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

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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,

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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. Symp- larly to determine whether caterpillars using insecticides. Monitor again to toms of virus infection are dark, soft, are abundant and to identify the most determine whether pest populations limp larval carcasses hanging from fo- effective time to spray them. Monitor- have declined or if natural enemies liage or twigs; these carcasses eventu- ing is important, because oakworm are increasing to levels that soon may ally degenerate into a sack of liquefied populations are cyclic in nature, caus- cause pest numbers to decline. contents. Whitish fungal growth may ing the pest to be common in some cover Beauveria-infected oakworms years and virtually absent in others. Frass monitoring. Frass collection is and produce an unpleasant odor. Outbreaks do not occur every year, another monitoring tool. The oakworm and treatment by spraying is of no caterpillar excretes characteristic When infected insects are broken, benefit during most years. If you hire droppings, called frass, that fall to the they release pathogen particles, which others to care for your trees, make ground beneath the tree. As the cat- infect other oakworms. Disease out- sure any decision to spray is based on erpillars grow, their dark fecal pellets breaks rapidly can reduce populations monitoring. Apply least-toxic pesti- increase in size. Greater numbers of under favorable conditions, although cides if young oakworm caterpillars pellets are produced either from an in- outbreaks are difficult to predict and become abundant. creased number of oakworms or from may not occur until oakworm popula- increased temperatures, which cause tions have become high. Monitoring caterpillars to feed faster. Oakworm Monitor to determine whether insect frass pellets are typically dark brown Cultural Controls populations are going up or down, with a highly sculptured surface and Good cultural care of oaks is an es- if control is warranted, and, if so, to may be observed lodged in bark crev- sential component of integrated pest properly time management efforts. ices, spider webs, and ground cover management and will enable trees to You also should monitor trees at least plants beneath infested oaks. once after taking control action to as- tolerate moderate levels of defoliation sess the effectiveness. without harm. Healthy trees can better To monitor, place 3 to 5 light-colored sustain some foliage feeding or defolia- sticky cards, shallow trays, or cups be- Oakworm monitoring. Regularly in- tion than trees under stress. neath the canopy at regular intervals, spect foliage for oakworms or damage. such as a 24-hour period each week. Because caterpillars usually are more Provide irrigation only when needed Place these frass traps when no rain or abundant in the western part of the to minimize drought stress. This will sprinkler irrigation and little wind are tree canopy, concentrate monitoring depend on whether precipitation has expected. Save the frass or record its there to provide an earlier and more been normal; soil type and conditions volume to compare with the amount sensitive indication of damage. No collected on other sampling dates or such as shallow soil, soil compaction, thresholds have been established, but from other oaks. or the presence of fill or pavement that as a guide inspect 25 young, lighter can divert rain water; location; oak green shoot terminals, representing Initially you may want to conduct species; and whether the tree recently the current season’s leaf flush. If you suffered any root injury. Protect roots observe more than 8 to 10 oakworms both frass monitoring and foliage in- and trunks from damage, and properly 1 spection. This provides a record of the more than /4 inch long, defoliation proportion of leaves eaten or the num- prune trees when needed. may occur if oaks are not sprayed. Al- ber of oakworms present and the cor- ternatively, a density of 25 oakworms In comparison with pests, the more per 100 shoot terminals has been sug- responding density or volume of frass. common and serious problems that gested as a treatment threshold. With experience, frass monitoring injure or kill oaks include inappropri- alone may estimate caterpillar density ate irrigation, physical injury to trunks When monitoring, look closely for the and damage and aid in deciding if and roots, and soil changes such as presence of predators, parasites, dis- control is needed. Frass monitoring compaction or changes in the grade. Do eased caterpillars, and other evidence also helps to estimate the relative age not fertilize oaks unless laboratory test- of biological control, and record this of most oakworms by comparing frass ing of properly collected leaf samples information. Evidence of natural en- pellet size. Average pellet lengths for reveals that a nitrogen deficiency exists. emy activity includes dead pupae or first, third, and fifth instar stages are eggs with holes from which parasites 1/100, 1/50, and 1/20 inches (0.3, 0.6, and 1.4 Many landscape trees are stressed or emerged, oakworms or unhatched millimeters) respectively. Certain in- dying because of poor growing condi- eggs that are discolored or darker than secticides such as Bacillus thuringiensis tions and inappropriate cultural care. normal indicating they may contain (Bt) are most effective against younger See publications such as Living Among parasites, or hatched caterpillar eggs caterpillars, when smaller frass pellets the Oaks listed in References. with no evidence of caterpillars or predominate.

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Table 2. Types of Pesticides and Some Products for Oakworm Control on Oaks. Commercial Product Name Pesticide Type Common Chemical Name Home Use Professional Use Microbial Bacillus thuringiensis subspecies aizawai NA Xentari Green Light BT Worm Killer, Safer Brand Dipel and other Microbial Bacillus thuringiensis subspecies kurstaki Caterpillar Killer products Green Light Lawn & Garden Spray Spinosad, Microbial Spinosad Conserve Monterey Garden Insect Spray Garden Safe Brand Multi-Purpose Garden Botanical Pyrethrins plus piperonyl butoxide Pyrenone Insect Killer, Spectracide Garden Insect Killer Botanical Pyrethrins plus rotenone NA Pyrellin E. C. Insect growth regulator Diflubenzuron NA Dimilin

Check current labels for permitted uses. NA—Not available or availability uncertain.

Chemical Control Depending on factors such as insecti- when oakworms are feeding ac- Several pesticides of low toxicity to cide choice, application coverage and tively. Young caterpillars are most people and natural enemies are avail- timing, oakworm abundance, and the susceptible to Bt. Because sunlight able to control oakworms. IPM-com- presence of nearby untreated trees, quickly decomposes Bt on foliage, patible pesticides include microbials, more than one application may be most oakworms hatching after the botanicals, and insect growth regula- needed to provide good control. For application are not affected. tors. Although some are available to example, during outbreak years and • A second application about 7 to 10 both homeowners and professional when applying Bt (discussed below), days after the first may be required. applicators (Table 2), most homeown- two sprays during each oakworm gen- • Some users add insecticidal soaps, ers lack the equipment and experience eration in Northern California may be oils, or pyrethrins to increase the to effectively treat large trees. needed to provide good control. Make efficacy of Bt sprays. applications when young and mid-in- • Be sure to follow label directions for When hiring a professional applica- star oakworms are the most prevalent mixing and applying. tor, discuss the specific pesticide to life stages, typically March through Spinosyns insecticides (i.e. spinosad) be applied and insist on using an April and July through August. Decide are produced by fermentation and are IPM-compatible one. Avoid using the actual application times by moni- by-products from the bacterium Sac- broad-spectrum insecticides such as toring oakworm populations. charopolyspora spinosa. Spinosyns are carbamates (carbaryl), organophos- toxic to most caterpillars, fly larvae, Microbials. Microbial insecticides phates (acephate and malathion), or thrips, and certain species of beetles are naturally occurring microorgan- pyrethroids (fluvalinate and perme- and wasps. They are one of the most isms or their by-products produced thrin), because these materials kill effective microbials and have rela- both pests and beneficials and may commercially for pest control. Bacillus tively low toxicity to people and the induce outbreaks of spider mites or thuringiensis, commonly called Bt, is adults of many natural enemies. pest insects. IPM-compatible pesti- the most widely used and has been cides provide good control of target used effectively against oakworm for Botanicals. Botanical pesticides are pests, reduce secondary outbreaks of many years. Unlike broad-spectrum derived from plants. One of the most other potential pests such as mites, insecticides that kill on contact, oak- common botanical insecticides, py- and minimize hazards to people and worms must eat Bt-sprayed foliage to rethrum, is from chrysanthemum pets. In part because of their more be killed. Bt destroys the oakworms’ flowers grown in Africa and South specialized and selective modes of digestive system and causes larvae to America. This botanical is comprised action, these pesticides often require stop feeding within about a day. Af- of insecticidal compounds including more knowledge, skill, and careful ap- fected oakworms often die within a several pyrethrins. The most effective plication to be effective. Some of these few days. Bt is not toxic to most non- products containing pyrethrins also pesticides can be mixed and applied caterpillar insects including natural include the synergistic compound together to increase their effective- enemies. Timing and thorough spray piperonyl butoxide, which strengthens ness. For example, an insecticide that coverage, especially on the underside the effect. Insects may be only tempo- provides immediate control such as of leaves, are crucial for effective ap- rarily paralyzed (knocked down) and pyrethrins can be combined with a plication. may recover from the effects of expo- pesticide, such as Bt or diflubenzuron, sure to pyrethrins unless piperonyl that acts more slowly to kill insects. • Apply Bt during warm, dry weather butoxide is added.

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Other Contact Sprays. During out- REFERENCES breaks when oakworms migrate to Burke, H. E. and F. B. Herbert. 1920. non-oaks, they can be thoroughly California Oak Worm. USDA Farmers sprayed with horticultural or nar- Bulletin 1076. row-range oil (Sunspray, Volck), in- secticidal soaps (Safer), or pyrethrins Harville, J. P. 1955. Ecology and Popu- and soap combined (Safer Yard & lation Dynamics of the California Oak Garden Insect Killer). Because these Moth Phryganidia californica Packard migrating oakworms cannot complete (Lepidoptera: Dioptidae). Microentomol- development on non-oaks and will die ogy 20 (4). without producing another generation, consider not treating non-oak species Johnson, S. G. 1995. Living Among the and exercising patience until the oak- Oaks: A Management Guide for Hom- worm outbreak subsides. eowners. Oakland: Univ. Calif. Nat. Res. Publ. 21538. ❖

For more information contact the University of California Cooperative Extension in your county. See your telephone directory for addresses and phone numbers.

AUTHORS: S. Swain, UC Cooperative Extension, Marin Co.; S. A. Tjosvold, UC Cooperative Extension, Santa Cruz Co.; and S. H. Dreistadt, UC Statewide IPM Program TECHNICAL EDITOR: M. L. Flint COORDINATION AND PRODUCTION: P. N. Galin and M. L. Fayard ILLUSTRATIONS: Figs. 1-4: J. K. Clark; Fig. 5. V. Winemiller; Fig. 6: lacewing larva and pirate bug by Celeste Green in Smith, R. F. and K. S. Hagen. 1956. Enemies of spotted alfalfa aphid. Calif. Agric. 10 (4): 8-10. Fig. 6: soldier bug, Brachymeria, Itoplectis, and tachinid fly from Burke, H. E. and F. B. Herbert. 1920. Fig. 6: yellowjacket courtesy WARNING ON THE USE OF CHEMICALS of U.S. Public Health Service. Pesticides are poisonous. Always read and carefully follow all precautions and safety recommendations given on the container label. Store all chemicals in the original labeled containers in a locked cabinet or shed, away from food or feeds, and out of the reach of children, unauthorized persons, pets, and livestock. Produced by UC Statewide IPM Program, Pesticides applied in your home and landscape can move and contaminate creeks, rivers, and oceans. University of California, Davis, CA 95616 Confine chemicals to the property being treated. Avoid drift onto neighboring properties, especially gardens containing fruits or vegetables ready to be picked. This Pest Note is available on the Do not place containers containing pesticide in the trash or pour pesticides down sink or toilet. Either use World Wide Web (www.ipm.ucdavis.edu). the pesticide according to the label or take unwanted pesticides to a Household Hazardous Waste Collection site. Contact your county agricultural commissioner for additional information on safe container disposal and for the location of the Household Hazardous Waste Collection site nearest you. Dispose of empty containers by following label directions. Never reuse or burn the containers or dispose of them in such a manner that they may contaminate water supplies or natural waterways.

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