Sensory Ecology of Arthropods Utilizing Plant Infochemicals
263
information that guides the carnivore to its host or prey (reviewed in Dicke and
Vet 1999).
Apart from the detectability problem, the inherent variability in plant volatile
production as described in Section 2.2.2 may complicate oriented searching of
prey or hosts even further. The olfactory system of carnivores therefore has to
accomplish the task of discriminating irrelevant volatile information (noise) from
relevant information that reliably indicates the presence of host or prey (signal).
The arthropod olfactory system can assess both quality and quantity of odor
bouquets (de Jong and Visser 1988, Smith and Getz 1994). Results at the
behavioral level suggest that parasitoids discriminate qualitative differences better
than quantitative differences (Vet et al. 1998).
We will subsequently focus on answering two crucial question~ that follow from
the foregoing discussion: does herbivory result in qualitative differences in plant
volatile production and do carnivores possess olfactory receptors (specifically)
sensitive to such compounds?
2.4.2 Plant Volatiles Produced in Response to Herbivory: Qualitative vs.
Quantitative Differences
The occurrence of qualitative differences in plant volatile production in response
to herbivory depends on the plant-herbivore interaction. Based on the information
presently available three categories can be distinguished that differ in the
specificity and consequently reliability of the volatile blend available to the
carnivore: (1) plant species which release the same compounds when damaged by
different herbivore species as when damaged artificially, with variation only in the
relative contribution of each compound to the blend; (2) plants that produce novel
compounds when damaged by herbivores compared to artificial damage while
different herbivore species lead to quantitative differences in the blend; (3) plants
that emit novel compounds when damaged by herbivores compared to artificial
damage and that emit qualitatively different blends in response to damage by
different herbivore species. Evidently, the informational specificity of the
interactions belonging to (3) and thus the reliability of the signal to the carnivore is
highest. Of ten tritrophic systems for which the composition of the volatile blends
in response to damage by different herbivore species have been determined, four
yielded qualitative differences. In some cases the novel compounds make up the
largest part of the bouquet (Fig. 2A).
The distinction of these three categories stresses differences in the specificity of
the olfactory information available to carnivores. It should not be seen as absolute,
however. An inherent problem in deciding on the chemical uniqueness of a certain
blend component is that any chemical analysis has its limits of detection and, as
noted above, the arthropod olfactory system may defy the sensitivity of our
analytical equipment. Carnivores foraging for herbivores on plants in category (I)
and (2) are expected to base their searching behavior on quantitative chemical
263
information that guides the carnivore to its host or prey (reviewed in Dicke and
Vet 1999).
Apart from the detectability problem, the inherent variability in plant volatile
production as described in Section 2.2.2 may complicate oriented searching of
prey or hosts even further. The olfactory system of carnivores therefore has to
accomplish the task of discriminating irrelevant volatile information (noise) from
relevant information that reliably indicates the presence of host or prey (signal).
The arthropod olfactory system can assess both quality and quantity of odor
bouquets (de Jong and Visser 1988, Smith and Getz 1994). Results at the
behavioral level suggest that parasitoids discriminate qualitative differences better
than quantitative differences (Vet et al. 1998).
We will subsequently focus on answering two crucial question~ that follow from
the foregoing discussion: does herbivory result in qualitative differences in plant
volatile production and do carnivores possess olfactory receptors (specifically)
sensitive to such compounds?
2.4.2 Plant Volatiles Produced in Response to Herbivory: Qualitative vs.
Quantitative Differences
The occurrence of qualitative differences in plant volatile production in response
to herbivory depends on the plant-herbivore interaction. Based on the information
presently available three categories can be distinguished that differ in the
specificity and consequently reliability of the volatile blend available to the
carnivore: (1) plant species which release the same compounds when damaged by
different herbivore species as when damaged artificially, with variation only in the
relative contribution of each compound to the blend; (2) plants that produce novel
compounds when damaged by herbivores compared to artificial damage while
different herbivore species lead to quantitative differences in the blend; (3) plants
that emit novel compounds when damaged by herbivores compared to artificial
damage and that emit qualitatively different blends in response to damage by
different herbivore species. Evidently, the informational specificity of the
interactions belonging to (3) and thus the reliability of the signal to the carnivore is
highest. Of ten tritrophic systems for which the composition of the volatile blends
in response to damage by different herbivore species have been determined, four
yielded qualitative differences. In some cases the novel compounds make up the
largest part of the bouquet (Fig. 2A).
The distinction of these three categories stresses differences in the specificity of
the olfactory information available to carnivores. It should not be seen as absolute,
however. An inherent problem in deciding on the chemical uniqueness of a certain
blend component is that any chemical analysis has its limits of detection and, as
noted above, the arthropod olfactory system may defy the sensitivity of our
analytical equipment. Carnivores foraging for herbivores on plants in category (I)
and (2) are expected to base their searching behavior on quantitative chemical