Western grapeleaf skeletonizer—Harrisina brillians
This introduced moth (family Zygaenidae) is a sporadic pest of grapevines. It also feeds on Boston ivy and Virginia creeper and uncommonly on Rosaceae such as almond, apricot, cherry, and rose.
Adults (moths) have a metallic bluish or greenish black body and wings. The body is about 3/5 inch long and the wingspan is 1 to 1-1/3 inches. These moths are active both day and night and during the day can be observed flying and laying eggs or mating on grape leaves.
Eggs are pale yellow to whitish, oblong, and about 1/40 inch (0.6 mm) in length. They occur separated slightly from each other in a group of several dozen on the underside of grape leaves.
The first instars (larvae) are about 1/16 inch (1.5 mm) long and the first two instars are cream colored. Third instars are yellowish green with brown bands. Fourth and fifth instars are yellow with two wide purple bands and several narrower blackish bands. Last (fifth) instars are about 2/3 inch long.
Pupae occur in an oblong grayish to whitish cocoon. Cocoons occur in bark crevices and organic litter on the ground.
Western grapeleaf skeletonizer develops through four life stages: egg, larva, pupa, and adult. In California's Central Valley, adult moths of the first generation emerge from overwintering pupae in April and May. After mating, adult females lay first-generation eggs that along with young larvae are present in late April through June. Larvae of the second generation are present in July and August and those of the third generation in September and October.
The larvae develop through five increasingly larger instars. First through third instars feed in a group with the individuals side-by-side on the leaf underside where they scrape the undersurface of leaves. Fourth and fifth instars feed in a more dispersed manner and chew all the way through leaves.
Development time from egg to reproductive adult is about 2 months. Western grapeleaf skeletonizer has 3 generations per year in the Central Valley and 2 generations in the cooler coastal regions.
The first through early fourth instar larvae feed on the lower leaf surface, leaving only the veins and the cuticle of the upper leaf surface. This leaves a brown to whitish paperlike appearance. The late fourth and fifth instars chew entirely through foliage and leave only the larger veins.
When abundant, larvae can completely defoliate a grapevine by July. When vines are severely defoliated, larvae then feed on grape clusters allowing pathogen entry, resulting in bunch rot disease. Defoliation can also cause fruit to become sunburned and atypically soft. Extensive defoliation may weaken vines and reduce fruit yield.
Larvae are also a problem for people who handle infested grapevines. Hairs on their body exude a stinging chemical that irritates the skin when the caterpillars are contacted.
Biological control. A granulosis virus commonly keeps populations of western grapeleaf skeletonizer below the levels that cause significant defoliation. This virus is transmitted from one generation to the next by disease-carrying adults that survived a low degree of infection during the larval stage. Symptoms of infection with this virus include:
- Eggs are widely scattered instead of more compactly laid in a group, and the average number of eggs per cluster is reduced.
- Eggs mostly fail to hatch.
- Larvae are sluggish and feed singly when young instead of in side-by-side groups
- Larvae consume only tiny patches of tissue rather than consuming a larger and growing area of the leaf.
- Larvae stop feeding, wander irregularly, and leave a visible trail of blackish liquidy excrement.
- Larval coloration and growth change; larvae shrink, develop more black coloration, and die without maturing to the next instar or pupal stage.
Two parasitoids (parasites) are also important natural enemies of western grapeleaf skeletonizer larvae. Apanteles harrisinae is a black wasp with yellowish legs. Adult females lay about three to nine eggs into each young caterpillar. After the caterpillar finishes feeding and spins its pupal cocoon, the parasitoid larvae mature, kill the host and emerge from it, then spin individual cocoons. This parasitoid has more generations per year than its host and it overwinters as a prepupa (mature larva) or pupa in its cocoon.
Ametadoria misella (=Sturmia harrisinae) is a black and gray tachinid fly. Adult females lay an oval white egg on fourth and fifth instars. The fly larva hatches immediately and chews into the caterpillar. Once the caterpillar pupates the fly larva completes its development and kills the host pupa. The oblong, dark reddish puparium (pupal covering) of the parasitic fly is formed within the silk of the host cocoon. This tachinid has at least two generations per year and overwinters as a larva inside a host pupa.
Control ants, reduce dust (e.g., periodically hose off small plants), and avoid the application of broad-spectrum, persistent insecticides for all pests on the vines to increase the effectiveness of parasitoids and predators. See Protecting Natural Enemies and Pollinators for more tips on conserving natural enemies.
Insecticides. Bacillus thuringiensis (Bt) or spinosad can be sprayed to thoroughly cover the underside of infested foliage if larvae and feeding damage of western grapeleaf skeletonizer are abundant and parasitism and symptoms of larval infection by granulosis virus have not been common. For best control, apply the insecticide when western grapeleaf skeletonizers are primarily early instars. If applying Bt, make a second application 7 to 10 days after the first. Adding horticultural oil to either insecticide will also cause it to kill moth eggs, help to control certain other pests such as leafhoppers and mites, and improve the persistence of spinosad.
Adapted from Grape Pest Management Third Edition from the University of California Agriculture and Natural Resources (also available as an eBook) and Pest Management Guidelines: Grape, University of California Statewide Integrated Pest Management Program (UC IPM).