herbivores. They can do so by increasing their sensory and behavioural
ability to perceive the signal, by efficient information-processing and
decision-making, by enhanced discrimination between signal and noise
and by other behavioural adaptations. The tritrophic approach to
parasitoid and predator behaviour is now a strong research interest (Dicke
and Vet, 1999; Sabeliset al., 1999).Plant cues
Through learning, parasitoids can specialize temporarily on available and
profitable host plants.Cotesia glomerata, a parasitoid ofPieriscater-
pillars, learns to fly to the host-plant species that is most profitable in
terms of host encounter rate (Geervlietet al., 1998a,b). When given a
three-oviposition experience on a plant species infested with a high host
density, followed by a three-oviposition experience on another plant
species with a low host density,C. glomeratafemales chose the plant
species on which they had experienced the highest host density when
given a choice between the two now equally infested host-plant species in
a wind-tunnel (Geervlietet al., 1998a). When the production of plant
volatiles varies reliably with herbivore density, plant volatiles can be a
suitable cue for predicting host density, an important characteristic of
patch quality. Non-profitable plants are neglected and the area to be inten-
sively searched is thus significantly reduced. This is especially beneficial
to parasitoids that are time-limited, i.e. are likely to die without having
laid their full complement of eggs.
Plant species vary significantly in the degree of specificity of volatile
information they produce after they are damaged (for references, see
Vet, 1999). Some plant species, such as broad bean plants, produce
qualitatively different odours when attacked by different herbivore
(aphid) species (Powellet al., 1998). Life seems simple for the parasitoid
Cotesia kariyai: maize plants produce different volatiles when suitable
and unsuitable host stages feed on them. Takabayashiet al. (1995) showed
that female wasps are attracted to maize plants infested by first- to fourth-
instar caterpillars (suitable host instars), while plants infested by fifth-
or sixth-instar larvae (non-suitable host instars) are not attractive. This
high degree of specificity of plant volatiles can greatly help parasitoids
to optimize plant choice decisions, i.e. to distinguish between plants
infested with host and non-host species or suitable and unsuitable host
stages. Other plant species, such as cabbage, merely produce a higher
quantity of odours when attacked by any herbivore, demanding a different
foraging strategy by parasitoids, such as the need to learn subtle quanti-
tative differences in odour composition or the use of visual cues. When
information from the plant is indistinct, parasitoids can be hampered in
their efficiency to locate suitable patches and hosts. This was shown for
the parasitoidC. glomerata, which attacksPieriscaterpillars on cabbage
(Voset al., 2001). Parasitoids waste search time by being attracted toFlexibility in Host-search and Patch-use Strategies 45