Plant-mediated indirect effects and the persistence of parasitoid-herbivore communities

We have examined the effects of herbivore diversity on parasitoid community persistence and stability mediated by nonspecific information from herbivo re-infested plants. First, we investigated host location and patch time allocation in the paras itoid Cotesia glomerata in environments where host and/or nonhost herbivore s were present on Brassica oleracea leaves. Parasitoids were attracted by i nfochemicals from leaves containing nonhost herbivores. They spent consider able amounts of time on such leaves. Thus, when information from the plant is indistinct, herbivore diversity is likely to weaken interaction strength s between parasitoids and hosts. In four B. oleracea fields, all plants con tained herbivores, often two or more species. We modelled parasitoid-herbiv ore communities increasing in complexity, based on our experiments and fiel d data. Increasing herbivore diversity promoted the persistence of parasito id communities. However, at a higher threshold of herbivore diversity, para sitoids became extinct due to insufficient parasitism rates. Thus, diversit y can potentially drive both persistence and extinctions.