THE PARASITOID COTESIA-GLOMERATA (HYMENOPTERA, BRACONIDAE) DISCRIMINATES BETWEEN FIRST AND 5TH LARVAL INSTARS OF ITS HOST PIERIS-BRASSICAE,ON THE BASIS OF CONTACT CUES FROM FRASS, SILK, AND HERBIVORE-DAMAGED LEAF TISSUE

Adult females of the larval parasitoid Cotesia glomerata (L.) respond to chemical cues associated with feeding damage inflicted on cabbage p lants by its host, Pieris brassicae (L.). The use of these infochemica ls by the parasitoid during selection of the most suitable host instar was investigated. The parasitoid can successfully parasitize first-in star host larvae, while contacts with fifth-instar larvae are very ris ky since these caterpillars react to parasitization attempts by biting , spitting, and hitting, resulting in a high probability of the parasi toid being seriously injured or killed. Observations of the locomotor behavior of individual wasps on leaves with feeding damage inflicted b y the first and the fifth larval instars and on host silk and frass sh owed that several cues affect the duration of searching by the parasit oids after reaching a leaf: cues on the margin of the feeding damage a nd cues in the host frass and silk. Whole frass, silk, and hexane extr acts of frass obtained from first-instar elicited parasitoid's searchi ng behavior significantly longer than frass, silk, and hexane extract of frass from the fifth instar. The results demonstrate that C. glomer ata can discriminate between first instars, which are more suitable ho sts, and fifth instars of P. brassicae without contacting the caterpil lars, by exploiting instar-related cues.