The impact of spiders and ground beetles on the abundance of insect pests and yields in soybean and vegetable agroecosystems

Conserving generalist predators is a component of many integrated pest management programs. However, because such predators prey upon both pests and other enemies of pests, it is difficult to predict the impact of particular generalist predators on pest populations. I performed large-scale, manipulative field experiments to reveal the extent to which spiders and carabid beetles, which are common generalist predators in agroecosystems, limit populations of insect pests in soybeans and vegetable crops.;In 1994 I reduced densities of spiders and carabid beetles in fenced, 7 x 7-m plots of soybeans. Comparing insect numbers and soybean yield in the experimental plots with open control plots provides no evidence that these generalist predators depress herbivorous insects in soybean.;In 1995, as background to performing field experiments in vegetable crops, I surveyed spider populations and measured habitat variables on 23 organic vegetable farms in central Kentucky. Landscape features, vegetable type and mulching technique influenced the distribution of one or more of six common spider families. Among the common spiders, only the sheet-web building Linyphiidae showed no correlation with environmental variables.;I conducted field experiments in 105-M2 mixed-vegetable gardens in 1996 and in 64-M2 gardens of Kennebec potato in 1997. The experiments employed a combination of mulching, fencing, hand-removal and live pitfall trapping to after densities of spiders and carabid beetles. Predation by spiders and carabid beetles reduced densities of squash bugs, Colorado potato beetles and potato leafhoppers. However, the magnitude of the reduction in pest densities varied between years and effects were often marginally significant statistically. When densities of insect pests were reduced, effects on vegetable yield were often absent. The only significant or marginally statistically significant effects were on the size of marketable cucumbers and the yield of Kennebec potatoes in 1996. Predation by spiders and carabids also affected densities of several other predators of insect pests: predaceous damsel bugs, big-eyed bugs and lacewings.;Future studies are needed to quantify further the indirect effects of spiders and carabids on other natural enemies, and to determine threshold pest densities below which spiders and carabids can limit pest populations.