Host Searching by Egg Parasitoids: Exploitation of Host Chemical Cues

Insect parasitoids are considered \u201ckeystone species\u201d in many ecosystems in terms of biodiversity, ecological impact and economic importance (Vinson 1985, LaSalle and Gauld 1993, Hawkins et al. 1999). In the last decades, several reviews have been published on the relationships among plants, hosts and parasitoids, which reflect a strong interest in these insects both as models for behavioral ecologists and as important organisms for classical and applied biological control programs (Hawkins et al. 1999, Vet 1999, Bale et al. 2008). The majority of these studies have considered the larval parasitoids, besides the extensive use of egg parasitoids in biological control (Hawkins et al. 1999). Insect eggs can be parasitized by about 15 families of Hymenoptera parasitoids, among which several may have potential for biological control application, such as Aphelinidae, Encyrtidae, Eulophidae, Eupelmidae, Mymaridae, Platygastridae, Pteromalidae, Scelionidae, Tetracampidae and Trichogrammatidae (Bin 1994). Three families, Mymaridae, Scelionidae and Trichogrammatidae are exclusively composed of egg parasitoids, whereas the other families are represented by species developing in different host stages, and they also include egg-larval parasitoids, egg-prepupal parasitoids and egg predators (Bin 1994, Vinson 1994). In this chapter we will focus only on egg parasitoids. Successful parasitism of herbivores by insect parasitoids arises through several phases during host searching, which lead wasp females into the close vicinity/contact of their hosts (Vinson 1998). During the host location process, females encounter and explore a great variety of stimuli, among which the chemical cues, named semiochemicals or infochemicals, play a relevant role (Godfray 1994, Vet and Dicke 1992, Vinson 1998). Female parasitoids are under selection pressure to efficiently invest their limited time on the location and exploitation of host derived stimuli, so that the appropriateness and usability of semiochemicals could be influenced by their reliability in indicating host presence and by the degree to which stimuli can be detected, as explained by the reliabilitydetectability theory (Vet and Dicke 1992). In developing this theory, it was argued that the level of reliability and detectability of a particular stimulus is inversely correlated, e.g. cues from the hosts may be highly reliable, but are less detectable compared to volatiles from plants, which have a much larger biomass. To get through the reliability detectability dilemma, wasp females can adopt three different strategies based on the exploitation of either: (1) cues originated from stages different from the one attacked (infochemical detour); (2) cues originated from the interaction of the plant and the herbivore (host-induced synomones); or (3) reliable but poorly detectable cues which were linked, through associative learning, with more detectable but unreliable cues (Vet and Dicke 1992)