Field parasitism rates of caterpillars on Brassica oleracea plants are reliably predicted by diffential attraction of Cotesia parasitoids

1. Herbivore-induced plant volatiles (HIPVs) play an important role in host location of parasitoid wasps and may benefit the plant by top–down control of its herbivorous attackers. Although many studies have shown that accessions of plants differ in attractiveness to parasitoid wasps under controlled laboratory studies, few studies have confirmed that the most attractive accessions also sustain highest parasitism rates in the field. Here, we tested whether in-flight preference of parasitoids for HIPVs from cultivars of Brassica oleracea in the laboratory reliably predicts the parasitism rates of herbivores feeding on these cultivars in the field. 2. In wind tunnel tests in the laboratory, we ranked cultivars of B. oleracea for the preference of two congeneric parasitoids (Cotesia glomerata and C. rubecula) for their HIPVs. The cultivars were then compared for their relative parasitism rates of caterpillars in the field. Throughout the growth season in the field, we infested the different cultivars with Pieris caterpillars on a weekly basis. The caterpillars were recollected after 3 days, dissected and scored for the rate of parasitism. 3. Cultivars of B. oleracea that we identified as most attractive to parasitoids in the laboratory also sustained highest proportions of parasitism in the field. The composition of the headspace of the B. oleracea cultivars damaged by P. rapae differs among these cultivars in the amounts of terpenoids and methyl salicylate emitted, which may be responsible for the differential attraction of parasitoids to the cultivars. 4. Our results show that intraspecific variation in HIPVs of plants is paralleled by differential parasitism of caterpillars in the field. The widely used laboratory assays on HIPV-based preferences of parasitoids provided reliable information on relative parasitism differences of herbivores as found in the field. 5. Thereby, our work confirms that through HIPVs plants attract parasitoids that effectively parasitize herbivores even under the complex and variable abiotic and biotic conditions in (agro-) ecosystem