Sensory Ecology of Arthropods Utilizing Plant Infochemicals
261
Several specialized species respond strongly to purified volatile compounds
characteristic for the host-plant family with which they are associated (Schoonhoven et al. 1998). Although only few studies have explicitly addressed this issue,
behavioral specialization has its equivalent at the sensory level as specialized
olfactory receptors for such taxon-specific compounds have been found (Honda et
al. 1987; Blight et al. 1989; Nottingham et al. 1991 ). In addition, all species
investigated have olfactory receptor neurons (ORNs) that respond to the generally
occurring GLVs (see above). All higher green plants emit the generally occurring
GLVs (see above and Schoonhoven et al. 1998). Both the precise range of
compounds and the ratios of the different components in the blend differ between
families, however. This leaves the possibility that, despite the fact that the
compounds as such are produced by many plant species, still the blends can
convey specific information to the arthropod because of quantitative differences
which allow a distinction between blends from different plant species or families
(Visser 1986).
It has been repeatedly concluded that chemoreception of GL V s is achieved by
so-called generalist ORNs (Ma and Visser 1978; Boeckh 1984; de Jong and Visser
1988). Such sensory neurons are assumed to respond to a wide range of
compounds, which may even belong to different chemical categories. Several
recent reports on herbivorous arthropods, however, identified receptors that are
orders of magnitude more sensitive to only some GL V s (Dickens 1990; Hansson et
al. 1999).
For the other major chemical category of generally occurring plant volatiles, i.e.,
terpenoids, specialized ORNs responsive to individual plant-derived terpenoids
have been found (Dickens 1990; Anderson et al. 1993; Wibe and Mustaparta
1996; Jonsson and Anderson 1999). Several arguments have been put forward to
question the existence or role of generalist receptors. One is that in older studies
on olfaction unrealistically high concentrations of odorants were applied that cause
a non-specific response in many cells. Also, only few studies determined doseresponse relationships which are necessary to evaluate differences in sensitivity. A
third argument is that the range of compounds emitted by plants is broad. No study
tested all candidate compounds exhaustively. Thus, key compounds exciting
specific ORNs may have been missed. It is important to stress that especially the
latter argument reflects the main difference with pheromone chemoreception: the
chemical complexity of the pheromone message is less complex compared to that
contained in the blend ofGLVs and terpenoids emitted by plants.
To understand the role of ORNs with low specificity to GLVs, more studies into
the specificity of olfactory cells in response to plant volatiles are required. However, attention paid to this and other physiological aspects of arthropod chemoreception has declined over the past decade. The focus of olfactory research has
shifted to the molecular characterization of olfactory receptors and their transduction pathways (Breer and Shepherd 1993; Clyne et al. 1999). The behavioral
effects of plant taxon-specific volatiles differs between generalist and specialist
herbivores. Specialist herbivores may show a strong attraction to plant-taxon
261
Several specialized species respond strongly to purified volatile compounds
characteristic for the host-plant family with which they are associated (Schoonhoven et al. 1998). Although only few studies have explicitly addressed this issue,
behavioral specialization has its equivalent at the sensory level as specialized
olfactory receptors for such taxon-specific compounds have been found (Honda et
al. 1987; Blight et al. 1989; Nottingham et al. 1991 ). In addition, all species
investigated have olfactory receptor neurons (ORNs) that respond to the generally
occurring GLVs (see above). All higher green plants emit the generally occurring
GLVs (see above and Schoonhoven et al. 1998). Both the precise range of
compounds and the ratios of the different components in the blend differ between
families, however. This leaves the possibility that, despite the fact that the
compounds as such are produced by many plant species, still the blends can
convey specific information to the arthropod because of quantitative differences
which allow a distinction between blends from different plant species or families
(Visser 1986).
It has been repeatedly concluded that chemoreception of GL V s is achieved by
so-called generalist ORNs (Ma and Visser 1978; Boeckh 1984; de Jong and Visser
1988). Such sensory neurons are assumed to respond to a wide range of
compounds, which may even belong to different chemical categories. Several
recent reports on herbivorous arthropods, however, identified receptors that are
orders of magnitude more sensitive to only some GL V s (Dickens 1990; Hansson et
al. 1999).
For the other major chemical category of generally occurring plant volatiles, i.e.,
terpenoids, specialized ORNs responsive to individual plant-derived terpenoids
have been found (Dickens 1990; Anderson et al. 1993; Wibe and Mustaparta
1996; Jonsson and Anderson 1999). Several arguments have been put forward to
question the existence or role of generalist receptors. One is that in older studies
on olfaction unrealistically high concentrations of odorants were applied that cause
a non-specific response in many cells. Also, only few studies determined doseresponse relationships which are necessary to evaluate differences in sensitivity. A
third argument is that the range of compounds emitted by plants is broad. No study
tested all candidate compounds exhaustively. Thus, key compounds exciting
specific ORNs may have been missed. It is important to stress that especially the
latter argument reflects the main difference with pheromone chemoreception: the
chemical complexity of the pheromone message is less complex compared to that
contained in the blend ofGLVs and terpenoids emitted by plants.
To understand the role of ORNs with low specificity to GLVs, more studies into
the specificity of olfactory cells in response to plant volatiles are required. However, attention paid to this and other physiological aspects of arthropod chemoreception has declined over the past decade. The focus of olfactory research has
shifted to the molecular characterization of olfactory receptors and their transduction pathways (Breer and Shepherd 1993; Clyne et al. 1999). The behavioral
effects of plant taxon-specific volatiles differs between generalist and specialist
herbivores. Specialist herbivores may show a strong attraction to plant-taxon