Plant volatiles and adult experience affect selection by Cotesia plutellae of host larvae on different plants

Laboratory experiments were conducted to examine selection by Cotesia plutellae between host larvae feeding on two plant species: Chinese cabbage, Brassica campestris ssp. pekinensis and common cabbage, Brassica oleracea var. capitata. When C. plutellae wasps were provided with equal numbers of Plutella xylostella larvae on both species of plants in one arena, the parasitoid parasitised 4-15 fold more host larvae on the Chinese cabbage than on the common cabbage and this preference did not change with host density. However, an experience of oviposition or searching on a leaf of the less-preferred plant, the common cabbage, significantly increased the preference for parasitising host larvae on this plant. Plant volatiles from Chinese cabbage were more attractive to C. plutellae adult females than those emanating from common cabbage. Feeding by P. xylostella larvae increased the attraction of both plant species to C. plutellae, but the infestation and the presence of P. xylostella larvae on the plants did not affect their relative levels of attraction to the parasitoid. In parallel to the increased parasitism on common cabbage following experience, an oviposition in host larvae on this less-preferred plant significantly increased the response to volatiles emanating from the plant. These results indicate that host plants may strongly influence the foraging behaviour of C. plutellae, but their differential attractions to the parasitoid may be significantly offset by the learning behaviour of the insect.