behavioural ecology. Since efficient exploitation of patches determines
the fitness of individual parasitoids and thus population dynamics,
patch residence times have been the subject of many experimental and
theoretical studies. Parasitoids that are able to react in a flexible way to
varying conditions are supposed to realize more offspring than para-
sitoids using rigid rules. Patch residence times can be influenced by
previous experience, host-related cues (kairomone concentration), egg
load, encounters with (un)parasitized hosts, predation risk and the pres-
ence of conspecifics (by direct encounter, superparasitism or recognizing
a mark).
Most of the early optimal foraging models consider animals as
deterministically acting organisms. Only after advanced statistical tools
became widely available did the viewpoint of researchers shift to how
decision-making by parasitoids was affected by different cues. The fitness
output of an individual parasitoid can depend heavily on its behaviour,
irrespective of whether this has evolved because of different spatial
distributions of hosts or because of encountering conspecifics. Since
each parasitoid has to compete with its conspecifics for an often limited
number of hosts, decision-making by parasitoids can, for a substantial
part, be determined by the behaviour of one or more conspecifics. We
shall give examples of early optimal foraging theory models, of factors
affecting the behaviour of individual parasitoids, of behavioural strategies
on different spatial distributions of hosts affecting lifetime fitness and of
parasitoids reacting to the presence of conspecifics.Early optimal foraging theory
In the early optimal foraging models, the parasitoids live in a static world
and are assumed to be omniscient and to forage optimally, i.e. they try to
produce as many offspring as possible. Charnov’s (1976) marginal-value
theorem shows elegantly how an omniscient predator or parasitoid
should adapt its patch residence time in prey or host patches according
to the expected encounter rates with prey or hosts in the full habitat.
The simple rule that results from the analysis of the model is to leave a
patch when fitness gain per time is higher outside than inside the patch.
This rule predicts that, in poorer habitats, patches are exploited more
fully than in richer ones. That this is the case for parasitoids has been
shown by Visseret al. (1992b):L. heterotomahaving experienced a poor
environment on the previous day superparasitize more than those that
experienced a rich environment beforehand. One of the main criticisms of
the early models concerns omniscience of the forager and a static world.
That parasitoids may sample their environment and thus gain experience
on which to base their foraging decisions is simply not considered. Since
the early models, researchers have tried to incorporate more reality. For
instance, they model foraging animals in dynamic worlds (Houston and
McNamara, 1999; Clark and Mangel, 2000). The outcome of these modelsFlexibility in Host-search and Patch-use Strategies 53