PLANT NATURAL ENEMY ASSOCIATION IN TRITROPHIC SYSTEM, COTESIA-RUBECULA-PIERIS-RAPAE-BRASSICACEAE (CRUCIFERAE) .3. COLLECTION AND IDENTIFICATION OF PLANT AND FRASS VOLATILES

To elucidate the identity of the volatile compounds that could be involved in the searching behavior of the parasitoid Cotesia rubecula Marshall (Hymenoptera: Braconidae), the volatiles released by cabbage and frass of Lepidoptera feeding on cabbage were collected and analyzed using a gas chromatograph-mass spectrometer. The volatiles emitted by intact cabbage were alpha-pinene, beta-pinene, myrcene, 1,8-cineole, n-hexyl acetate, cis-3-hexen-1-yl acetate, and dimethyl trisulfide. Mechanical damage on an intact plant induced the release of two more compounds, trans-2-hexenal and 1-methoxy-3-methylene-2-pentanone. Current feeding by larvae of Pieris rapae L. (Pieridae) induced the plant to release all the compounds released after mechanical damage and additionally 4-methyl-3-pentenal and allyl isothiocyanate. Current feeding by larvae of Plutella xylostella L. (Plutellidae) induced the plant to release all the compounds present after mechanical damage and additionally allyl isothiocyanate. The volatiles emitted after feeding by the lepidopterans had ceased were the same as those emitted by cabbage damaged by mechanical means. The blend of volatiles emitted by frass was comprised of plant chemicals, mainly sulfur compounds. Frass of P. rapae emitted allyl isothiocyanate, methyl isothiocyanate, methyl propyl sulfide, dimethyl trisulfide, S-methyl methane thiosulfinate, 4-methyl-3-pentenal, trans-2-hexenal, and 2,3-dihydro-4-methyl furan. Frass of P. xylostella emitted only dimethyl trisulfide and S-methyl methane thiosuifinate. The blend of volatiles emitted by frass is herbivore-species specific.