Final Report: 2021- 2022

David G. James, Department of Entomology, Irrigated Agriculture Research and Extension Center, Washington State University, Prosser, WA 99350

Tel: 509 786 9280. Email: david_james@wsu-edu

Summary: Leafminer damage on wine grapes was first reported from vineyards in the Tri-Cities area in September 2020. We determined that the damage was caused by a new, undescribed species of leafmining moth in the genus Phyllocnistis. This species is the first leafminer pest to be recorded damaging commercial wine grapes in North America.

The commercially available pheromone for the Citrus leafminer, surprisingly, was found to be an effective lure for the Grape leafminer and facilitated our surveying and monitoring in 2021. We deployed pheromone-baited white sticky traps in single vineyards in eleven eastern WA AVA’s during July-September 2021. Grape leafminers were detected in six AVA’s (Walla Walla, Candy Mountain, Horse Heaven Hills, Columbia Valley, Red Mountain and Snipes Mountain). Most of these detections comprised low numbers (1-20) except Columbia Valley (208) which is the AVA where the original detection of this insect occurred in 2020. These results suggest that the center of Grape leafminer distribution and abundance in eastern Washington viticulture in 2021 was focused on the Tri-Cities area within the Columbia Valley.

Two vineyards in the Tri-Cities area that harbored substantial populations of Grape Leafminers in 2020, were selected for regular monitoring in 2021. Leafminers were far more abundant at one vineyard in 2021 with hundreds of leafminer adults trapped weekly during the season, with a maximum of almost 1500 (3 traps) in late June. In both vineyards numbers of adult leafminers increased rapidly in June (maximum of 1481 in one trap on June 22), before falling quickly at the end of the month into July. The decline in leafminer numbers may have been associated with the historically high temperatures that occurred in eastern WA in late June 2021. Leafminer larvae living between the upper and lower leaf epidermis likely were ‘cooked’ in the sun.

Our research has shown that the Grape Leafminer, a new, undescribed species, can occur in large populations in eastern Washington vineyards with much visible evidence of leaf damage. However, as of 2021, it was only abundant in a few vineyards in the Tri-Cities area in the Columbia Valley AVA. It is possible or even likely that this pest will spread in the future and for populations to increase region-wide. However, it seems likely that Grape Leafminer will principally be a cosmetic leaf pest issue. Affected leaves look unsightly, but economic damage to mature grapevines is unlikely. The impact of Grape Leafminer on nursery and newly-established young grapevines could be more significant, however, and needs further study.

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Pest & Disease //