BEHAVIOR AND INDIRECT INTERACTIONS IN FOOD WEBS OF PLANT-INHABITING ARTHROPODS

With the increased use of biological control agents, artificial food w ebs are created in agricultural crops and the interactions between pla nts, herbivores and natural enemies change from simple tritrophic inte ractions to more complex food web interactions. Therefore, herbivore d ensities will not only be determined by direct predator-prey interacti ons and direct and indirect defence of plants against herbivores, but also by other direct and indirect interactions such as apparent compet ition, intraguild predation, resource competition, etc. Although these interactions have received considerable attention in theory and exper iments, little is known about their impact on biological control. In t his paper, we first present a review of indirect food web interactions in biological control systems. We propose to distinguish between nume rical indirect interactions, which are interactions where one species affects densities of another species through an effect on the numbers of an intermediate species and functional indirect interactions, defin ed as changes in the way that two species interact through the presenc e of a third species. It is argued that functional indirect interactio ns are important in food webs and deserve more attention. Subsequently , we discuss experimental results on interactions in an artificial foo d web consisting of pests and natural enemies on greenhouse cucumber. The two pest species are the two-spotted spider mite Tetranychus urtic ae and the western flower thrips, Frankliniella occidentalis. Their na tural enemies are the predatory mite Phytoseiulus persimilis, which is commonly used for spider mite control and the predatory mites Neoseiu lus cucumeris and Iphiseius degenerans and the predatory bug Orius lae vigatus, all natural enemies of thrips. First, we analyse the possible interactions between these seven species and we continue by discussin g how functional indirect interactions, particularly the behaviour of arthropods, may change the significance and impact of direct interacti ons and numerical indirect interactions. It was found that a simple fo od web of only four species already gives rise to some quite complicat ed combinations of interactions. Spider mites and thrips interact indi rectly through resource competition, but thrips larvae are intraguild predators of spider mites. Some of the natural enemies used for contro l of the two herbivore species are also intraguild predators. Moreover , spider mites produce a web that is subsequently used by thrips to hi de from their predators. We discuss these and other results obtained s o far and we conclude with a discussion of the potential impact of fun ctional indirect and direct interactions on food webs and their signif icance for biological control. Exp Appl Acarol 22: 497-521 (C) 1998 Kl uwer Academic Publishers.