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The hypothesis that generalist predators limit associated prey populations was tested using spiders, a group classically thought to be an insignificant component of the agroecosystem. Two habitat manipulations (addition of mulch and flowers) in separate and combined treatments were utilized to enhance spider numbers in a mixed vegetable system. Compared to control plots, significantly higher spider densities were observed in the plots to which mulch alone or both mulch and flowers had been added. Insect damage to the plants was significantly lower in the plots to which mulch had been added as well, which correlates with the lower numbers of pest insects in plots containing mulch. That spiders were the probable cause of the effect was demonstrated in plots containing both mulch and flowers but from which spiders had been systematically removed. Removing spiders from plots with mulch and flowers removed the effect of this treatment on pest numbers and plant damage. In addition, 84% of the predators observed foraging in the study areas during timed watches were spiders and 98% of the predation events observed were by spiders. In a separate experiment, spiders were added to a subset of individually bagged broccoli plants that had been infested previously with known numbers of herbivorous pests. Plant damage in bags lacking spiders averaged 93.3%, whereas damage averaged 31.8% in bags to which spiders had been added.

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