Actor Arnold Schwarzenegger is famous for the legendary Hollywood phrase “I’m B-a-c-k” yet also on the return is the voracious western grapeleaf skeletonizer (WGS) pest in Napa County, Calif. grape country following an eight-year hiatus.
Agricultural officials on June 24 discovered an adult male WGS in a vineyard pheromone-baited trap on Tubbs Lane in Calistoga.
“This is a destructive and serious pest,” says Greg Clark, Napa County Agricultural Commissioner.
The western grapeleaf skeletonizer, Harrisina metallica, is native to Arizona and New Mexico and was first discovered in California in the 1940s. It eventually spread throughout the Golden State, especially in the Central Valley.
“There was a time when this moth pest was very common in the San Joaquin Valley, especially in raisin vineyards,’ said Kent Daane, University of California Cooperative Extension specialist based at the Kearney Agricultural Research and Education Center in Parlier.
Nat DiBuduo of Allied Grape Growers in Fresno remembers previous pest finds.
“We (grape industry) successfully rid ourselves of it in the San Joaquin Valley. It is a very hungry pest and if not controlled can destroy a vine and vineyard. However it can be controlled.”
The last WGS find in Napa County was in 2007 on Mt. Veeder Road.
Clark asks commercial grape growers and gardeners to keep their eyes peeled for all WGS larval (caterpillar) stages.
“All larval life stages are voracious feeders that cause extensive damage to grape leaves, including partial or complete defoliation of grapevines,” Clark said. “Excessive feeding can damage fruit, lead to secondary fungal damage, and grape cluster rot.”
Clark stated, “We do not want this pest to become established in Napa County.”
After the find, Ag Commission pest detection trappers deployed 25 additional traps within a mile radius of the Calistoga find.
Helpful WGS guide available
The University of California Cooperative Extension’s North Coast Farm Advisor Lucia Varela, Santa Rosa, and Viticulture Farm Advisor Monica Cooper, Napa, have released a “Monitoring Guide for Western Grapeleaf Skeletonizer” to help vineyard owners, growers, and workers understand the life stages and feeding habits of the skeletonizer pest.
Varela and Cooper say look at the grape leaf underside for the pest.
Damage is caused by 1st-5th stage larvae. Early symptoms include three “cleared” areas. Pale or whitish capsule-shaped eggs are laid in mass and third-stage larvae feed as a group.
Fourth-stage larvae feed side-by-side while fifth-stage larvae feed individually.
Pest damage is easy to detect. All that is left after the larvae feeding cycle is a distinctive lacy-skeletonized leaf with only the main veins remaining.
If the pest is found in Napa County, contact the Ag Commissioner’s office at (707) 253-4357 or by e-mail at [email protected]. You can also reach the UCCE Napa County office at (707) 253-4221 or [email protected].
Adult, larvae information
According to the UC IPM website, western grapeleaf skeletonizer moths fly during the day and are metallic blue or greenish black in color. The wing span is 1 to 1.3 inches with a .6-inch body length.
Typically, cooler coastal regions have two generations of the moth per year while three generations are typical in the Central Valley.
The five larval stages are distinguishable by color. The first two stages are cream colored while third stage larvae are brownish. Larvae with two purple and several blackish banks are signs of the 4th and 5th stage larvae.
At maturity, larvae crawl under loose bark or into ground litter to spin a dirty, whitish cocoon to pupate.
Field workers should be cautious in the vineyard as larvae have long black poisonous spines which can cause skin welts.
According to UC, leaf damage from the 1st-4th larvae instar results in only the leaf veins and upper cuticle remaining, giving a whitish paper-like appearance. The leaf eventually turns brown.
The late fourth- and all fifth-stage larvae skeletonize the leaves, leaving only the larger veins.
In large numbers, larvae can defoliate vines by July. When this occurs, larvae feed on grape clusters causing bunch rot, defoliation-caused fruit sunburn, and quality loss.
The moth is not a long-distance flyer so it does not exist in all grape-growing regions.
UC says the pest can be controlled with insecticides.
“Imidacloprid is very effective on this pest,” said David Haviland, UC Cooperative Extension entomologist and pest management farm advisor in Kern County.
With imidacloprid and biocontrol, Haviland says the western grapeleaf skeletonizer is rarely seen in commercial vineyards. The pest is still prevalent in urban settings with grapevines in backyards.
“I get many calls every year about what homeowners can do to control the black and white ‘worms’ defoliating their grapevine,’ he said.
Biological control includes the Apanteles harrisinae and Amedoria misella (Sturmia harrisinae) parasites which attack larvae. Thousands of these parasites were released in the past in the San Joaquin Valley. Amedoria misella was commonly released in SJV vineyards.
According to UC, a granulosis virus for control has been successful in several areas in certain areas with good success. With the virus, eggs within clusters are scattered instead of compactly laid, and egg numbers are reduced and fail to hatch.
The UC IPM website says biological control and sprays of Bacillus thuringiensis and the Entrust formulation of spinosad are acceptable for use in organically certified grapes.
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To prevent the spread of the pest, growers, vineyard managers, wineries, and residents who transport farm equipment or wine grapes into Napa County should inspect these items to ensure they are clean of this and other wine grape pests.