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CALIFORNIA OAKWORM Integrated Pest Management for Home Gardeners and Landscape Professionals The California oakworm (Phryganidia suspend from limbs, leaves, trunks, or californica, family Dioptidae) is one of objects near trees. Inside the colorful many species of caterpillars that feeds pupal case, also called a chrysalis, the on oaks. It is the most important oak- oakworm develops into a moth. feeding caterpillar (Fig. 1) throughout its range, which extends along the Other Leaf-eating Species. The fruit- coast and through the coastal moun- tree leafroller (Archips argyrospila) is tains of California. Damage is most the most common defoliator of oaks common on coast live oak (Quercus in the warmer Central Valley of Cali- agrifolia) in the San Francisco Bay and fornia. Larvae are green with brown Monterey Bay regions. Populations 3 vary unpredictably year to year from Figure 1. California oakworm larva or black heads and /4 to 1 inch long very high to undetectably low. Healthy at maturity (Fig. 2). When disturbed, oaks generally tolerate extensive loss red centers that become pinkish to they often wiggle vigorously and drop of leaves (defoliation) without serious brownish gray before hatching into from leaves while suspended on silken harm, so treatment to control oak- the larval stage (caterpillars). threads. Larvae feed on buds and worms usually is not recommended. developing leaves, webbing them to- Despite their common name, young gether to form a protective case. Initial IDENTIFICATION oakworms are not wormlike but are damage includes leaf skeletonization. The adult, called an oak moth, is a uni- small, yellowish green caterpillars As the larvae mature, they may con- form tan to gray or silvery color and is with large, brown heads and dark distinguished by its prominent wing stripes on their sides. Older caterpil- sume entire leaves. 1 veins. The body is about /2 inch long, lars vary in color, commonly dark 1 and the wingspread is about 1 /4 inches. with prominent, lengthwise yellow or Tussock moths (Orgyia species) and Unlike females, males have feathery olive stripes. tent caterpillars (Malacosoma species) antennae. also feed on oaks throughout the state. Caterpillars range from 1/10 inch long Unlike the greenish, relatively smooth The female lays tiny, round eggs in when newly hatched to about 1 inch surface of California oakworm and groups of about two or three dozen, when fully grown. In the pupal stage fruittree leafroller larvae, tent caterpil- mostly on the underside of leaves. The they are white, yellowish, or pinkish lar and tussock moth larvae are quite eggs initially are white but develop with black markings, 1/2 inch long, and hairy (Figs. 3-4). Figure 2. Fruittree leafroller larva Figure 3. Tent caterpillar larva Figure 4. Tussock moth larva EST OTES Publication 7422 PUniversity of California N Agriculture and Natural Resources April 2009 April 2009 California Oakworm LIFE CYCLE Two oakworm generations a year typi- Eggs on cally occur in Northern California as underside described below and in Table 1. A of leaf Chrysalis third generation sometimes occurs at (pupa) warmer and inland sites and in North- ern California in years of uncommonly warm, dry winters. Insect develop- ment in Southern California and other warmer sites is especially variable, (actual size) and oak moths may appear almost any Adult time from March through November. (actual size) Fall-to-Spring (Overwintering) Gen- Mature larva eration. During fall, females lay eggs chewing leaf that hatch within a few weeks. The emerging oakworms overwinter as (actual size) young caterpillars on the lower leaf surface of evergreen oaks. They de- velop through five, increasingly larger stages (instars) and mature into pupae Figure 5. Life cycle of the California oakworm. during May or early June in Northern California. Overwintering caterpil- lars rarely occur on deciduous oaks, DAMAGE may cause defoliation in July through Young oakworm caterpillars skel- because they shed their leaves in the September. During years when popula- etonize the leaf surface of native fall along with any oakworm eggs and tions are high, oakworms may notice- oaks, while older caterpillars chew larvae, causing any oakworms at- ably defoliate virtually every oak in a tempting to overwinter there to die. all the way through the leaf. Partially chewed leaves may turn brown and neighborhood, sometimes contiguously across acres of oak woodlands. Trees Summer Generation. Moths emerge die. Viewed from a distance, the from pupal cases during June and July. canopies of damaged trees may ap- under stress from drought or other fac- Oak moths may be seen during sum- pear brown or gray overall because of tors may decline if defoliated. Healthy mer fluttering around oaks in the late the dead leaves and because chewed- trees can tolerate oakworm damage. afternoon. Female moths lay eggs that away foliage makes branches more Because defoliating outbreaks last usu- soon hatch into a new generation of visible than normal. In some years, ally only 1 or 2 years, oakworms rarely oakworms that feed from July through overwintering oakworms can com- cause repeated defoliations that can September, pupate, then emerge as oak pletely defoliate trees by May or June. severely harm or kill otherwise healthy moths in fall (Fig. 5). The subsequent summer generation trees. Table 1. The Seasonal Occurrence of Life Stages of the California Oakworm. Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Fall to Spring Generation Adults (moths) Eggs Larvae Pupae Summer Generation Adults (moths) Eggs Larvae Pupae ◆ 2 of 6 ◆ April 2009 California Oakworm Even when tree health is not threat- Biological Control deflated skins of tachinid-killed oak- ened, caterpillars and moths may Predators, parasites, and natural worms often remain attached to twigs become a nuisance when populations outbreaks of disease sometimes kill or bark. are high. The caterpillars may move enough oakworms to control popula- from defoliated oaks and travel on tions. Predators of All Immature Stages other surfaces in large numbers, such Predators. Predators include birds, as across lawns and up walls of build- Lacewing ings. In the process, they sometimes spiders, and predatory insects. larva enter homes and form harmless, but Green lacewing larvae (Chrysopa and annoying, pupae attached to house- Chrysoperla species), pirate bugs (Orius hold surfaces such as drapes and walls. species), the spined soldier bug (Podi- sus maculiventris), and yellowjackets Pirate bug Outbreak Populations. Oakworms ( Vespula and Dolichovespula species) are occasionally become abundant in the important oakworm predators (Fig. 6). landscape, with high (outbreak) popu- Green lacewing larvae, adults and lations occurring at irregular intervals nymphs of pirate bugs, and soldier followed by several years with no ap- bugs feed on oakworm eggs, larvae, Soldier bug parent damage. If most of the leaves and pupae, impaling prey with their on an oak have been eaten, many of tubular, sucking mouthparts. Yellow- the caterpillars may become malnour- jackets carry oakworms back to their ished and eventually die without de- nests, where the wasp larvae eat them. veloping into adults. Parasitic Wasps. Two small wasps When outbreaks occur, oakworms can ( Itoplectis behrensii, family Ichneu- Yellowjacket behave differently than during years monidae, and Brachymeria ovata, family when they are less common. Once they Chalcididae) are reported to be the have extensively defoliated an oak, most important parasites (Fig. 6). In caterpillars may drop from the tree on both species, the wasps’ immature Parasites of Pupae silken webs and crawl away, seeking stages—egg, larva, and pupa—oc- a new supply of food growing nearby. cur within oakworm pupae. An ir- They may feed temporarily on non-oak regularly rounded hole chewed by an Brachymeria species, sometimes extensively chew- emerging adult wasp can be seen in ing and defoliating them. Although parasite-killed oakworm pupae. The this may be alarming and annoying, adult Brachymeria ovata is stout, black 1 these oakworms are unable to mature and yellow, and about /4 inch long on non-oak plants, and most will die with enlarged rear basal leg segments. before becoming moths. The Itoplectis behrensii adult is slender with a long, narrow abdomen and a Itoplectis 1 2 MANAGEMENT body length of about /4 to /3 inch. Pesticide sprays applied to control This wasp is mostly black but has long oakworm usually are not warranted legs that are orange with yellow and black bands. to protect the health or survival of oak Parasites of Larvae trees. If you believe trees need protec- Parasitic Flies. At least two parasitic tion from defoliation because they tachinid flies (Actia flavipes and Hy- are stressed or because defoliation or phantrophaga virillis [formerly Zenillia abundant insects are not aesthetically virillis], family Tachinidae) kill oak- Tachinid fly tolerable, regularly inspect foliage for worm larvae (Fig. 6). The female fly oakworms and spray least-toxic pes- lays one or more eggs on an oakworm. ticides only when caterpillars reach The emerging larvae bore down and levels warranting treatment. Control feed inside the oakworm. As the para- oakworm and protect oaks with an sitized caterpillar begins to pupate, Figure 6. Natural enemies of the Cali- integrated pest management (IPM) the fly larvae inside kill their host fornia oakworm and the stage of the program that relies primarily on the and emerge to form oblong, reddish pest they attack. conservation of natural enemies and to dark brown pupal cases that may on good cultural practices. be seen on or beneath oaks. The dry, ◆ 3 of 6 ◆ April 2009 California Oakworm Pathogens. Naturally occurring patho- MANAGEMENT IF OAKS damage. If you have an increasing gens including a nuclear polyhedrosis MIGHT BE SPRAYED number of pests but also many natu- virus (NPV) and a fungus (Beauveria If considering spraying, monitor regu- ral enemies, wait a few days before bassiana) often kill oakworms.

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