I’ve received questions about a couple of questions about two different yet similar insect pests: grape tumid gallmaker (GTG) and phylloxera. Both of these insects cause galls that develop around larvae that feed on foliar (and fruit, in the case of GTG) tissue which can be quite concerning, but often are not an economic threat. That said, I’ve seen vines with over 50% cluster damage from GTG, and young vines with significant leaf distortions from phylloxera that overall vine growth is likely impacted. Treatment options should be the same for either pest. Although GTG may not technically be on the label for If you decide on treatment, there are a few materials of choice, which Tim Martinson outlined well in an older post that is still relevant today. Basically, Movento is the gold standard, and Dr. Ann Hazelrigg at the UVM Plant Diagnostic Clinic recently confirmed that it killed larvae inside GTG galls from one area vineyard. Admire Pro and Assail are labeled and effective against phylloxera, but have no label for GTG.
If galling is extensive, removal of damaged leaves may not reduce the damage to the plant. Galled leaves are still photosynthesizing, and, for phylloxera anyway, hatched nymphs (“crawlers”) have likely left the galls to infest younger leaves already. As those leaves expand, expect to see more galls, but not necessarily as bad as the ones on early-season leaves. I’ve seen some pretty scarred-up leaves, especially on ‘Frontenac’, that have ripened a crop just fine. If the damage is looking too severe (and I don’t have a specific threshold), then treatment with one of the systemic materials listed above is your best management option.
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