In the warmer central valley of California, another caterpillar, the fruit-tree leafroller, is a more common defoliator of oaks. Larvae are light green with black heads. They commonly drop to the ground on silken threads, wriggling vigorously when disturbed.
Life Cycle
Two generations a year typically occur in northern California; there is sometimes a third generation in southern California or northern California in years of uncommonly warm, dry winters. The oakworm overwinters as young larvae on the lower leaf surface. Larva develop through five instar stages, and mature in May or early June in northern California; emerged moths often flutter around oaks in the late afternoon in June and July. These first generation female oakmoths lay eggs that hatch into larvae from July through September. Large populations of the first generation generally do not occur on deciduous oaks because these trees drop their leaves in the fall, causing the overwintering generation larvae and eggs to die. Second generation moths occur in October and November when they lay eggs that hatch into the overwintering larvae.
Damage
Young larvae skeletonize the leaf surface, while mature larvae chew all the way through the leaf. Defoliation on live oaks may become extensive in May or June from the overwintering larvae. Larvae from eggs of first generation adults may cause noticeable defoliation in July through September. Healthy trees can better tolerate damage than trees under stress from drought or other factors.
Management
Oaks tolerate feeding by moderate populations of oakworms, and control is not needed when damage is low. Healthy and well-cared for oaks tolerate extensive defoliation without serious harm.
If trees need protection from defoliation because they are stressed or of high aesthetic value, regularly inspect foliage for larvae and spray only when caterpillars are abundant. Monitoring is important because California oakworm populations are cyclic in nature causing the pest to be common in some years and virtually absent in others. Outbreaks requiring treatment do not occur every year. Control with an integrated program that incorporates good cultural practices, conservation of natural enemies and use of less toxic pesticides.
Monitoring
Monitor to determine whether insect populations are going up or down. Regularly inspect foliage for oakworms or damage. Concentrate monitoring in the west part of tree canopy to determine early damage. Frass collection is another monitoring tool. The California oakworm excretes characteristic droppings, called frass, that fall to the ground beneath the plant. As larvae grow, fecal pellets increase in size; greater amounts are generally produced with an increase in the number of larvae or an increase in temperature, which causes caterpillars to feed faster.
Place three to five light-colored sticky cards, shallow trays, or cups beneath the canopy at regular intervals, such as for 24 hours each week. Place these frass traps when no rain or sprinkler irrigation is expected. Save the frass or record its volume for comparison with the amount collected on other sampling dates or at other locations.
When monitoring for pests, also look closely for the presence of predators, parasites, and other evidence of biological control. Evidence of natural enemy activity includes pupae or eggs with holes from which parasites emerged, unhatched eggs that are darker than normal indicating they may contain parasites, or hatched caterpillar eggs with no evidence of caterpillar damage. If you have an increasing number of pests but also many natural enemies, wait a few days before using insecticides. Monitor again to determine whether pest populations have declined, or natural enemies are increasing to levels that may soon cause pest numbers to decline.
Biological Control
Predators, parasites, and natural outbreaks of disease sometimes kill enough oakworms to control populations. Predators include spiders, bigeyed bugs, pirate bugs, lacewing larvae, ground beetles, damsel bugs, assassin bugs, and birds. Two mini-wasps provide the greatest control while two fly parasites also contribute to control.
Cultural Controls
Good cultural care of trees is an essential component of integrated pest management allowing landscapes to tolerate moderate levels of defoliation without harm to the trees. Provide proper fertilizer and water, depending on soil type, location and oak species. Protect roots and trunks from damage, and properly prune trees when needed.
Chemical Control
You get the greatest benefit from an integrated pest management program if you apply insecticides that conserve natural enemies. The biological insecticide Bacillus thuringiensis (Bt) leaves most natural enemies unharmed. Unlike broad-spectrum insecticides that kill on contact, oakworms must eat sprayed foliage to be infected. Bt destroys the oakworms’ digestive system and causes larvae to stop feeding within about a day.
Timing and thorough spray coverage are very important for effective application. Apply Bt during warm, dry weather when oakworms are feeding actively. Thoroughly treat foliage when larvae are first observed to be chewing completely through the leaf or at the leaf edge. Because sunlight quickly decomposes Bt on foliage, most oakworms hatching after the application are not affected. A second application about 7 to 10 days after the first may be required. Follow label directions for mixing and applying.
More toxic pesticides are generally not recommended for oakworm larvae control; however, if you decide to use them, apply selectively to oaks or those portions that are heavily infested. Carbaryl, acephate, and pyrethroids rapidly reduce populations of most caterpillar species.