Answer: Although leafhoppers infest most vineyards in California, they may not require chemical treatment because vines can tolerate fairly high populations without harm, and predators and parasites may be able to maintain leafhopper populations below tolerance levels. It takes some time for the leafhopper population to become high enough to attract predatory insects. It sounds as though your grapevines have reached that threshold.
Unfortunately, there are not many insecticides registered for homeowner use. Insecticidal soaps will work, but applied this late in the year will spot the bloom on the grape skins. It isn't harmful, but doesn't look very attractive.
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