate. Look for signs of leaf feeding or fecal pellets around the base of the tree for activity or worms raining down on you.
Trees that are in known areas of infestation can be treated preventatively in the early spring with TREE-äge at least 4-6 weeks prior to activity. Trees with current canopy infestations should be treated with ACE-jet for rapid response and then sequentially with TREE-äge for two years of control.
Early spring applications provide the most protection from feeding damage but later treatment will also stop mid to late summer infestations very well. Be sure to encourage watering of trees in naturalized areas that may have low soil moisture or natural rainfall in order to help upward distribution of the material into the canopy. Use of a soil surfactant such as NutriRoot™ in combination with watering will ensure deeper water penetration into the root system and ultimately better translocation of the systemic insecticide.
Larvae in treated trees will immediately stop feeding and fall from the tree within hours (ACE-jet) to days (TREE-äge). Trees with more than 25% defoliation may generate more leaf tissue with adequate soil moisture. Consider recording or mapping the location of outbreaks as infestations are often sporadic.
Main photo California oak worm taken by Dawn Fluharty, Arborjet
Tussock moth (larvae) taken by Herbert A. ‘Joe’ Pase III, None, Bugwood.org