Description of the Pest
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.
Organically Acceptable Methods
Biological control and sprays of Bacillus thuringiensis and the Entrust formulation of spinosad are acceptable for organically certified grapes.
Monitoring and Treatment Decisions
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— ). 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.
|Common name||Amount per acre**||REI‡||PHI‡|
|(Example trade name)||(hours)||(days)|
|Not all registered pesticides are listed. The following are ranked with the pesticides having the greatest IPM value listed first—the most effective and least harmful to natural enemies, honey bees, and the environment are at the top of the table. When choosing a pesticide, consider information relating to air and water quality, resistance management, and the pesticide's properties and application timing. Always read the label of the product being used.|
|(Intrepid 2F)||10–16 fl oz||4||30|
|MODE-OF-ACTION GROUP NUMBER1: 18|
|COMMENTS: Do not apply more than 48 fl oz/acre per season.|
|MODE-OF-ACTION GROUP NUMBER1: 28|
|(Delegate WG)||3–5 oz||4||7|
|MODE-OF-ACTION GROUP NUMBER1: 5|
|COMMENTS: A stomach poison; most effective when ingested. To protect honey bees, apply only during late evening, night, or early morning when bees are not present.|
|(Success)||4–8 fl oz||4||7|
|MODE-OF-ACTION GROUP NUMBER1: 5|
|COMMENTS: Apply when eggs first hatch to target the young larvae. A stomach poison; most effective when ingested. Heavy infestations require a second application in 4 or 5 days. To protect honey bees, apply only during late evening, night, or early morning when bees are not present.|
|(Prokil Cryolite 96)||6–8 lb||12||30|
|MODE-OF-ACTION GROUP NUMBER1: 8C|
|COMMENTS: Wine and raisin grapes: limit of two applications per season. Table grapes: One application only and not after fruit formation. If used on wine grapes or grapes that may be sold to a winery for export, observe their restrictions on applications. A stomach poison that must be consumed by larvae so thorough coverage is important. Less harmful to natural enemies than carbaryl and provides long residual action.|
|F.||BACILLUS THURINGIENSIS ssp. KURSTAKI#|
|(various products)||Label rates||4||0|
|MODE-OF-ACTION GROUP NUMBER1: 11A|
|COMMENTS: Only effective against young larvae. Provides fairly good control, has a short residual, and is not harmful to natural enemies. If coverage is not satisfactory or if all the eggs have not hatched, requires a second treatment.|
|(Assail 70WP)||1.1 oz||12||3|
|MODE-OF-ACTION GROUP NUMBER1: 4A|
|COMMENTS: To protect honey bees, apply only during late evening, night, or early morning when bees are not present.|
|(Agri-Mek SC)||1.75–3.5 fl oz||12||28|
|MODE-OF-ACTION GROUP NUMBER1: 6|
|COMMENTS: Do not make more than two applications per growing season. Dust on leaves will inhibit absorption of this material. Effectiveness is also reduced by sulfur burn on leaves. To protect honey bees, apply only during late evening, night, or early morning when bees are not present. Certain formulations emit high amounts of volatile organic compounds (VOCs); use low-VOC formulations. Regulations affect use for the San Joaquin Valley from May 1 to October 31, 2019.|
|(Admire Pro - Soil)||7–14 fl oz||12||30|
|(Admire Pro - Foliar)||1.0–1.4 fl oz||12||0|
|MODE-OF-ACTION GROUP NUMBER1: 4A|
|COMMENTS: Do not exceed 0.5 lb a.i. of imidacloprid/acre per year. To protect honey bees, apply foliar sprays only during late evening, night, or early morning when bees are not present.|
|MODE-OF-ACTION GROUP NUMBER1: 22A|
|**||Apply with enough water to provide complete coverage.|
|‡||Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (PHI) is the number of days from treatment to harvest. In some cases the REI exceeds the PHI The longer of two intervals is the minimum time that must elapse before harvest.|
|*||Permit required from county agricultural commissioner for purchase or use.|
|#||Acceptable for use on organically grown produce.|
|1||Rotate chemicals with a different mode-of-action Group number, and do not use products with the same mode-of-action Group number more than twice per season to help prevent the development of resistance. For example, the organophosphates have a Group number of 1B; chemicals with a 1B Group number should be alternated with chemicals that have a Group number other than 1B. Mode-of-action group numbers are assigned by IRAC (Insecticide Resistance Action Committee).|