Infestations by grape berry moth can vary greatly from year to year and are often uneven in a vineyard. The damage by first generation larvae can be serious since a single larvae can destroy a dozen or more potential berries by feeding on buds and flowers. Late first and all second generation larvae feed only on the berries. The second generation is potentially more damaging than the first. A single larvae can destroy a half dozen berries in a cluster, and several larvae may inhabit a single cluster.
Injured berries ripen prematurely, split open and shrivel.
Webbing produced by larvae prevent the berries from dropping. Larvae feeding directly reduces yield and contaminates the crop. More importantly, feeding by larvae creates infection sites for fruit rots and feeding by fruit flies. At harvest, severely infested bunches may contain several larvae, and many berries which are completely hollowed out. In many cases, the bunch is covered with bunch rot fungi, infested with fruit flies and appears unhealthy.