How Can I Control California Oakworm?

Do you want to try controlling by preserving and introducing natural enemies of California oakworm?

Do you wish to preserve or introduce predators of California oakworms?

Do you wish to preserve or introduce parasitic wasps?

Do you wish to preserve or introduce parasitic flies?

Do you want to control California oakworm using cultural practices?

Predators include birds, spiders, and predatory insects. Green lacewing larvae (Chrysopa and Chrysoperla species), pirate bugs (Orius species), the spined soldier bug (Podisus maculiventris), and yellowjackets (Vespula and Dolichovespula species) are important oakworm predators. Green lacewing larvae, adults and nymphs of pirate bugs, and soldier bugs feed on oakworm eggs, larvae, and pupae, impaling prey with their tubular, sucking mouthparts. Yellowjackets carry oakworms back to their nests, where the wasp larvae eat them.

Two small wasps (Itoplectis behrensii and Brachymeria ovata) are reported to be the most important parasites. In both species, the wasps’ immature stages—egg, larva, and pupa—occur within oakworm pupae. An irregularly rounded hole chewed by an emerging adult wasp can be seen in parasite-killed oakworm pupae. The adult Brachymeria ovata is stout, black and yellow, and about 1/4 inch long with enlarged rear basal leg segments. The Itoplectis behrensii adult is slender with a long, narrow abdomen and a body length of about 1/4 to 2/3 inch. This wasp is mostly black but has long legs that are orange with yellow and black bands.

At least two parasitic tachinid flies (Actia flavipes and Hyphantrophaga virillis) kill oakworm larvae. The female fly lays one or more eggs on an oakworm. The emerging larvae bore down and feed inside the oakworm. As the parasitized caterpillar begins to pupate, the fly larvae inside kill their host and emerge to form oblong, reddish to dark brown pupal cases that may be seen on or beneath oaks. The dry, deflated skins of tachinid-killed oakworms often remain attached to twigs or bark.

Naturally occurring pathogens including a nuclear polyhedrosis virus (NPV) and a fungus (Beauveria bassiana) often kill oakworms. Symptoms of virus infection are dark, soft, limp larval carcasses hanging from foliage or twigs; these carcasses eventually degenerate into a sack of liquefied contents. Whitish fungal growth may cover Beauveria-infected oakworms and produce an unpleasant odor. When infected insects are broken, they release pathogen particles, which infect other oakworms. Disease outbreaks rapidly can reduce populations under favorable conditions, although outbreaks are difficult to predict and may not occur until oakworm populations have become high.

Healthy trees can better sustain foliage feeding or defoliation than trees under stress. Provide irrigation only when needed to minimize drought stress. Protect roots and trunks from damage, and properly prune trees when needed. The more common and serious problems that injure or kill oaks include inappropriate irrigation, physical injury to trunks and roots, and soil changes like compaction or changes in the grade. Do not fertilize oaks unless laboratory testing of properly collected leaf samples reveals that a nitrogen deficiency exists.

Pesticide sprays applied to control oakworm usually are not warranted to protect the health or survival of oak trees. If you believe trees need protection from defoliation because they are stressed or because defoliation or abundant insects are not aesthetically tolerable, regularly inspect foliage for oakworms and spray least-toxic pesticides only when caterpillars reach levels warranting treatment. These pesticides include microbials (Bacillus thuringiensis and spinosad), botanicals (pyrethrin), and insect growth regulators (Diflubenzuron).