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.
Damage
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.
Management
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.
Biological Control
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.
Pesticides and Natural Enemy Releases
Not all registered pesticides are listed. The following are ranked by their IPM value, with the most effective and least harmful to natural enemies, honey bees, and the environment listed at the top of the table. When choosing a pesticide, consider information related to water and air quality, resistance management, and the pesticide's properties and application timing. Use PestManage to compare summarized management options for different pests in the same crop. Always carefully read the label of the product being used and take all necessary precautions when handling pesticides.
| Rank | Active ingredient | Example trade name | Group | Group Order | MoA 1 | Amount per acre | REI (hours) ‡ | PHI (days) ‡ | Comments | Selectivity 2 | Bees 3 | Predatory mites 4 | Predators 5 | Parasitoids 5 | Residue duration 6 | Leaching(fish) 7 | Adsorbed runoff(fish) 8 | Solution runoff(human) 9 | Leaching(human) 10 | Solution runoff(human) 11 | Last updated 12 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
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various products # | Application Timing Varies (See UC IPM Pest Management Guidelines and Label) | 8 |
11A |
Label rates | 4 | 0 | Only effective against young larvae. Pro... | narrow | low | low | low | none | — | — | — | — | — | 07/2015 | ||
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Agri-Mek SC | Application Timing Varies (See UC IPM Pest Management Guidelines and Label) | 8 |
6 |
1.75–3.5 fl oz | 12 | 28 | Do not make more than two applications p... | moderate | moderate | low | moderate/high | moderate | high | low | extra high | low | intermediate | 07/2015 | ||
|
|
Assail 70WP | Application Timing Varies (See UC IPM Pest Management Guidelines and Label) | 8 |
4A |
1.1 oz | 12 | 3 | To protect honey bees, apply only during... | broad | — 13 | — 14 | — | moderate | very low | very low | low | very low | low | 07/2015 | ||
|
|
Altacor | Application Timing Varies (See UC IPM Pest Management Guidelines and Label) | 8 |
28 |
2.0–4.5 oz | 4 | 14 | — | narrow | low | low | low/moderate | short | — | — | — | — | — | 07/2015 | ||
|
|
Admire Pro | Application Timing Varies (See UC IPM Pest Management Guidelines and Label) | 8 |
4A |
1.0–1.4 fl oz | 12 | 0 | Do not exceed 0.5 lb a.i. of imidaclopri... | narrow | — | low | — | — | — | — | — | — | — | 07/2015 | ||
|
|
Admire Pro | Application Timing Varies (See UC IPM Pest Management Guidelines and Label) | 8 |
4A |
7–14 fl oz | 12 | 30 | Do not exceed 0.5 lb a.i. of imidaclopri... | narrow | — | low | — | — | — | — | — | — | — | 07/2015 | ||
|
|
Avaunt | Application Timing Varies (See UC IPM Pest Management Guidelines and Label) | 8 |
22A |
3.5–6 oz | 12 | 7 | — | narrow | low | low | low | moderate | low | low | intermediate | very low | low | 07/2015 | ||
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|
Intrepid 2F | Application Timing Varies (See UC IPM Pest Management Guidelines and Label) | 8 |
18 |
10–16 fl oz | 4 | 30 | Do not apply more than 48 fl oz/acre per... | narrow | low | low | low | none | very low | low | low | very low | low | 07/2015 | ||
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Delegate WG | Application Timing Varies (See UC IPM Pest Management Guidelines and Label) | 8 |
5 |
3–5 oz | 4 | 7 | A stomach poison; most effective when in... | narrow | low/moderate | moderate 15 | moderate/high | moderate 16 | low | low | low | very low | low | 07/2015 | ||
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Entrust # | Application Timing Varies (See UC IPM Pest Management Guidelines and Label) | 8 |
5 |
1.5–2.5 oz | 4 | 7 | Apply when eggs first hatch to target th... | narrow | low | moderate 15 | low/moderate | short | very low | low | low | very low | low | 07/2015 | ||
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|
Success | Application Timing Varies (See UC IPM Pest Management Guidelines and Label) | 8 |
5 |
4–8 fl oz | 4 | 7 | Apply when eggs first hatch to target th... | narrow | low | moderate 15 | low/moderate | short | very low | low | low | very low | low | 07/2015 |
Legend
- ‡ a b Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without personal protective equipment. Preharvest interval (PHI) is the number of days from treatment to harvest. In some cases, the REI exceeds the PHI. The longer of the two intervals is the minimum time that must elapse before harvest.
- # a b Acceptable for use on certified organic crops. Check with your certifier to confirm before application.
- 1 ↩ Group numbers for insecticides and miticides are assigned by the Insecticide Resistance Action Committee (IRAC). Insecticides with unknown modes of action are assigned mode-of-action group numbers (MoAs) that begin with UN. Rotate pesticides 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; insecticides with a 1B group number should be alternated with insecticides that have a group number other than 1B.
- 2 ↩ Range of insect and mite groups affected by a pesticide. Broad means the pesticide affects most groups of insects and mites; narrow means the pesticide affects only a few specific groups.
- 3 ↩ Risk of harm to honey bees. For more information, see Bee Precaution Pesticide Ratings.
- 4 ↩ Risk of harm to predatory mites. Toxicities are generally to western predatory mite, Galendromus occidentalis. Where differences have been measured in toxicity of the pesticide-resistant strain versus the native strain, these are listed as pesticide-resistant strain or native strain.
- 5 a b Risk of harm to parasitoids and general predators. Toxicities are averages of reported effects and should be used only as a general guide. Actual toxicity of a specific insecticide depends on factors including the application rate, environmental conditions, and the life stage and species of a parasitoid or predator.
- 6 ↩ Length of time residue affects natural enemies. Short means hours to days; moderate means days to 2 weeks; and long means many weeks or months.
- 7 ↩ Risk of harm to fish from leaching, based on USDA Natural Resources Conservation Service Windows Pesticide Screening Tool (WIN-PST).
- 8 ↩ Risk of harm to fish from adsorbed runoff, based on USDA Natural Resources Conservation Service Windows Pesticide Screening Tool (WIN-PST).
- 9 ↩ Risk of harm to fish from solution runoff, based on USDA Natural Resources Conservation Service Windows Pesticide Screening Tool (WIN-PST).
- 10 ↩ Risk of harm to humans from leaching, based on USDA Natural Resources Conservation Service Windows Pesticide Screening Tool (WIN-PST).
- 11 ↩ Risk of harm to humans from solution runoff, based on USDA Natural Resources Conservation Service Windows Pesticide Screening Tool (WIN-PST).
- 12 ↩ Date information was last updated in the UC IPM Pest Management Guidelines.
- 13 ↩ May cause an increase in spider mite numbers.
- 14 ↩ Acute toxicity is low but reproductive capacity is reduced.
- 15 a b c Toxic to some natural enemies (lacewing and syrphid fly larvae, predatory beetles, and thrips) when sprayed and up to 5 to 7 days after.
- 16 ↩ Residual is moderate if solution is between a pH of 7 and 8.
Important Links
UC IPM Pest Management Guidelines: Grape
UC ANR Publication 3448
L.G. Varela (emeritus), UC IPM and UC Cooperative Extension Sonoma County
D.R. Haviland, UC IPM and UC Cooperative Extension Kern County
W.J. Bentley (emeritus), UC IPM and Kearney Agricultural Research and Extension Center, Parlier
L.J. Bettiga, UC Cooperative Extension Monterey County
K.M. Daane, Kearney Agricultural Research and Extension Center, Parlier
R.J. Smith (emeritus), UC Cooperative Extension Sonoma County
L.R. Wunderlich, UC Cooperative Extension Central Sierra
F.G. Zalom (emeritus), Entomology, UC Davis
Acknowledgement for Contributions to Invertebrates
M.C. Battany, UC Cooperative Extension San Luis Obispo County
J. Granett (emeritus), Entomology, UC Davis
P.A. Phillips (emeritus), UC Cooperative Extension Ventura County
A.H. Purcell (emeritus), Environmental Science, Policy, and Management, UC Berkeley
