INFLUENCE OF GROUND COVER ON SPIDER POPULATIONS IN A TABLE GRAPE VINEYARD

1. Cover crops and/or resident ground vegetation have been used in Cal ifornia Vineyards to increase the number of predators and decrease the number of pestiferous herbivores. The most common resident predators in vineyards are spiders (Araneae). Several observational studies sugg est that the addition of cover crops results in an increase in spider density and a decrease in insect pest densities. 2. To test experiment ally the effects of cover crops and/or resident ground vegetation (her eafter collectively referred to as ground cover) on spider populations , a 3-year study was undertaken in a commercial vineyard. Large, repli cated plots were established with and without ground cover during the growing season. Spider species diversity was analysed on the vines and on the ground cover. 3. On the vines, there was no significant differ ence in spider species richness or the total number of spiders in plot s with and without ground cover. There were differences in the relativ e abundance of two spiders between treatments, with one species (Trach elas pacificus [Chamberlin & Ivie]) more abundant in plots with ground cover and another (Hololena nedra Chamberlin & Ivie) more common on v ines in plots with no ground cover. Annual variation in spider abundan ce was greater than variation due to ground cover treatment. 4. On the ground cover, the spider species diversity was considerably different from that found on the vines above, suggesting that there is Little m ovement of spiders between the ground cover and the vines. Enhancement of ir: pacificus populations on vines with ground covers may be a res ult of prey species movement between the ground cover and the vines. S pider abundance was sparse on the bare ground. 5. The maintenance of g round cover increased spider species diversity in the vineyard as a wh ole (vine and ground cover). However, the relatively small changes in spider abundance on the vines indicate there are limitations in the us e of ground covers for pest management with respect to generalist pred ators.