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Joop J.A. van Loon, Marcel Dicke
information. In order to overcome the limited specificity contained in such a
message a neural link has to be established by associative learning between actual
host or prey encounters (eliciting contact-chemosensory and tactile activity) with
the quantitative olfactory profile perceived just prior to and during this encounter.
This type of learning is widespread in hymenopteran parasitoids (Vet et al. 1995).
It effectively leads to "temporary specialization".
From behavioral studies it appears that several carnivorous species are well able
to discriminate between odor mixtures that differ quantitatively even when lacking
previous experience (for review see Dicke 1999). In the majority of cases,
carnivores were shown to discriminate between plants infested by different
herbivores, either innately or after previous feeding or oviposition experience
(reviewed by Dicke and Vet 1999 and Dicke 1999). Specialist carnivores may rely
largely on innate responses to olfactory cues while generalist feeders may employ
associative learning (Vet et al. 1995), a difference that holds even for closely
related species (Geervliet et al. 1998 ).
2.4.3 Olfactory Characteristics of Arthropod Carnivores
The olfactory system of arthropod carnivores has hardly been explored in any
detail for the aspects of specificity and sensitivity. We found six studies on
parasitoids (Lecomte and Pouzat 1985; Baehrecke et al. 1989; Ramachandran and
Norris 1991; Sal om et a!. 1991; Li et a!. 1992; Du et a!. 1998) and five studies on
predators (T0mmeras 1985; Sant'ana and Dickens 1998; Dickens 1999;
Weissbecker et al. 1999, 2000). All these studies but one (T0mmenis 1985) have
employed electro-antennogram (EAG) recording, a valuable tool for a first
screening of olfactory responses to volatiles. In these studies, the choice of the
compounds has partly been based on the commercial availability or from the
viewpoint of a structure-activity approach rather than on their actual occurrence in
the headspace of herbivore-damaged plants or on their ecological significance.
Several damage-related compounds have been identified in a growing number of
studies over the past five years. These include butyrate- and valerate-esters of (Z)3-hexen-1-ol (Whitman and Eller 1990, 1992); homoterpenes (Dicke and Vet
1999; Turlings and Fritzsche 1999) and methylsalicylate (Dicke et al. 1990; Duet
al. 1998, Dickens 1999). These have not been extensively tested yet. In several
cases parasitoids or predators display a significantly higher olfactory sensitivity to
some damage-related compounds than the herbivore they attack (Ramachandran
and Norris 1991; Dickens 1999; Weissbecker et al. 1999, 2000).
The EAG technique does not allow conclusions on the specificity of single
olfactory neurons. Until now only one study addressed predator ORN specificity.
Also, this study focussed on prey (bark beetle) pheromones rather than on hostplant volatiles (T0mmeras 1985). Although it has been demonstrated for quite a
few tritrophic systems that parasitoids or predators are attracted to herbivoredamaged plants and that the plant is the producer of the synomones (Dicke and Vet
Joop J.A. van Loon, Marcel Dicke
information. In order to overcome the limited specificity contained in such a
message a neural link has to be established by associative learning between actual
host or prey encounters (eliciting contact-chemosensory and tactile activity) with
the quantitative olfactory profile perceived just prior to and during this encounter.
This type of learning is widespread in hymenopteran parasitoids (Vet et al. 1995).
It effectively leads to "temporary specialization".
From behavioral studies it appears that several carnivorous species are well able
to discriminate between odor mixtures that differ quantitatively even when lacking
previous experience (for review see Dicke 1999). In the majority of cases,
carnivores were shown to discriminate between plants infested by different
herbivores, either innately or after previous feeding or oviposition experience
(reviewed by Dicke and Vet 1999 and Dicke 1999). Specialist carnivores may rely
largely on innate responses to olfactory cues while generalist feeders may employ
associative learning (Vet et al. 1995), a difference that holds even for closely
related species (Geervliet et al. 1998 ).
2.4.3 Olfactory Characteristics of Arthropod Carnivores
The olfactory system of arthropod carnivores has hardly been explored in any
detail for the aspects of specificity and sensitivity. We found six studies on
parasitoids (Lecomte and Pouzat 1985; Baehrecke et al. 1989; Ramachandran and
Norris 1991; Sal om et a!. 1991; Li et a!. 1992; Du et a!. 1998) and five studies on
predators (T0mmeras 1985; Sant'ana and Dickens 1998; Dickens 1999;
Weissbecker et al. 1999, 2000). All these studies but one (T0mmenis 1985) have
employed electro-antennogram (EAG) recording, a valuable tool for a first
screening of olfactory responses to volatiles. In these studies, the choice of the
compounds has partly been based on the commercial availability or from the
viewpoint of a structure-activity approach rather than on their actual occurrence in
the headspace of herbivore-damaged plants or on their ecological significance.
Several damage-related compounds have been identified in a growing number of
studies over the past five years. These include butyrate- and valerate-esters of (Z)3-hexen-1-ol (Whitman and Eller 1990, 1992); homoterpenes (Dicke and Vet
1999; Turlings and Fritzsche 1999) and methylsalicylate (Dicke et al. 1990; Duet
al. 1998, Dickens 1999). These have not been extensively tested yet. In several
cases parasitoids or predators display a significantly higher olfactory sensitivity to
some damage-related compounds than the herbivore they attack (Ramachandran
and Norris 1991; Dickens 1999; Weissbecker et al. 1999, 2000).
The EAG technique does not allow conclusions on the specificity of single
olfactory neurons. Until now only one study addressed predator ORN specificity.
Also, this study focussed on prey (bark beetle) pheromones rather than on hostplant volatiles (T0mmeras 1985). Although it has been demonstrated for quite a
few tritrophic systems that parasitoids or predators are attracted to herbivoredamaged plants and that the plant is the producer of the synomones (Dicke and Vet