Finally, although we have discussed the various host-acceptance deci-
sions separately, we want to stress that it is unlikely that this is how
parasitoids behave. Often, a simultaneous decision on host acceptance,
clutch size and sex ratio may be needed. For example, autoparasitoids lay
male eggs on hosts that already contain a conspecific larva but female
eggs on unparasitized hosts (Viggiani, 1984). Hunter and Godfray (1995)
showed that, under abundant host densities, when parasitoids are egg-
limited, equal numbers of males and females are produced. Under low
host densities, the sex ratio reflects the ratio of the two host types. Thus,
clearly, the decision whether to accept or reject a host of a specific type
depends on the sex ratio already produced on the patch. Hence,
host-acceptance and sex-ratio decisions are not taken sequentially but
simultaneously.Decisions of developing parasitoids
Host acceptance may be the end of the mother parasitoid’s decision
trajectory in respect of a single host, but it is not the end of flexibility in
parasitoid behaviour. After oviposition, decision-making is ‘transferred
to’ the developing parasitoids. They now have to decide: (i) how to inter-
act with siblings or other competitors (see Strand, Chapter 7, this volume);
and (ii) when and how quickly to devour the host individual. Natural
selection will act strongly on both processes and it goes beyond the scope
of this chapter to discuss larval behaviour in detail. We want to mention
only developmental timing, since this is a novel aspect for parasitoid
behavioural ecology. The growth strategy of a parasitoid larva may have
severe fitness consequences. The outcome will depend on the costs and
benefits of a basic trade-off between developmental speed and adult
wasp size at emergence. Early emergence can be highly advantageous in a
growing population, while a large size at emergence can ensure greater
survival, fecundity and foraging success. The developmental strategy has
rarely been approached from an optimality point of view. Hemerik and
Harvey (1999) show with a simulation model that developing parasitoids
ofVenturia canescenshave to trade off their size as adults (which deter-
mines lifetime reproductive success) and development time. Especially
in nutritionally suboptimal hosts, there is strong selection for longer
development times, and a flexible growth strategy allows developing
parasitoids to respond adaptively to such variation in host quality.Patch Leaving
After having found a patch, a parasitoid exploits this patch by encounter-
ing hosts, which it might accept for oviposition or host feeding.
Eventually, exploitation of a patch ends with a parasitoid leaving the
spot. The decision of when to leave a patch is a classical question in52 L.E.M. Vetet al.