Spatial scale, neighbouring plants and variation in plant volatiles interactively determine the strength of host-parasitoid relationships

摘要

Species-specific responses to the environment can moderate the strength of interactions between plants, herbivores and parasitoids. However, the ways in which characteristics of plants, such as genotypic variation in herbivore induced volatiles (HIPVs) that attract parasitoids, affect trophic interactions in different contexts of plant patch size and plant neighbourhood is not well understood. We conducted a factorial field experiment with white cabbageBrassica oleraceaaccessions that differ in the attractiveness of their HIPVs for parasitoids, in the context of different patch sizes and presence or absence of surroundingBrassica nigraplants. Parasitism rates of experimentally introducedPieris brassicaecaterpillars and the presence of naturally occurringPierisspp. caterpillars in the plots were assessed throughout the growing season. The abundance ofPieriscaterpillars was neither affected by cabbage accession nor plot size. Later in the season, whenB. nigraplants had senesced, fewer caterpillars were found on cabbage plants in plots with aB. nigraborder. Parasitism rates fluctuated over the season, and were not affected by plot size. However, theB. nigraborder negatively affected parasitism rates on the accession that is less attractive to the parasitoidCotesia glomerata, but not on the more attractive accession. Our results show that plant variation in HIPVs can differentially influence herbivores and parasitoids depending on characteristics of the surrounding vegetation context. These findings underscore the importance of considering the interaction between focal plant traits and neighbourhood context to reliably predict trophic cascades.