Spiders as Beneficials in Grape Agro-ecosystems

Spiders represent an entirely predacious group of organisms whose potential importance as beneficials in agro-ecosystems has generally been overlooked. The goal of this research was to investigate the potential importance of spiders as a mortality factor for grape insect pests. Spider populations were sampled at frequent intervals in a number of Fresno County vineyards. Except for one site, all vineyards utilized “soft” pesticide alternatives. Furthermore, the sites represented a range of environments including vineyards near and distant from riparian habitats. Spiders were abundant in vineyards managed with the use of “soft” pesticides (e.g., B.t. & soaps) in the Fresno County area. Three species in the families Agelenidae and Clubionidae were particularly common. Species in the family Salticidae were less abundant, although consistently present. Spider populations “built” in the canopy and persisted through the growing season with little fluctuation over short time intervals. The relative abundance of spiders in vineyard canopies suggests that they are of considerable potential importance as biological control agents in vineyards, having been grossly overlooked. It is suggested that the negative impact of pesticide use on spider populations may be extensive. Our findings indicate the need for additional research directed at defining the impact that spiders have on specific insect pests of vineyards. In addition, studies are needed to investigate the effects of ground cover management, pesticide use, and other cultural factors on spider population dynamics.