With vineyards in many areas of California at or approaching bloom, University of California IPM guidelines recommend that growers with a history of damage from omnivorous leafroller, grape leaffolder and western grapeleaf skeletonizer as well as orange tortrix or light brown apple moth in coastal regions plan to treat for these caterpillars once the vines begin flowering.

Even if these pests haven’t been a problem in the past, it’s also a good time to monitor flower clusters or leaves for any damage to determine the need for treatment this season.

The type of damage they cause varies by species. Grape leaffolder and western grapeleaf skeletonizer, for example feed on foliage. In fact, heavy populations can defoliate the wines, while Omnivorous leafroller, orange tortrix, and light brown apple moth feed on leaves as well as flowers, and developing berries. However, the primary damage they cause is by feeding on fruit, opening the way for rot organisms to enter.

To prepare for bloom, UC IPM specialists recommend starting to monitor for webbing on leaves caused by omnivorous leafroller, orange tortrix or light brown apple moth early in the period of rapid shoot growth. This can help map out areas of concern to monitor for the caterpillars during bloom. Unroll leaves with orange tortrix or light brown apple moth and look for leafroller larva, pupa, or parasite cocoons. Check for leaves skeletonized by western grapeleaf skeletonizer.

After bloom, continue monitoring for omnivorous leafroller, orange tortrix, and light brown apple moth by examining fruit clusters and start monitoring leaves for grape leaffolder and western grape leaf skeletonizer following these UC IPM guidelines:

-Omnivorous leafroller

Examine 10 flower/fruit clusters in the center of each of 20 vines, for a total of 200 clusters. If you see webbing and frass, look for caterpillars. Note the number of clusters infested with omnivorous leafroller.

At bloom, treat if any larvae are found.

After bloom, treat if 2 or more clusters are infested.

-Orange tortrix (Coastal regions only) 

From bloom until bunch closure, examine 10 flower/fruit clusters in the center of each of 20 vines, for a total of 200 clusters.

If you see webbing and frass, look for caterpillars. Note the number of clusters infested with orange tortrix.

If you find an average of 0.5-1 larva/vine, treatment may be warranted if parasites are not present.

-Grape leaffolder

Count the number of rolled leaves per vine.

Unroll leaves and look for both healthy and parasitized grape leaffolder larvae.

Treatment may be warranted if population levels are increasing. Treat before larvae roll leaves.

-Light brown apple moth

From bloom until bunch closure, examine 10 flower/fruit clusters in the center of each of 20 vines, for a total of 200 clusters.

If you see webbing and frass, look for caterpillars. Caterpillars must be properly identified by the Agricultural Commissioner's Office.

From bloom to bunch closure treat if any larvae found is identified as light brown apple moth.

-Western grapeleaf skeletonizer

Check for skeletonized leaves.

If present, look for caterpillars and evidence of granulosis virus.

If larvae are found and no granulosis virus is evident, treat soon after bloom.

Treatment after veraison for omnivorous leafroller, orange tortrix, and light brown apple moth is limited in effectiveness and not recommended. However, veraison monitoring for all these caterpillars will reveal any larval damage going into harvest and potential problems next  year

At harvest, check fruit clusters for damage by omnivorous leafroller, orange tortrix, and light brown apple moth to assess this year's management program and to plan for next year. In table grapes, also assess grape leaffolder damage.

A sample form for monitoring for caterpillars along with photos of them and the damage they cause at bloom and veraison are available in the online UC Pest Management Guidelines for grapes:  www.ipm.ucdavis.edu