none Pieris brassicae; SILK-; FAECES-; Frass; FOOD-PLANTS; Brassica oleracea var. gemmifera (Brussels sprouts); Contact cues for hymenopteran parasites; HYMENOPTERAN-PARASITES; Cotesia glomerata; Larval instars discrimination, contact cues from frass, silk & plants Cotesia glomerata; LEPIDOPTERAN-HOSTS; Pieris brassicae; Larval instars discrimination, contact cues from frass, silk & plants Adult females of the larval parasitoid Cotesia glomerata (L.) respond to chemical cues associated with feeding damage inflicted on cabbage plants by its host, Pieris brassicae (L.). The use of these infochemicals by the parasitoid during selection of the most suitable host instar was investigated. The parasitoid can successfully parasitize first-instar host larvae, while contacts with fifth-instar larvae are very risky since these caterpillars react to parasitization attempts by biting, spitting, and hitting, resulting in a high probability of the parasitoid being seriously injured or killed. Observations of the locomotor behavior of individual wasps on leaves with feeding damage inflicted by the first and the fifth larval instars and on host silk and frass showed that several cues affect the duration of searching by the parasitoids after reaching a leaf: cues on the margin of the feeding damage and cues in the host frass and silk. Whole frass, silk, and hexane extracts of frass obtained from first-instar elicited parasitoid's searching behavior significantly longer than frass, silk, and hexane extract of frass from the fifth instar. The results demonstrate that C. glomerata can discriminate between first instars, which are more suitable hosts, and fifth instars of P. brassicae without contacting the caterpillars, by exploiting instar-related cues.