Parasitism of the herbivore Pieris brassicae L. (Lep., Pieridae) by Cotesia glomerata L. (Hym., Braconidae) does not benefit the host plant by reduction of herbivory

Models of tritrophic interactions often imply that attraction of herbivore natural enemies by the plant constitutes a defence. Parasitoid attack on he rbivores is assumed to result in a reduction in herbivory and/or an increas e in plant fitness. Coupled with the active involvement of the plant in pro ducing signals, this can be seen as indirect mediation of wound-induced def ence. The assumption that parasitism of Pieris brassicae L. by the parasito id Cotesia glomerata L, reduces leaf consumption of cabbage is tested. The effect of parasitism on the growth and feeding rates of P. brassicae larvae compared to unparasitized larvae. Either the defensive mechanism, if it ex ists: is more subtle, or the natural enemy is using an incidental chemical produced as a by-product of the herbivores feeding action as a cue for host -finding, with no increaction effects. This has implications for the study of tritrophic interactions, particularly in the context of agricultural sys tems and the assumption that in IPM strategies, parasitoids are a plant ben efit.