Western Grapeleaf Skeletonizer

The metallic bluish or greenish black western grapeleaf skeletonizer moths fly during the day. Body length is about 0.6 inch and the wingspan is 1 to 1.3 inches. There are three generations per year in the Central Valley and two generations in the cooler coastal regions. Adult moths of the first generation in the Central Valley emerge from hibernating pupa in early spring to June. The pale yellow or whitish capsule-shaped eggs are laid in clusters on the underside of grape leaves. After hatching, the larvae line up and feed side-by-side on the leaf underside until the early fourth instar stages. There are five larval stages. The first two stages are cream colored, the third stage is brownish, and the fourth and fifth stages are yellow with two purple and several blackish bands. Larvae have conspicuous tufts of long black poisonous spines that cause skin welts on field workers. The fifth or last larval stage is about 0.6 inch long. When mature, larvae crawl under the loose bark or into ground litter and spin a dirty, whitish cocoon to pupate.

First through the early fourth instar larvae feed on the lower leaf surface, leaving only the veins and upper cuticle. This gives leaves a whitish paperlike appearance; eventually the entire leaf turns brown. The late fourth and all fifth stage larvae skeletonize the leaves, leaving only the larger veins. When abundant, larvae can defoliate vines by July. When vines are severely defoliated, larvae will then feed on grape clusters, which can result in bunch rot. Defoliation can also result in sunburn of the fruit and loss of quality. Defoliation after harvest may weaken vines by affecting stored reserves. Larvae also can cause problems for workers at harvest because hairs on their bodies can irritate the skin if they are brushed against.

Western grape leaf skeletonizer does not occur in all grape-production areas because the moths are not long-distance fliers and this pest has been slow to spread in California since its first appearance in the 1940s. In areas where it does occur, granulosis virus usually keeps populations below economically damaging levels. When the virus is insufficient, western grapeleaf skeletonizer is easily controlled with insecticides that are also effective on other caterpillars, leafhoppers, or thrips.

Two insect parasites, Apanteles harrisinae and Ametadoria misella (=Sturmia harrisinae), attack western grapeleaf skeletonizer larvae. Thousands of these parasites have been released in the San Joaquin Valley, and Ametadoria misella is common in many vineyards in the San Joaquin Valley.

A granulosis virus, endemic in southern California, has been introduced in selected areas with excellent success. It is extremely infectious when it is introduced into an outbreak population of western grapeleaf skeletonizer. Symptoms of populations infected with the virus include: (1) eggs within clusters are scattered instead of compactly laid, and the number of eggs is reduced; (2) most eggs fail to hatch; (3) larvae consume tiny patches of tissue rather than consuming entire areas of the leaf; (4) diseased larvae are sluggish and feed solitarily instead of in tight groups and usually tend to wander irregularly, leaving a visible trail of liquid excrement; and (5) larval growth and coloration change, and larvae shrink and eventually die. This virus is transmitted from one generation to the next by disease-carrying adults that survive a low degree of infection in the larval stage.

Biological control and sprays of Bacillus thuringiensis and the Entrust formulation of spinosad are acceptable for organically certified grapes.

If the granulosis virus is not present, the amount of leaf damage will increase with each generation. Monitor end and border vines during the first generation. This can be done at bloom when monitoring for other caterpillars; see MONITORING CATERPILLARS. Record results on a monitoring form (example form—PDF). If larvae are found and the virus is not present, treat soon after bloom. If needed later in season, treat when young larvae are found.

Check table grapes for sunburned fruit, a possible sign of defoliation caused by western grape leaf skeletonizer.

• Caterpillar monitoring form (PDF)
• Photos of virus-infected western grapeleaf skeletonizer