as foraging environments, as the source of
variation in natural-enemy searching
behaviour (Waage and Hassell, 1982; Vet
and Dicke, 1992; Godfray, 1994; Vinson,
1998). Very limited consideration has been
given to intraspecific variation in the nat-
ural enemy’s genetic composition or behav-
ioural state.
More recent studies show that foraging
responses among individuals of a parasitoid
population, even to the precise same set of
stimuli, can be quite variable. Further, the
behaviour of a given female parasitoid is
often plastic and can vary considerably,
depending on the history of that individual
(e.g. Wardle and Borden, 1986; Lewis and
Tumlinson, 1988; Vet et al., 1990, 1995; Vet
and Dicke, 1992; Steidle and van Loon,
2002). Therefore, researchers hoping to use
natural enemies for biological control of
pests must appreciate that an effective end
result will be a product of the diversity and
plasticity of the naural enemy’s population
interacting with environmental parameters
of the foraging arena.
In this chapter, we explore sources of
variation in the responsiveness of para-
sitoids to various foraging cues, with
emphasis on the intrinsic causes for this
variation, and the importance to biological
control programmes of a proper matching
of the parasitoids’ genotypic and pheno-
typic behavioural traits with the type envi-
ronment in which they must forage. We
include considerations of genotypic diver-
sity, the influence of different physiological
states on the responses by individuals and
the plasticity of individual parasitoids
caused by preadult and adult experiences
(see Chapter 3 for elaboration of the latter
subject of parasitoid learning). A model is
proposed for collectively assessing these
sources of variation and their sum effect on
parasitoid foraging behaviour. This model
can, with adaptations, also be used for
predators. Information about foraging
behaviour of predators is, however, much
more limited that that of parasitoids, and
that is the reason why this chapter is mainly
focused on parasitoids. Finally, we discuss
ways that this information might be used to
improve biological control.
Need for Understanding Variations in
Parasitoid Foraging Behaviour
Animal behaviourists often emphasize
interspecific diversity, particularly when
illustrating how animals adapt to the vari-
ety of problems that they encounter
(Alcock, 1984). Intraspecific diversity is also
recognized as a common and important fea-
ture of animal behaviour, including forag-
ing behaviour (Roughgarden, 1979; Hoy,
1988). Intraspecific differences in foraging
behaviour typically involve differences
among individuals and differences in the
behaviour of a given individual from one
foraging occasion to the next (Papaj and
Prokopy, 1989). Behaviourists generally
agree that these differences are caused by
the selection for mechanisms that enable
individuals to cope effectively with varying
circumstances under which food resources
must be obtained (Matthews and Matthews,
1978; Roughgarden, 1979; Alcock, 1984; Vet
et al., 1995).
Interspecific variation of parasitoids has
been the subject of considerable discussion
relative to biological control (e.g. Waage and
Hassell, 1982; Bellows and Fisher, 1999),
whereas intraspecific variation has received
little attention in the design and implementa-
tion of biological control programmes
(Caltagirone, 1985; Hoy, 1988). The regi-
mented production process used with con-
ventional pesticides has perhaps dulled our
appreciation of biological knowledge needed
for the production and use of biological
organisms versus chemical formulations
(Lewis, 1981; Lewis et al., 1997). We must
remember that evolution by natural selection
does not stop at the species level but oper-
ates at the individual level. Thus, unlike
chemical compounds or other products, the
definition of a species or even a strain of a
parasitoid does not mean that the individu-
als within the species are a product of one
‘blueprint’ or single set of performance char-
acteristics. Furthermore, individual organ-
isms are quite plastic, and their behavioural
traits can be altered substantially by the con-
ditions to which they are exposed. The chal-
lenge for biological control specialists is to42 W.J. Lewiset al.