Spiders compose the dominant predator group in California vineyards, and are assumed to play a role in control of leafhoppers (Erythroneura spp.). A 3-year study was carried out to understand the influence of ground covers on leafhopper densities. Results show leafhopper densities were typically 15-25% lower in treatments with permanent ground covers. However, spider density did not differ significantly among treatments. Spider species composition did vary; for example, Trachelas pacificus, was significantly more abundant on vines above ground covers. Levels of parasitism did not vary consistently among treatments (this was similar for plots adjacent to prune refuges). Vine vigor was significantly lower in treatments with permanent ground covers. It is suggested that lowered leafhopper density in ground cover treatments was largely a result of reduced vine vigor, resulting in a poorer quality host plant. Data are discussed with respect to changes in spider species composition and migration patterns.
Species 1: Homoptera Cicadellidae Erythroneura variabilis (variegated leafhopper)
Species 2: Araneae Clubionidae Cheiracanthium inclusum
Species 3: Araneida Corinnidae Trachelas pacificus
Keywords: nutrient competition, groundcover
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