Plants can benefit in terms of fitness gain from carnivores that attack the herbivores (Van Loon et al. 2000; Fritzsche-Hoballah and Turlings 2001). Herbivores, however, are under selection to be inconspicuous, and are small in comparison to the plants they are feeding on. Consequently, carnivorous arthropods often depend on plant cues to locate their herbivorous victims (Turlings et al. 1990; Steinberg et al. 1992; Vet and Dicke 1992; Geervliet et al. 1994). Even though host-derived stimuli are potentially more reliable for host location, their use is often limited by low detectability, especially at longer distances (Vet and Dicke 1992). Therefore, carnivorous arthropods are usually more strongly attracted by plant-derived volatiles as compared to volatiles derived from their herbivorous victims (Turlings et al. 1990, 1991; Steinberg et al. 1993; Geervliet et al. 1994; Dicke 1999). Attraction by volatiles from host-infested plants and by EFN was shown for egg-as well as larval parasitoids and predators (e.g., Blaakmeer et al. 1994; Geervliet et al. 1997; Lou et al. 2005; Hilker and Meiners 2006; Choh et al. 2006; Mumm and Hilker 2006). Induced levels of EFN also increase the abundance of ants, wasps and flies (Kost and Heil 2005), and reduce the amount of leaf damage (Heil 2004).
The major signal-transduction pathway involved in attraction of natural enemies seems to be the JA-pathway (Dicke and Van Poecke 2002). The involvement of JA in induced attraction of members of the third trophic level has been demonstrated both by manipulation of JA on the level of the plant's phenotype and the plant's genotype. JA-treated plants attract natural enemies of herbivorous arthropods and have increased parasitism rates in field (Gols et al. 1999; Thaler 1999; Ozawa et al. 2004). Moreover, jasmonate-deficient plants are less attractive to natural enemies than control plants when attacked by herbivores (Thaler et al. 2002).
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