How To Hunt for Hiding Hosts: the Reliability-Detectability Problem in Foraging Parasitoids

HOW TO HUNT FOR HIDING HOSTS: THE RELIABILITY- DETECTABILITY PROBLEM IN FORAGING PARASITOIDS by LOUISE E. M. VET, FELIX L. WÄCKERS and MARCEL DICKE (Department of Entomology, Wageningen Agricultural University, P.O. Box 8031, 6700 EH Wageningen, The Netherlands) SUMMARY Foraging parasitoids may use stimuli that are derived from their host or from the food of their host, often plants. But how usable are 2nd and 1st trophic level stimuli and what is their relative importance in parasitoid foraging? It is argued that foraging parasitoids are facing a reliability-detectability problem: host-derived stimuli are the most reliable in indicating host presence, accessibility and suitability but they are generally hard to detect. Plant stimuli, on the other hand, are easier to detect but arc generally less reliable indicators. Parasitoids have evolved different non-exclusive strategies to solve this problem. (1) Infochemical detour: parasitoids resort to information from other, more detectable, host stages than the one under attack. (2) Herbivore-induced synomones: parasitoids use specific plant produced volatiles that are released upon damage by a specific herbivore species. In the present paper we put most emphasis on a third strategy (3) Associative learning: through associative learning parasitoids link easy-to- detect stimuli to reliable but hard-to-detect stimuli. Specific mechanisms by which associative learning can improve foraging success are discussed. KEY WORDS: Tritrophic interactions, parasitoids, host location, associative learning, foraging, infochemicals, behaviour. INTRODUCTION Due to the impressive variety of specialized lifestyles found in insect parasitoids, generalizations about their foraging behaviour may be considered difficult to make. Certain characteristics of the host's ecology can be important in the evolution of specific foraging mechanisms in parasitoids and in determining the usefullness of dif- ferent sensory modalities in this foraging (VET & VAN ALPHEN, 1985; VET & DICKE, 1992). For example, the feeding habit of the host (such as concealed versus exposed) may influence the relative usefulness of olfactory, visual, and host-vibration stimuli in host finding (GLAS & VET, 1983; SUGIMOTO Bt al., 1988; WACKERS & LEWIS, 1991). However, in spite of the lifestyle diversities, insect parasitoids have certain aspects of foraging in common. Parasitoids, together with other natural enemies of insects such as arthropod predators, con- stitute the third trophic level of a multitrophic system with the host