Is olfactory learning related to the “ecological relevance” of an odour to improve host location?
To overcome the problem of identifying the right cues for host location, it is thought (reliability-detectability hypothesis) that parasitoids have learned to associate reliable cues from the host with HIPVs, which are detectable over a greater distance than odour cues from the host (Vet and Dicke, 1992). Hence, learning to recognise HIPVs can be seen as an evolutionary adaptation to improve host location and thus fitness.
In the current study, learning increased the responsiveness of C. vestalis to a generic HIPV linalool, while 1-nonanol, an odour not associated with their tritrophic system (Durrant, 2007), resulted in avoidance learning. It has previously been shown that generalists are more “flexible” in changing their preference than specialists (Geervliet et al., 1998b). Generalists, contrary to specialists, have a variety of hosts, and learning different cues of their host-plant complex would therefore be beneficial. For a specialist it is essential to be able to discriminate HIPVs from non-HIPVs and thus, learning of non-HIPVs would be irrelevant and costly for them. This suggests that their olfactory system is fine tuned for specific HIPVs of their host-plant complex, which remain constant. In turn, innate preference to “relevant” HIPVs may also be relatively high and improved through learning.
However, there have been cases in which specialist parasitoids were shown to learn new or “ecologically irrelevant” odours (DeJong and Kaiser, 1991). This has also been demonstrated in C. vestalis, where females were able to learn vanillin (Durrant, 2007). On the other hand, it has been previously discovered that HIPVs can elicit avoidance behaviour too (Durrant, 2007; Snoeren et al., 2010). This suggests that olfactory learning in parasitoids might not only depend on host specialisation and the “ecological relevance” of an odour, but also on the
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functional group, type and concentration of odour (Smith and Menzel, 1989; Getz and Smith, 1990; Daly et al., 2001; Meiners et al., 2002; Ngumbi et al., 2009).
Furthermore, it has been argued that the degree of host specialisation alone might not be a good indicator to what extent animals rely on innate cues (Steidle and Van Loon, 2003). Indeed, it was found that innate responses can also be important for generalists (Steidle and Van Loon, 2003; Peñaflor et al., 2011).
Responses to HIPVs: a cue for hosts and reproduction?
It has been shown in the past that female parasitoids are able to learn odours through oviposition (Potting et al., 1999). Learning in males, however, has received less attention (Goh and Morse, 2010). In this study, EAG and behavioural studies were used in both genders to test the hypothesis whether HIPVs are important for males as well as females. Indeed, measurements of antennal responses of C. vestalis have shown that males perceived all of the compounds tested (section 2.3.1). Furthermore, males learned linalool, but not 1- nonanol in the HP conditioning group. Copulation did not appear to be essential for learning, as males did not succeed in copulation. Hence, just the presence of the female was sufficient to induce learning in males. However, a high number of animals did not make a choice, which suggests that linalool might be less attractive to males (Girling et al., 2006) than to females. This might be caused by the quality of reward (the lack of copulation), and it can be hypothesised that the association would have been stronger if copulation had taken place.
In summary, it can be assumed that HIPVs play a different role for males and females. The reproductive success of males is dependent on when and where females are available, while females need to locate the host for successful reproduction. Especially in solitary specialist species, the chance to encounter females is less than in generalists. Therefore, being able to detect and associate an odour with the presence of a female (in the case of the male), or the presence of a host (in the case of the female); or avoid those which are not an indicator for female/host presence can be of an advantage. From these results it can be suggested that C. vestalis males use HIPVs as a cue to locate females and females prefer volatiles indicating host presence.
60 Food conditioning: differences in neural pathways depending on conditioning paradigm?
Appetitive food conditioning is widely used in the insect world (Guerrieri et al., 2011; Roussel et al., 2011; Salloum et al., 2011; Sanford and Tomberlin, 2011; Simões et al., 2011). Successful food conditioning has been shown in previous studies in other parasitoid species (Lewis and Takasu, 1990; Wäckers et al., 2002; Wäckers et al., 2006). In this study, however, neither males nor females were able to learn via food conditioning, regardless of the odour. Animals showed more response to linalool, but both odours failed to induce a significant association in any of the conditioning groups. A lack of learning the odour 1-nonanol has also been observed in previous studies (Meiners et al., 2002). Interestingly, females trained to 1-nonanol in the FOOD group showed a higher preference to the odour than females in the HP group. This reinforces the possible aversive effect of 1- nonanol on females in the HP group. One explanation might be that the odour needs to be relevant to the natural context. Hence, HIPVs could be better learned with oviposition conditioning, and floral odours with food conditioning. This might also indicate different pathways for food and oviposition conditioning, which differs to the suggestion of Bleeker (Bleeker et al., 2006b), who suggested that oviposition learning could be mediated by the VUMmx1 neuron, which is known to mediate PER (proboscis extension conditioning) in bees (Hammer, 1993).