Moth Monday: California Oak Moth

Welcome to the first post in a semi-regular feature I call “Moth Monday.” When possible I will include images of the adult, caterpillar, and pupa stages. I will strive to update previous posts if I am able to obtain images of adults or larvae that I did not have at the time of the original posting. Note that this feature is irregular in part because my family’s “weekend” is Sunday and Monday rather than Saturday and Sunday.

Rarely am I able to complete the life cycle of any insect in images, even over months if not years, let alone in one day at one location, but this improbable event occurred on March 25, 2011 in Carpinteria, California. My wife and I (we were dating at the time) were visiting her uncle and his wife, and strolling the neighborhood we came across one tree that was full of every life stage of the California Oak Moth, Phryganidia californica.

This species is legendary for its periodic population outbreaks that can, in epidemic years, result in the complete defoliation of oak trees along the California coast. California Live Oak, Quercus agrifolia, is the preferred host, but other species are on the menu. Incidental hosts include eucalyptus, chestnut, Tan Oak (Lithocarpus densiflorus), and azalea. The California Oak Moth also occurs in southwest Oregon where it eats Giant Chinkapin (Castanopsis chrysophylla) and Canyon Live Oak, as well as Tan Oak.

Despite the visually devastating effects of its caterpillars, the moth is rarely, if ever, responsible for the outright death of trees. We tend to severely underestimate the resilience of plants in general to withstand the attacks of insects and other herbivores.

Female moths lay their eggs in loose clusters on the foliage, limbs, or trunks of the host. Eggs laid on the underside of leaves late in the season are often able to overwinter there.

The caterpillars that hatch begin skeletonizing the leaves, grazing on the tissue between veins on the undersurface.

Older caterpillars are eventually able to consume all but the major leaf veins. The caterpillars undergo five molts, eventually reaching about 25 millimeters in length before they pupate.

The pupa may be mistaken for a butterfly chrysalis, as there is no silken cocoon encasing it, and it is boldly marked in black and white or yellow. Pupae are generally suspended by silken threads from the host tree or any vertical object close by.

The caterpillars and/or pupae are hosts themselves for a number of parasites. Among them are the tachinid flies Actia flavipes, Hyphantrophaga virillis, and a species of Ceranthia. An ichneumon wasp in the genus Mesochorus is also recorded from California Oak Moth caterpillars (Carmean, Miller, and Scaccia, 1989). There is also a nuclear polyhedrosis virus (NPV) and a fungus (Beauveria bassiana) that kill the caterpillars. The virus leaves the caterpillars hanging as limp, lifeless brown corpses from foliage, while the fungus may manifest itself as a white, powdery coating on the larvae, with an unpleasant odor as an accompaniment (S. Swain, et al., 2012).

The adult moths are dull brown, with a wingspan varying from 25-35 millimeters. They fly weakly, but well enough to disperse effectively. Males have pectinate (comb-like) antennae, the better to locate a female by the pheromones (scents) she releases to attract potential mates. Rarely do these moths fly to lights at night. You are much more apt to see them randomly during daylight hours in places where populations are abundant.

The California Oak Moth is also known as the California Oakworm, which is perhaps a better name since it is the caterpillars that do the damage. Formerly placed in its own family, the Dioptidae, it has recently been reclassified as one of the “prominent” moths in the family Notodontidae (subfamily Dioptinae).

Anyone living in the western half of California, from the Oregon state line to the Mexican border, is likely to encounter this species. During population booms it finds its way inland from the coast to central valleys. There are two generations each year in northern California, and three generations per year in southern California.

Sources: Carmean, David, Jeffrey C. Miller, and Brian Scaccia. 1989. “Overwintering of Phryganidia californica in the Oregon Cascades and Notes on its Parasitoids (Lepidoptera: Dioptidae),” Pan-Pac. Entomol. 65(1): 74-76
Essig, E.O. 1958. Insects and Mites of Western North America. New York: The Macmillan Company. 1050 pp.
Furniss, R.L. and V.M. Carolin. 1977. Western Forest Insects. Washington, DC: U.S. Department of Agriculture Forest Service Miscellaneous Publication No. 1339. 654 pp.
Powell, Jerry A. and Charles L. Hogue. 1979. California Insects. Berkeley: University of California Press. 388 pp.
Swain, S., S.A. Tjosvold, and S.H. Dreistadt. 2012. “Pest Notes: California Oakworm,” UC IPM Online.