Surprisingly, little is known about the role of general host volatiles for searching behavior of insect parasitoids. Carbon dioxide released from respiring host insects could be a highly detectable cue, especially when host insects occur in high abundances. While other parasitic organisms, like trematode cercariae, are well-known for using carbon dioxide to locate their hosts (Haas 1992, Haas et al. 2002), the role of carbon dioxide for insect parasitoids remains unclear. Several insect species have been shown to be able to respond and orientate along carbon dioxide gradients: herbivorous insects (Stange et al. 1995, Langan et al. 2001, 2004, Johnson & Gregory 2006); those living on decaying organic material like Drosophila (Faucher et al. 2006); and blood-sucking insects (Gillies 1980, McCall 2002, Barrozo & Lazzari 2004). Furthermore, it is unknown whether the carbon dioxide gradient established by respiration of herbivorous insects on a plant is even fortified by reduced photosynthesis activity of plant tissue close to feeding damage (Welter 1989, Zangerl et al. 2002, Haile & Higley 2003). Thus, even though orientation by carbon dioxide might provide some information for generalist parasitoids on the presence of hosts, to the best of our knowledge, carbon dioxide sensitivity in parasitoids of herbivorous insects has not been studied so far (Fig. 5.1).
General and widespread host cues may be released from faeces or honeydew of herbivorous insects (Steidle & van Loon 2003, Buitenhuis et al. 2004, and references therein), although they may also release specific volatiles or specific ratios of volatiles that provide valid information on the host species for both larval and egg parasitoids. For example, egg parasitoids with a narrow host range, but with hosts feeding on a wide variety of plants, recognize specific patterns of chemicals released from adult host faeces regardless of the plant species used by the herbivore (Hilker & Meiners 1999). Hydrocarbons on the surface of host scales are also widespread chemical cues used by several egg parasitoids (Jones et al. 1973, Lewis et al. 1975, Shu et al. 1990, Paul et al. 2002). However, they probably serve as foraging cues over very short distances or upon contact only.
Several host specific cues, such as pheromones, are used as kairomones by egg and larval parasitoids (Steidle & van Loon 2003). Many egg parasitoids have been shown to use sex, anti-aphrodisiac, or oviposition marking pheromones of their hosts to find microhabitats where host eggs could be expected (Rutledge 1996, Powell 1999, Nufio & Papaj 2001, Anderson 2002, Fatouros et al. 2005a). This use of cues from other stages, other than the one used as an oviposition site by the female parasitoid, has been labeled 'infochemical detour' (Vet & Dicke 1992). Such a detour is worthwhile, as eggs per se hardly release any volatiles (Kaiser et al. 1989), except those adsorbed on the egg surface or those released from compounds attaching eggs to the plant (Frenoy et al. 1992, Renou et al. 1992, Bin et al. 1993). The specificity of response to host pheromones can be extremely high, as seen with the eulophid wasp Chrysonotomyia ruforum that attacks pine sawfly eggs. Females of this wasp only respond to those stereoisomers that the sawflies use as intraspecific mating signals (Hilker et al. 2000) (Fig. 5.1).
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