DENSITY-RELATED FORAGING BEHAVIOR IN CLOSTEROCERUS-TRICINCTUS, A PARASITOID OF THE LEAF-MINING MOTH, CAMERARIA-HAMADRYADELLA

1. We examined the foraging behaviour of the parasitoid wasp, Clostero cerus tricinctus (Ashmead) (Chalcidoidea: Eulophidae), as it visited l arvae of the leaf-mining moth, Cameraria hamadryadella ( Lepidoptera: Gracillariidae), in an outbreak population. 2. We tracked females of C . tricinctus, recording the time spent searching for mines and handlin g host larvae. The density of leaf-mines (host larvae) and their condi tion were recorded for each leaf visited. A subset of leaves visited b y C. tricinctus was enclosed in fine mesh bags so that foraging succes s could be determined by rearing or dissection. The average density of mines and the average leaf-area mined was estimated for a random samp le of leaves from each tree. 3. The selection of leaves upon which to forage appears to be density-dependent. C. tricinctus visits leaves wi th leaf-mine densities twice the average, and when switching leaves la nds directly on leaf-mines 5 times more often than expected assuming r andom landings. 4. The total time spent foraging on a leaf, the averag e time spent handling hosts, and the total search time within leaves t end to decline on leaves with many hosts, but the observed declines ar e not statistically significant. 5. The proportion of leaf-mines visit ed within a leaf is strongly inversely density-dependent. 30% of visit s to leaf-mines are re-visits and 29% of handling time is spent re-han dling previously visited hosts. Furthermore, only 21% of visits to min es lead to successful parasitism. We suggest that self-interference an d the avoidance behaviour of the host may reduce the number of visits of leaf-mines by C. tricinctus within a leaf. 6. The effect of the str ongly inversely density-dependent foraging investment within leaves is to offset the observed density-dependent pattern of leaf visitation m aking the overall spatial pattern of visitation by C. tricinctus to mi nes of C. hamadryadelia inversely density-dependent. 7. We suggest tha t the uncertainty of C. tricinctus surviving on multiply mined leaves because of density-dependent host mortality due to intraspecific compe tition in high-density host populations, the rarity of high-density ho st populations, and the rarity of multiply-mined leaves in low-density host populations combine to select against an aggregative response wi thin leaves by C. tricinctus.