With veraison upon us, I just wanted to remind everyone about the continued threat from Japanese beetles. The adult beetles feed on the soft leaf tissue between veins, which often results in a skeletonized appearance. Below are images from Petit Verdot that Cain observed recently in Virginia that have been severely fed on by Japanese beetles. The functional leaf area on these vines have been significantly reduced.
In Virginia, Boucher & Pfeiffer (1988) determined that a 11% reduction in leaf area after veraision resulted in a reduction in fruit quality. So while grapevines can tolerate a reasonable amount of leaf area loss without detrimental effects on the crop (see: Managing Japanese beetles), if feeding gets out of control, injury to the leaves can cause considerable impact on the grape quality. No Japanese beetle or leaf damage thresholds have been established for grapes, so chemical management is still a judgment call 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.
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