An Introduction for Colorado Growers Bob Hammon
CSU Extension, Tri River Area, Grand Junction office
2775 Highway 50, Grand Junction CO 81502
Western Grapeleaf Skeletonizer (WGS), Harrisina brillians, appeared in the vicinity of Montrose (Montrose County) during the summer of 2002. As of the end of August 2002, the infestations appear to be limited to Virginia creeper (Parthenocissus quinquifolia) and Boston ivy (Parthenocissus tricuspidata), but the potential for infestations in grapes exists. This web page is designed to introduce grape growers and home gardeners to the life history and management options of WGS. It will be updated as our knowledge of this pest under Colorado conditions develops.
Western grapeleaf skeletonizer is a major defoliating pest of grapes in parts of California. It is a native of the southwestern United States and northern Mexico. Several species of grapeleaf skeletonizers exist, with Harrisina americana existing as a pest of southeastern US grapes. There are three generations of WGS per year in California. Multiple generations have been observed in the Montrose area in 2002, but the exact number is unknown.
WGS adults are a bluish-black moth with a wingspan of about 5/8 inch, but can be smaller when larval food is in short supply. Eggs are pale yellow to whitish, laid in masses of ten to more than 200. The eggs do not touch one another in the masses. There is another currently unidentified moth which will lay egg masses on western Colorado grapes, but these eggs are laid directly touching one another. Virtually all eggs are laid on the underside of the leaves. In vineyards, most first generation eggs are laid on the inner mature leaves under the canopy. Second and third generation eggs are laid on the leaves of the outside shoots. The position on the outer portion of the plant makes scouting simple by lifting the shoots and looking for masses on the undersides of the leaves.
All eggs in a cluster tend to hatch simultaneously. The first of five instars is pale white, and about 1/16 inch long. The larvae of the first three instars line up side by side in a circular row to feed. First instar larvae feed on the underside of the leaf, and leave an area that appears as a white spot on the upper leaf surface. This area can be 1 ½ inch across. The distinct coloration is shown in the picture, and appears in third instar larvae.
The larvae are strongly gregarious, and feed side by side through the early portion of the larval cycle. When ready to molt, larvae of the first three instars move away from their feeding site. They may move to an undamaged leaf or stem. When sampling only damaged leaves, larvae may not be found, so it is important to search the entire plant for larvae if damage is found.
Early instar larvae feed on the leaf undersides, leaving the upper cuticle and veins intact. The white, damaged leaves are easily seen in WGS surveys. The gregarious feeding habit is lost in fourth and fifth instar larvae, and these scatter over the plant. A few later instar larvae can skeletonize entire leaves.
Many people are sensitive to contact with WGS larvae. The long, dark hairs on the bodies are poisonous when broken. Skin welts, similar to those produced by stinging nettles, can appear on sensitive individuals after contact with late instar larvae.
Pupae occur in silken cases, and may be under bark or in trash around the base of plants. WGS overwinters in the pupal stage. Emergence of adults and egg laying does not occur until after the flush of shoots and leaves in the spring.
Second and third generation larvae can completely defoliate vineyards if left untreated. Crop yield and quality can be adversely affected by WGS infestations, so it is important to scout for larvae early in the season and take action if eggs or larvae are found. Management techniques for Colorado conditions are unknown at this time, so caution is advised. If WGS is found in a vineyard, action should be taken.
Monitoring guidelines developed for California are based on counting adult WGS in measured areas within vineyards. These guidelines will have to be tested under Colorado conditions if infestations develop in vineyards. This pest is easily controlled with insecticides. Both conventional insecticides and biological formulations of Bacillus thuringensis are effective. In most cases, one properly timed application may give season long control. Several biological control agents have been released in California, and this long-term solution will have to be initiated in Colorado after the infestation develops.
This page was updated on April 26, 2014