Significant Grape Pest Found in Premium California Wine County
May 24, 2022 
Western grapeleaf skeletonizer larvae.
Photo by Whitney Cranshaw
Insect trapper Jesse Guidi discovered a single adult Western grapeleaf skeletonizer in an insect trap near Dollarhide Road, Pope Valley. Napa County (CA) Agricultural Commissioner Tracy Cleveland is asking growers and gardeners to watch for all larval (caterpillar) stages of this injurious moth.
“This is a destructive and serious pest,” Cleveland says. “All larval life stages are voracious feeders that cause extensive damage to grape leaves, including partial or complete defoliation of grapevines. Excessive feeding can damage fruit and lead to secondary fungal damage. We do not want this pest to become established in Napa County.”
Guidi discovered the adult Western grapeleaf skeletonizer on a yellow-panel sticky trap generally used to monitor for the glassy-winged sharpshooter. The find was made May 12, and reported this week. Pest detection trappers in the Agricultural Commissioner’s Office are increasing trap density by deploying 25 additional traps within a mile radius of the Pope Valley find.
Although the pest is not native to Napa County, it has been found here a number of times in the past, most recently in the same area adjacent to Tubbs Lane in June 2018. Native to Arizona and New Mexico, it was first discovered in California in the 1940s and eventually spread throughout the state, particularly in the Central Valley.
Damage caused by the Western grapeleaf skeletonizer is relatively easy to detect. When it feeds on grapevines, it leaves only the veins behind, producing a very distinctive, lacy skeletal appearance.
This pest may also be found on Boston Ivy and Virginia Creeper. The larval stage is distinguished by colored bands around the body. For more information and photos, check the University of California Integrated Pest Management (UCIPM) website.
“If you suspect you may have found a Western grapeleaf skeletonizer caterpillar or adult moth, please bring it in to our office immediately, or contact us in order to help identify it,” Cleveland adds. “We also strongly encourage all growers, vineyard managers, wineries, and residents, who may be transporting farm equipment or wine grapes into Napa County to inspect these items to ensure that they are free from Western grapeleaf skeletonizer or any wine grape pests.”
For more information, call the Napa County Agricultural Department at (707) 253-4357.
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