Spiders in Vineyard Agro-Ecosystems
The overall goal of this research project is to continue to elucidate the ecological roles, along with the potential economic value, of spiders in vineyard agro-ecosystems. Key objectives include determining which spider species in the grapevine canopy are also associated with vineyard cover crops, and further delineating the patterns of abundance and distribution of important vineyard spider species compared with population patterns of key insect pests upon which they may feed. One clear pattern beginning to emerge is an inverse relationship between spider and leafhopper densities in vineyards. Our findings demonstrate that when spiders are abundant, leafhoppers generally tend to stay below economically damaging levels. Another noteworthy discovery involves an apparent correlation between western grapeleaf skeletonizer (WGLS) mortality and occurrence of the clubionid spider Trachelas pacificus. Corrugated cardboard bands wrapped around vine trunks are very attractive to WGLS larvae seeking pupation sites. These bands also harbor large numbers of Trachelas. especially juveniles. Data suggest an inverse correlation between WGLS and Trachelas numbers in the cardboard bands (high spiders / low WGLS, and vice versa). The most abundant spiders sampled in the grapevine canopy from early June through October belonged to the family Clubionidae (two-clawed hunting spiders): Trachelas pacificus and Chiracanthium inclusum. along with the thendiid spider Thendion. It should be noted that the most abundant spiders commonly detected in both cover crops (pitfall trap samples) and the grapevine canopy (shakecloth samples) were Trachelas and the much smaller micryphantid spiders. Sampling trials conducted “around-the-clock” (6A, 12N, 6P, 12M) during both 1992 and 1993 revealed no clear pattern of differential spider activity or variable likelihood of collecting particular spider species at different times of the day.