several different habitats may learn (as will
be discussed later) to prefer the habitat in
which it encounters suitable hosts (Vet, 1983;
Vet et al., 1995).
Only recently have we begun to appreci-
ate the extent to which parasitoids can learn
and the importance of this plasticity to bio-
logical control considerations (van Alphen
and Vet, 1986; Vet et al., 1995). Several studies
have shown that many species of parasitoids
are able to acquire by experience an
increased preference for and ability to forage
in a particular environmental situation.
There is evidence that a parasitoid may
acquire some modifications in its foraging
traits during the immature stage (Thorpe
and Jones, 1937; Vinson et al., 1977; Vet, 1983;
Luck and Uygun, 1986; van Emden et al.,
1996). However, the clearest cases and those
with the greatest effects have thus far been
shown to be from the experience of the adult
parasitoid (Vinson et al., 1977; Vet, 1983;
Wardle and Borden, 1986; Drost et al., 1988;
Sheehan and Shelton, 1989; Vet et al., 1995;
Steidle and van Loon, 2002). The learning
process is often associative learning, where
the parasitoid learns to effectively use a pre-
viously weak or neutral cue for host foraging
by associating it with the host or a product of
the host (Lewis and Tumlinson, 1988;
Turlings et al., 1989; Vet et al., 1990, 1995). In
this case, close-range, reliable and uncondi-
tional stimuli can serve as reinforcers for the
longer range and more variable conditional
stimuli (Lewis and Tumlinson, 1988; Vet et
al., 1990, 1995). This learning process can
begin at or just before eclosion, based on the
host products associated with the para-
sitoid’s cocoon (Vet, 1983; Hérard et al.,
1988b). Thereafter, the parasitoid’s foraging
responses are modified continually accord-
ing to the foraging circumstances encoun-
tered (Vet et al., 1990, 1995).
The conditions under which these two
alternative adaptive variances of a species,
fixed and unfixed, are most likely to occur
have been discussed for other organisms
(Bradshaw, 1965; Papaj and Rausher, 1987).
We hypothesize that, in general, the occur-
rence of fixed versus unfixed foraging
behaviour of parasitoids is determined by
the combination of two basic features of the
foraging environment. These features are: (i)
the extent of differences between various
host, habitat and other foraging situations
encountered by the individuals; and (ii) the
consistency with which each different forag-
ing situation is available to the parasitoids
within and among generations. Large differ-
ences among the characteristics of foraging
situations should favour parasitoids with the
genetically fixed alternative, because unfixed
individuals must adjust to the widely differ-
ent situations by learning. On the other
hand, genotypes that are fixed for a particu-
lar foraging situation would need a depend-
able availability of that circumstance over
generations. Thus, inconsistencies in the for-
aging situations favour individuals with a
plastic behaviour (unfixed) that can be modi-
fied for the various circumstances encoun-
tered. A chart of the expected occurrence of
these behavioural types relative to various
foraging situations is presented in Table 4.1.
As an apparent result of the interacting
effect of these selection forces, parasitoid
individuals often and perhaps most com-
monly show a combination of the fixed and
unfixed types of behavioural traits (Vet, 1983;
Drost et al., 1986, 1988; Vet et al., 1995). This
combination is accomplished by having an
inherent rank order of preferences for the
various cues used to locate hosts (e.g. the
preference for different host-plant odours to
which the parasitoid responds). However,
the rank order can be modified within gener-
ations by learning based on the circum-
stances encountered by the individuals
(Chapter 3; Vet et al., 1990). We propose that
this initial inherent rank order can vary sub-
stantially among individuals of the species.
Further, the frequency of occurrence of a
given rank order would be determined by its
profitability over generations.
Another major factor that contributes to
variations in the foraging behaviour of para-
sitoids is their general physiological state. A
number of authors have shown that the for-
aging behaviour of female parasitoids can be
altered by their physiological state relative to
other needs and conditions (Chapter 5;
Nishida, 1956; Hérard et al., 1988a; Nordlund
et al., 1988; Wäckers, 1994). Naturally, a para-
sitoid faces varying situations in meeting its44 W.J. Lewiset al.