leading role in the biological control of insect pests. In natural ecosystems
they also play a crucial role, both in number of species and in number of
individuals. Taxonomists estimate that parasitoids make up 20–25% of all
10 million to 100 million insect species that probably exist. After plants
and herbivores, parasitoids are the third trophic level (or higher) and this
high position in the food web, combined with their often specialized host
association, makes them vulnerable: disturbance of natural habitats can
easily reduce population size or cause species extinction. Parasitoids are
considered to function as keystone species in almost every terrestrial
ecosystem (LaSalle and Gauld, 1993). Together with other natural ene-
mies, such as predators, they keep our planet green by limiting herbivore
populations (Hairstonet al., 1960).
Like all parasites, insect parasitoids can only develop in or on another
organism. However, insect parasitism differs from classic parasitism in
several ways. First, the adult parasitoid is a free-living insect and the
parasitic lifestyle of parasitoids is thus limited to the immature stages
only. In parasitoids, it is generally the adult female that makes foraging
and ‘infection’ decisions that influence the fitness of her progeny. This
contrasts with classic parasites, where each individual infective stage
makes its own foraging and infection decision and host infection is
initiated by the stage that will become established. Secondly, because the
host has been killed by the time of the adult parasitoid’s emergence, the
emerging female wasp is challenged to search for new hosts to produce
her offspring. This task demands an active process of host searching by
the female wasp in an environment that is highly variable in space and
time.
These characteristics have played a major role in shaping the host-
searching strategies of parasitoids. Searching for new hosts is not an easy
task, since hosts are small organisms in an often very complex natural
environment and they are under strong selection to remain inconspicuous
to their enemies. In some cases, parasitoids can solve this problem by
spying on the communication system of their host. Some parasitoids use
the host’s sex pheromone or aggregation pheromone to locate potential
sites where host females may be laying eggs. But to most parasitoids this
solution is not available and they are thus forced to use indirect cues to
find their victims. Such indirect cues, such as odours from the food of
their host, create the problem of unreliability. A potential host-food plant
may be easy to find, but the location of the plant does not guarantee
the presence of a suitable host insect. Reliable host cues are undetectable
and detectable indirect cues are unreliable. This reliability–detectability
problem has constrained the evolution of parasitoid foraging behaviour
(Vetet al., 1991; Voset al., 2001).
Another important difference between insect parasitoids and other
parasites is the parasitoid’s high degree of behavioural flexibility. We
define behavioural flexibility as a change over time in the behaviour of an
individual, due to changes in the animal’s physiological state (hunger, egg
load) or to different learning processes (e.g. classical associative learning).40 L.E.M. Vetet al.