Have Your Vines Been Skeletonized-

Jennifer Hashim

UCCE Farm Advisor, Kern County

Viticulture - Grapes & Kiwis

October 3, 2002

It's officially fall and the leaves on grapevines are turning yellow. Thus, begins the cycle of leaf senescence and dormancy. Over the last couple of weeks I have received an influx of calls from homeowners regarding worms defoliating their backyard grapevines. The call goes something like this: "Help! My vines are being eaten by yellow, black and purple caterpillars. What can I spray them with?"

The larvae described are called Western grapeleaf skeletonizer (WLGS), Harrisina brillians. WGLS is common to the western US and was first found in 1941 in California, near San Diego, where it severely defoliated wild grapevines. WGLS has been a nuisance of sorts in commercial table grape vineyards because if vines are partially or completely defoliated before harvest occurs, the fruit may sunburn badly and as a result reduce yield. Defoliation after harvest is less damaging, but there may be consequences regarding stored carbohydrate reserves in the following season.

One reason homeowners may not have seen the pest in the past is that field trials in cooperation with entomology departments at various University of California campuses as well as the California Department of Food and Agriculture were extremely successful controlling it (in the mid-80's) with a biological disease that affects the gut of larvae called a granulosis virus. Diseased larvae feed erratically and the virus inhibits food assimilation. Over the last couple of years, we have seen WGLS populations rise in the Bakersfield area, which indicates that the virus in nature has died out (with the sick caterpillars) and should be reintroduced.

Here are a few notes on its life cycle:

Eggs: Capsular-shaped eggs are 0.4 mm wide and 0.6 mm long. They are whitish to pale yellow in color. Eggs are laid on the underside of the leaves in clustered groups, a single egg deep.

Larvae: Larvae are strongly gregarious and feed together side-by-side in the first three instars. Larvae have five instars before pupating. Second instar larvae are 3-5 mm long and are yellowish-white in color with black hairs on the body segments. Third instar larvae are 6-8 mm long and the fourth and fifth instars are from 11-16 mm long. The older larvae have seven circular bands on the body that are black, purple and yellow in color. Fully-grown larvae spin silken cocoons into the bark of the vine in which to pupate. **A word of caution: Larvae have many long, dark hairs that are moderately poisonous. If there is skin contact with such hairs, one can develop a moderate rash accompanied by a burning sensation.**

Adults: Male and female moths are bluish-black in color and are active during the morning.

There are approximately three generations per year in the San Joaquin Valley. Currently, the third generation larvae are moving to enter pupal diapause. The first adults will emerge in Kern County around late April and early May of next year. If you have had this problem over the summer you should begin thinking about early season control of larvae next spring. There are several pesticides (Sevin, Kryocide) that can control larval stages in commercial vineyards. However, any material containing carbaryl, sold for control of caterpillars, found at the garden supply store should take care of WGLS on backyard vines. Please be sure to be safe, read the label and apply accordingly.
For more information and electronic color pictures of the Western grapeleaf skeletonizer, please visit: http://www.ipm.ucdavis.edu/PMG/r302301011.html