Mechanisms underlying the effects of spiders on pest populations

Assemblages of spider species can make significant reductions in pest numbe rs that are of value to the farmer A group of spider species with complemen tary niches leaves few refuges for the pest in space or time. Spiders usual ly exert an influence on pest numbers in concert with other natural enemies , and spiders are sometimes the dominant component. In addition to killing pests by direct attack, spiders cause pest mortality by dislodging them fro m plants or trapping them in webs. If the pest is distasteful, or if it is the dominant prey type available, spiders may kill more than they consume, which increases the rate of pest kill per unit of spider food demand. The i mplications for pest control, of various types of interaction between spide rs and other natural enemies, are explored in this paper. Interactions with specialist natural enemies usually result in complementary effects,, enhan cing pest control. Specialists reduce the density of pests to levels where spiders can prevent resurgence. Specialists foraging on the crop may flush pests off the plant to be killed by ground-zone spiders. Although hyperpred ation (i.e., predators killing other predators) may disrupt biological cont rol occasionally, it is considered that the wide range of competitive inter actions between natural enemies, in general, promotes diversity and stabili ty of the natural enemy community and generates a robust basis for pest con trol.