Detection of informative odor 531 The importance of contrast cues

Parasitic wasps must be able to distinguish the volatiles signaling the presence of hosts from background noise. The effectiveness of signals, whether they are visual, auditory, or chemical in nature, will be influenced by the degree of contrast with the background. For example, with visual cues, reduced background contrast would be optimal for camouflage, while marked contrast would be most effective in the case of aposematic coloration. Background volatiles may have a positive effect if they provide a 'sharper view' of the specific informative volatile (blend), or may be negative if they mask detection of the crucial, informative scent.

To date, the effects of background odor on herbivorous insects have received the greatest attention. When repellent components from non-host plants are present in the background, they may interfere with the positive cues that herbivores use to locate their host plants (Isaacs et al. 1993, Hori & Komatsu 1997, Yamasaki et al. 1997, Held et al. 2003, Mauchline et al. 2005). In addition, components from non-host plants, with no repellent properties per se, may also act as masking volatiles (Visser & Ave 1978, Thiery & Visser 1986, Nottingham et al. 1991, Yamasaki et al. 1997, Costantini et al. 2001). Masking effects of non-host plants have often been mentioned as one possible reason for the lower abundance of specialized herbivores in habitats with high plant diversity (Bukovinszky 2004, and references therein).

The positive influence of background odors on the response to chemical cues has been reported for a number of different insect species. Drosophila melanogaster adults responded better to minute changes of carbon dioxide when the background odor included vinegar rather than pure air (Faucher et al. 2006). Similarly, the response of corn rootworm Diabrotica virgifera virgifera larvae to CO2 was enhanced when the gas was presented in combination with corn volatiles (Hibbard & Bjostad 1988). Background dependent changes in olfactory sensitivity have also been seen in the housefly Musca domestica (Kelling et al. 2002). There are also several examples showing that both the release of, and response to, sex pheromones in herbivorous insects is modified by the presence of host plant volatiles (McNeil & Delisle 1989, Dickens et al. 1993, Landolt & Phillips 1997, Ochieng et al. 2002, Said et al. 2005).

There is no question that background odors will affect the use of volatiles by foraging parasitoids, but this phenomenon has not received a great deal of attention. The egg parasitoid Chrysonotomyia ruforum, a eulophid wasp specializing on pine sawflies, is attracted to oviposition-induced pine odors (Hilker et al. 2002a) that have higher quantities of (E)-¡¡-farnesene than control trees (Mumm et al. 2003). However, (E)-^-farnesene presented alone does not attract the parasitoid. It only elicits a positive response when presented in combination with non-induced pine odor (Mumm & Hilker 2005). Thus, the change in (E)-^-farnesene quantities will only serve as a reliable cue for the parasitoid if this is detected within a specific odor context. Modified responses related to background odors may also be learnt, as seen with Leptopilina heterotoma, a parasitoid of Drosophila larvae. The response to yeast odor was enhanced in the presence of a green leaf volatile, but only if the para-sitoid had previously experienced the green leaf volatile together with host kairomones (Vet & Groenewold 1990).

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