The western grape leafhopper and the variegated leafhopper are common pests of table grape vines. Leafhoppers feed on the grape vines' leaves, leaving yellow and white spots behind them. Small populations of leafhoppers are not detrimental to table grape vines--table grape vines can tolerate significant populations of leafhoppers. Large infestations, however, can cause the vine to lose a considerable amount of leaves. This defoliation may expose the fruit to sunburn, delay its ripening or stunt the growth of the vine.
Allow the leafhoppers' natural predators time to control the population. Wait at least until the second season of infestation to allow natural predators time to cull the population on their own.
Remove any weeds growing in the vineyard and the area surrounding it before spring. Leafhoppers may be breeding on their leaves as well.
Spray your grape plants with a narrow range oil or kaolin clay if populations warrant it. Examine 20 randomly selected vines in each block for first-generation nymph infestations four weeks after bud break (or whenever you notice the small, wingless nymphs crawling around the leaves). On each vine, examine one leaf that is three to four leaves above the base of the shoot. If you find an average of more than 15 leafhopper nymphs (taking into account all 20 leaves), use the spray. Follow the manufacturer's instructions and spray the entire vine, covering all of its foliage.
Prune any leaves that harbor leafhopper eggs during fruit set, and for two weeks afterward. The eggs are housed in small (less than 1 mm in length) blisters on the underside of the leaves. According to University of California entomologists, this will eliminate 30 to 50 percent of the population.