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Oakworm Infestation

Although the oakworm devastation is alarming looking, it is not an alarming situation for the oaks. “Defoliation seldom, if ever, kills affected trees according to the USDA publication, “A Field Guide to Insects and Pathogens of California Oaks” (an excellent resource!).  Copied below are pages 14 & 15 describing California oakworm/oakmoth. The “Importance” paragraph is moved to the fore. Note also that oakworm infestation is unrelated to Sudden Oak Death (SOD).

Wendi Haskell’s email (about the tree guys who wanted to cut down her oakworm eaten oaks saying that they were dead) brought up a very good point:  One should not take advice from tree services wanting to make a quick buck (who are these guys?!).  Be careful, get expert advice, and err on the side of caution.

This infestation is messy and annoying.  If you want to stop the worms in their tracks, as per Mike and Sharon Ward’s email, you can spray your trees with BT – stands for bacillus thuringiensis.  McClenahan and other tree services can spray this for you with their high powered sprayer that easily reaches the tree canopy.  It seems that spraying is not necessary due to the fact that defoliation from oakworm rarely kills a tree.

From the USDA publication, “A Field Guide to Insects and Pathogens of California Oaks”:

California oakmoth

Phryganidia californica (Dioptidae)


Within its range, this is probably the most serious defoliator of oaks in California. When defoliation is severe, tree appearance is degraded and frass production may be a nuisance. In years with high oakworm populations, all trees in affected areas may be infested. However, defoliation seldom, if ever, kills affected trees.


The California oakworm is generally found in coastal areas from Del Norte to San Diego County, and inland to near Riverside in the south and Davis in the north. It occurs on most oak species present within its range, as well as on tanoak.


Young larvae feed between veins on the lower leaf surface. Although the upper leaf surface is left intact, it dries out and turns brown. Larvae in later instars chew completely through the leaf blade, often leaving only major leaf veins. Small frass pellets drop from the canopy as larvae feed. In outbreak years, individual trees or groups of trees may be almost entirely defoliated, typically by late summer or early fall.

Agent Description

Eggs are round and clustered in groups of 20 or more. Eggs are initially white, but develop red centers that become pinkish to brownhish before hatching occurs. Larvae (fig.8) are black with lengthwise yellow stripes, and are about 3 cm long at maturity, the head is large, globose, and brown. Pupae are white or yellow with black markings. Adults are tan to gray moths with prominent wing veins. Moths are about 15mm long with wingspans of about 25 mm (fig.9).


Young larvae overwinter on the lower leaf surfaces of evergreen oaks. In northern California, overwintering larvae mature in May to June. First generation adults are present in June and July and lay eggs for the second generation of larvae. The second larval generation defoliates trees from July through September. Second generation adults are present in October and November and lay eggs that give rise to overwintering larvae. Evergreen or live oaks are attacked by both spring (first) and summer (second) generations; deciduous oaks normally avoid attack by the spring (first) generation. Development is more variable in southern California where three generations can occur per year and moths may be present at various times between March and November. Oakworm population levels cycle over a period of years, so that numbers may range from very high to nearly absent in any given year