Prey Control by an Assemblage of Generalist Predators: Spiders in Garden Test Systems

Abstract

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.

Spiders as biological controllers in the agroecosystem

“…This strategy would also help to maintain the diversity of the agroecosystem. Our theoretical analysis is then in agreement with field studies indicating the positive role spiders play as biological controllers in rice fields and fruit orchards, [13,15,17], in which the method of spider augmentation led up to 60% reduction of pesticide use [10] without harming the final production. [18] 0.002 ha kg −1 week −1 µ 0.02-0.11 [5] 0.064 week −1 θ not a constant [5] 0.35 -…”

Local and Landscape Correlates of Spider Activity Density and Species Richness in Urban Gardens

“…Ground spiders do not rely on webs for capturing prey, but instead move actively over the ground and chase or pounce on their prey (Uetz and Unzicker 1976). Spiders are sensitive to both microclimate and habitat structure, and mulch may provide suitable burrowing habitat for groundhunting spiders that need specific humidity and temperature conditions (Riechert and Bishop 1990, Uetz 1991, Bell et al 2001. Covering the surface of bare soil with mulch reduces soil water evaporation by decreasing soil temperature, preventing vapor diffusion, absorbing water vapor into the mulch, and reducing wind speed gradients at the soil-atmosphere level (Hatfield et al 2001).…”

“…Alternatively, increased tree height may provide more canopy cover to spiders. Canopy cover and mulch can provide temperature and humidity regulation for species-specific habitat requirements (Riechert and Bishop 1990, Hatfield et al 2001, Lin 2010.…”

Arboreal spiders in coffee agroecosystems: Untangling the web of local and landscape influences driving diversity

“…Spiders are generalist predators that can offer the important biocontrol services in 66 agriculture (Riechert 1984, Riechert 1990, Riechert 1997, Symondson 2002. Spiders prevent 67 and suppress pest outbreaks in arable crops (Symondson et al 2002, Riechert 1984.…”

“…They are 68 unique among predators because they are largely generalist consumers (Wise 1993), which 69 allows them to persist when pest numbers are low by feeding on alternative prey items within the 70 agroecosystem (Settle 1996, Symondson 2002. In some cases diverse assemblages of spiders 71 can provide greater pest suppression than simple assemblages (Riechert 1984, Riechert 1990, 72 Riechert 1997, Symondson 2002. Given the importance of spiders in providing biocontrol, and …”