Ground cover is used in some vineyards to improve soil structure and help manage insect pests; previous studies have shown lower leafhopper (Erythroneura spp.) densities on vines grown with ground cover. We undertook a 2-yr study to determine why ground cover is associated with reduced leafhopper densities. Ground cover consisted of a fall-planted cover crop of purple vetch (Vicia benghalensis) and barley (Hordeum vulgare), which senesced in May and was replaced by a complex of resident vegetation comprised primarily of the grasses Echinochloa spp., Digitaria sanguinalis, and Setaria spp., as well as common knotweed (Polygonum aviculare). We compared three treatments during the growing season: Cover, No Cover, and Cover/Exclusion. Cover/Exclusion was similar to Cover treatment but with barriers to impede arthropod movement between ground cover and vines. We measured leafhopper density and egg parasitism, spider density and diversity, and grapevine vigor, and found that mid- and late-season leafhopper densities were significantly lower in Cover versus No Cover. Neither leafhopper egg parasitism nor spider density on the vines or ground cover could explain these differences; however, grapevine vigor was significantly lower in Cover than No Cover, and provides the best correlation to leafhopper density. Late-season leafhopper density was highest in the Exclusion treatment but cannot be explained by changes in grapevine vigor. Individual spider species composition and density on the grapevine canopy varied significantly among treatments: Trachelas pacificus (Chamberlin and Ivie) was higher in the Cover treatment, Hololena nedra Chamberlin and Ivie, Cheiracanthium inclusum (Hentz), and Neoscona oaxacensis (Keyserling) were lower in the Exclusion treatment, and Oxyopes spp. was higher in the Exclusion treatment. We suggest the lower densities of leafhoppers in the Cover treatment resulted from poorer host plant quality because of the competition between ground cover and grapevines. The higher late-season leafhopper densities in the Exclusion treatment may be due to changes in spider species composition, and subsequently, differences in rates of predation on leafhopper nymphs.