Functions Of Infochemicals In The Host Selection Of Plutella Xylostella,and Its Parasitoid,cotesia Plutellae

Some aspects of the interactions in the tritrophic system of two Brassica plants, the diamondback moth, Plutella xylostella, and the parasitoid, Cotesia plutellae, were studied in the laboratory. First, the host selection of P. xylostella and the host selection and host-selection behaviour of C. plutellae were examined by choice tests using two host plant species. Next, the behavioural responses of P. xylostella to various plant volatiles, and the behavioural reponses of C. plutellae to various plant and host volatiles were bioassayed. Finally, volatiles collected from the headspace of various plant treatments were collected and identified. In addition, the effects of host plants on performance of P. xylostella and C. plutellae were examined. The results are summarized as follows:(1) The oviposition-selection of Plutella xylostella between two host plants, the common cabbage, Brassica oleracea L. var. capitata cv. Jingfeng No.l, and and the Chinese cabbage, B. campestris L. ssp. pekinensis cv. Zaoshou No. 5, were studied by choice tests, using adult females reared from radish, Rhaphanus sativus L. cv. Yidianhong, as the test insects. The results showed adult females preferred to lay more eggs on the Chinese cabbage than on the common cabbage, both in the laboratory and in the field. Adult females preferred common cabbage infested by conspecific larvae to intact common cabbage, and laid more eggs on the former. In contrast, adult females laid fewer eggs on Chinese cabbage infested by conspecific larvae than on intact Chinese cabbage. Wax on common cabbage was shown to affect the oviposition preference by P. xylostella. Removal of wax by various solvents inceased the oviposition by the insect on common cabbage, but the number of eggs laid on common cabbage was still lower than that on the Chinese cabbage.(2) The role of plant volatiles from the common cabbage and the Chinese cabbage in the host-selection behavior of P. xylostella adult females was investigated by Y-rube olfactometer. Volatiles from the intact Chinese cabbage were more attractive to adult females than those from the intact common cabbage. There is no significant difference between the two Brassica sepecies when they were mechanically-damaged. Plant volatiles from common cabbage infested by conspecific larvae were more attractive to adult females than those from Chinese cabbage infested by conspecific larvae. Removal of wax increased the attractiveness of common cabbage to adult females.IX(3) In choice tests with the common cabbage and the Chinese cabbage as the test the plants and the radish as the host plant for rearing the test insects, C. plutellae adult females exhibited longer residence time and higher percentage parasitism of host larvae on P. xylostella-infested Chinese cabbage than on P. xylostella infested common cabbage. Furthermore, the preference of C. plutellae to Chinese cabbage was not affected by host density. Rearing conditions during pre-adult development could affect the host-selection behavior of C. plutellae adult females. Adult females reared from hosts on radish showed preference of parasitism of host larvae on Chinese cabbage, females reared from hosts on common cabbage did not show preference of host larvae on the two test plants, while females reared from hosts on Chinese cabbage showed high preference of parasitism of host larvae on the Chinese cabbage. An oviposition experience or contact with a host-infested leaf prior to a bioassay significantly increased the percentage parasitism on that plant species. Parasitization of host larvae without contact of the host-infested leaves did not affect the percentage parasitism by the adult females.(4) In comparison with intact Brassica plants, C. plutellae females were attracted to the volatiles from mechanically-damaged plants, plants infested by P. xylostella larvae and plants previously infested by P. xylostella larvae. The attractiveness was not significantly different between volatiles from common cabbage damaged by mechanical means, infested by P. xylos...