Japanese beetle season is upon us. These beetles feed on over 300 species of plants including tree fruits, small fruits, vegetables, ornamentals, and of course, grapes. Japanese beetles can be a severe pest of grapes during the summer, feeding mainly on foliage and (thankfully) rarely on the berries. These beetles are most active on warm, sunny days and tend to congregate in groups on vines to feed and mate near the top of the canopy. The adult beetles feed on the soft leaf tissue between veins, which often results in a skeletonized appearance. Research from up North has shown that grapevines, depending on age and variety, can tolerate a reasonable amount of leaf area loss without detrimental effects on the crop (see: https://www.isaacslab.ent.msu.edu/Images/talks/Isaacs%20Viticulture%202010%20JB%20for%20web.pdf). However, no Japanese beetle or leaf damage thresholds have been established for grapes, so growers should rely on their judgment and experience to determine whether beetle abundance and/or injury warrants chemical control. See Table 1 below for a list of some of the commonly recommended chemicals for Japanese beetle control. Abundance is often highest at vineyard borders, so focused management for these beetles may be an option to minimize cost.
Additionally, Japanese beetles appear to show preference for grape varieties with smooth, thin leaves which are unfortunately characteristic of many wine grape varieties (e.g., Chardonnay, Traminette, and Vidal Blanc). Thus, focus for monitoring and management should be given to young vineyards and areas with a history of Japanese beetle injury. Note that the standard monitoring traps for Japanese beetles are highly efficient at attracting beetles to your vineyard, but are terrible at trapping them. Thus, these traps are NOT recommended.
Management recommendations can be found at: https://www.smallfruits.org/SmallFruitsRegGuide/Guides/2016/BunchGrapeSprayGuide2016.pdf
Table 1. Recommended insecticides for Japanese beetle management.