OIKOS 100: 269–282, 2003 Increased risk of parasitism as ecological costs of using aggregation pheromones: laboratory and field study of Drosophila -Leptopilina interaction Bregje Wertheim, Louise E. M. Vet and Marcel Dicke Wertheim, B., Vet, L. E. M. and Dicke, M. 2003. Increased risk of parasitism as ecological costs of using aggregation pheromones: laboratory and field study of Drosophila -Leptopilina interaction. – Oikos 100: 269 – 282. Information conveyance plays an important role in parasitoid-host interactions. Several sources of information are available for searching parasitoids and exploita- tion of that information during the different phases of host location depends on its reliability, detectability and accuracy. One source of information especially suitable for exploitation by parasitoids is a host aggregation pheromone, because this often combines all three aspects. In laboratory and field experiments we studied the behavioural responses of the parasitoid Leptopilina heterotoma to the aggregation pheromone of the fruit fly Drosophila melanogaster, both for substrate selection and the behaviour on host substrates. Our results show that substrates with increasing dose of the host’s aggregation pheromone attract increasingly more parasitoids, whereas we found no significant effects of pheromone on parasitoid searching behaviour on the substrates. Parasitoid searching behaviour on substrates was influenced by other host cues (e.g. larval excrements, traces of adults other than aggregation pheromone), which is discussed in relation to the expectations from reliability-detectability theory. The responses of the parasitoids were further influ- enced by substrate quality (i.e. yeast concentration) and the microscale distribution of pheromone. In several field experiments, the fraction of fruit fly larvae that was parasitised was significantly higher in substrates with aggregation pheromone than in control substrates, indicating an ecological cost to the use of aggregation pheromones in adult D. melanogaster. B. Wertheim, L. E. M. Vet, and M. Dicke, Lab. of Entomology, Wageningen Uni., P.O. Box 8031, NL-6700 EH Wageningen, The Netherlands. Present address of BW: Dept of Biology, Uni. College London, Darwin Building, Gower Street, London WC1E 6BT, United Kingdom (B.Wertheim@ucl.ac.uk). Second address of LEMV: Nether - lands Institute of Ecology (NIOO-KNAW), P.O. Box 1299, NL-3600 BG Maarssen, The Netherlands. Many behaviours of animals convey information on their whereabouts, their identity and their activities. Such information plays an important role in the inter- action with their natural enemies. Depending on the source of information, and on the costs and benefits to all participants, selective forces shape the information conveyance (Dicke and Vet 1999, Dicke and van Loon 2000). Information conveyance is also important in the interaction between insect parasitoids and their hosts (Godfray 1994). Parasitoids can exploit inevitable cues that originate directly from the host itself, e.g. frass, movement vibrations or feeding traces (Casas 1989, Meyho ¨ fer et al. 1994, Mattiacci et al. 1999). In these situations, directional selection acts on the host to minimise the information conveyance, to become as inconspicuous as possible. A more complex situation arises when the parasitoid is eavesdropping on deliber- ate cues for intraspecific communication between the Accepted 15 July 2002 Copyright © OIKOS 2003 ISSN 0030-1299 OIKOS 100:2 (2003) 269