Parasitoids have been shown to be excellent model systems for
answering evolutionary questions on behaviour, and we are convinced
that much of the progress in theoretical behavioural ecology can be
attributed to these small insects. Many behavioural ecologists have used
parasitoids to couple theory with experiment and to address field-derived
questions with fairly simple behavioural experiments in the laboratory. It
is due to the direct link between foraging and fitness in insect parasitoids
that this trajectory from field to laboratory experiment is relatively narrow
and therefore somewhat justifiable. The great emphasis on laboratory
experiments, rather than field observation, is also due to the fact that the
behaviour of parasitoids is hard to study under field conditions. There are
several reasons behind this. First of all, parasitoids are generally very
small and lively insects and thus difficult to track visually. In addition, it
is often hard to define a patch under field conditions. The chemical world
that is perceived by the insect often remains ambiguous to the observer.
But the most severe limitation is probably the great flexibility of para-
sitoid behaviour itself. Each parasitoid from a natural population has
its own history, which, to a great extent, determines its behavioural
decisions, as we have shown throughout our chapter. When observing a
naturally foraging parasitoid, it will be impossible to know all the para-
meters that have influenced its behaviour. An example is host-acceptance
behaviour, where parasitoids show an amazing flexibility. They take into
account the suitability of the host (host quality), encounter rates with
hosts, levels of competition, life expectancy and many more variables.
This suggests that the underlying response mechanisms (which translate
the external and internal cues the parasitoid receives into host-acceptance
decisions) must be quite complex. Natural selection will shape these
mechanisms such that, averaged over a large range of natural conditions,
they result in adaptive decisions. It will be our challenge for the future
to unravel these mechanisms (as presented in Fig. 3.2), at the sensory-
physiological and neurobiological level, in combination with behavioural
experimentation and a theoretical approach. At the same time, we need to
study genetic variation in traits and the selective forces acting on them.
Only this integration of proximate and ultimate studies will lead to a
complete understanding of parasitoid foraging behaviour. Undoubtedly,
technical developments will make it more feasible in the future to
track parasitoids during their lifetime under field conditions, which
will help us to further interconnect field and laboratory findings.
Furthermore, molecular biology is now offering us the tools to study
gene–environment interactions and to walk the path from genotype to
phenotype. It will certainly open new and important avenues for gaining
insight into the function and mechanism of behavioural variation in
insect parasitoids.
Whatever our limitations, our goal remains to understand the real
world in which parasitoids have evolved and function and how they
manage to adapt to varying circumstances. As we have shown in
this chapter, many other organisms, such as plants, non-hosts andFlexibility in Host-search and Patch-use Strategies 59