What is infestation on grape vine leaf? Near Lake Erie
These bumps or nodules on the grape leaves are called galls. They are formed by small insects called midges. The midge deposits eggs in the leaf tissue. The formation of the gall is the plant's reaction to the eggs in the tissue. The egg develops into a larva which will exit the gall, fall from the leaf to the ground and burrow into the soil. Once in the soil, the larvae will pupate then develop into an adult and start the process all over again.
Like most galls, grape gall midges seldom cause any real problems and generally do not reduce the number of grapes produced. Therefore, using chemicals is not recommended, especially because of the risk of killing native pollinators and honey bees. Pruning galls is the best management practice – especially before you see any exit holes in the gall.
Following are a few links to sites with more information: