In general, leafhoppers feed by penetrating the underside of leaves and sucking out the sap. Hosts include numerous varieties of grasses and many other vascular plant species. Some leafhopper species are considered “agricultural pests” for the damage they do to crops such as potatoes, lettuce, beets, and as in this case, grapes. Their saliva can cause leaf curling, yellowing of leaves, as well as stunting and distortions of the plants the leafhoppers feed from.
I have read (but not confirmed) that there are more species of leafhoppers than all the species of amphibians, reptiles, birds and mammals combined. Although there have been about 20,000 species recognized, some scientists suspect there could be well over 100,000 species.