As shown in Fig. 4.1, the physiological
state of the parasitoid relative to other needs
can be considered as a gateway that filters
the detection and responses to host-foraging
stimuli based on priority of the needs. We
feel that, until recently (Chapter 5), the influ-
ence of physiological state on the foraging
behaviour of parasitoids is an area that has
generally been recognized as important but
one that has seldom been studied in a sys-
tematic way as needed to develop the tech-
nology important to ensuring effective and
consistent host-foraging behaviour.
Variations in Responses at Different
Points in the Host-selection Sequence
It is well recognized that foraging for hosts by
parasitoids involves a series of steps that
draw them progressively closer to their host
(Salt, 1935; Flanders, 1953; Doutt, 1964). These
steps were reviewed and amended by Vinson
(1976, 1984a, 1998). Lewis et al.(1975b) pro-
posed a basic sequence in which various
behavioural acts are identified with the
respective stimuli that elicit the responses.
Much about the full repertoire of behaviours
and specific stimuli is yet to be explained
(Steidle and van Loon, 2002). Visual, tactile
and chemical stimuli are all involved to some
extent, but chemical stimuli appear to play a
major and often dominant role for many para-
sitoids (e.g. Vet and Dicke, 1992).
Much more information has accumulated
about mediating stimuli and close-range for-
aging responses than about long-range
responses of parasitoids (Vinson, 1984b;
Lewis et al., 1985; Vet and Dicke, 1992). This
was especially true before more recent
advances in the knowledge of parasitoid
responses to airborne odours were published
(e.g. Drost et al., 1986; Hérard et al., 1988a;
Lewis and Tumlinson, 1988; Vet and Dicke,
1992; Geervliet et al., 1998). The lack of
knowledge about the long-range foraging
behaviour of parasitoids has been due to the
greater ease with which the close-range
behaviour could readily be studied in small
laboratory containers. We offer arguments
that behaviour-mediating stimuli and intrin-
sic parasitoid conditions that influence
longer-range parasitoid foraging responses
are more variable than close-range
responses. This greater variation in the medi-
ating stimuli and conditions required for
response adds complexities to the methods
and procedures required to study long-range
behaviours effectively. Consequently, we
seem to have avoided studies of parasitoid
foraging at long range, although the infor-
mation is certainly needed in designing more
effective biological control.
As parasitoids negotiate a sequence of
cues and approach increasingly closer to the
host, there is a greater availability of direct
host cues upon which to rely. Thus, there is
less need for exploring, sorting and assessing
indirect cues. As a hypothetical example, let
us consider that a parasitoid may choose a
habitat to search based on recognizable
odour characteristics from a specific type of
plant. Subsequently, specific parts of the
plants, such as the buds or young fruit, may
be scanned because of their general attrac-
tion. Upon detecting indications of an
infested plant, such as damaged tissue, the
parasitoid may hover close for more careful
examination. If the smell of a potential host
is perceived, the parasitoid may land and
carefully antennate the surrounding area,
particularly by-products, such as faeces and
silk, indicating the presence of a candidate
host. If antennation results in contact with a
fresh-recognition kairomone, probing with
the ovipositor may occur, followed by ovipo-
sition if a host is actually encountered. This
hypothetical foraging sequence is only a gen-
eral example for a parasitoid of a phy-
tophagous host, and the number and exact
types of cues in the sequence would vary
among parasitoid species and types of hosts
(Vinson, 1984b, 1998). We present this exam-
ple to aid in illustrating some general conclu-
sions that we shall propose.
We shall contend here that there is greater
phenotypic plasticity (learning) at the long-
range phase of the foraging sequence. In
support of this argument, let us first consider
the adaptive value of various behaviours
involved in the foraging sequence of a para-
sitoid. The ultimate measure of success of the
foraging responses – and thus the reference
point from which natural selection operatesVariations in Foraging Behaviour 49