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 may be directly associated with vineyard cover crops, and further delineating the patterns of abundance and distribution of important spider species in vineyards. One clear pattern beginning to emerge is an inverse relationship between spider and leafhopper densities in vineyards. Our findings in this and other studies agree with several other vineyard spider researchers in demonstrating that when spiders are abundant, leafhoppers generally tend to stay below economically damaging levels. The two most abundant spiders sampled from mid-June to the end of November in 1992 belonged to the family Clubionidae (two-clawed hunting spiders): Trachelas pacificus and Chiracanthium inclusum. Juveniles for these two species were roughly three times as abundant as adults in samples taken throughout the season. The next most abundant spider was Theridion (family Theridiidae), which was also of particular interest by being detected only in the grapevine canopy (i.e., almost never from the cover crops between vine rows). It should be noted that the most abundant spiders commonly detected in both cover crops and canopy were Trachelas and the micryphantids. Another particularly noteworthy discovery during the 1992 season involves an apparent correlation between western grapeleaf skeletonizer (WGLS) mortality and Trachelas pacificus occurrence. Corrugated cardboard bands wrapped around the base of vines are very effective in concentrating WGLS larvae seeking pupation sites. Bands which individually contained up to 70 WGLS pupae typically were found to be free of spiders. However, it was also not uncommon to find bands with only 5 to 10 WGLS pupae; in virtualy all these cases a large number of Trachelas juveniles were also found residing in those bands. Additional data collected in “round-the-clock” sampling trials during 1992 indicated that in estimating population densities for several important spider species, the actual time of day when samples are taken can be of considerable importance.