composition (Stephens and Krebs, 1986). As it falls well outside the scope
of this chapter to give a comprehensive review of this field, we shall
restrict ourselves to emphasizing the flexibility in host-acceptance
behaviour of parasitoids.
Host acceptance is strongly affected by the number of eggs retained by
the parasitoid (egg load) (Rosenheim and Rosen, 1991), but also by the
expected number of hosts that will be encountered during the remainder
of a parasitoid’s life. Ideally, a parasitoid should die just after laying its
last egg. When parasitoids are exposed to conditions that indicate low sur-
vival probabilities, they accept hosts more readily. Roitberget al. (1992)
mimicked the ‘end of season’ by rearing the parasitoidL. heterotoma
under the autumn photoperiod and found that females accepted low-
quality hosts (in this case, parasitizedDrosophilalarvae) more often than
the control group, which were reared under the summer photoperiod.Superparasitism
Many species of parasitoids exploit hosts that have a patchy distribution.
When searching for hosts, such patches are depleted as more and more
hosts will get parasitized. Females encounter these parasitized hosts
and can either reject or superparasitize them. Theory predicts that this
decision in solitary parasitoids strongly depends on the number of
other females that are searching on the same patch (Van der Hoeven and
Hemerik, 1990; Visseret al., 1992a) as superparasitism becomes more
profitable when more females search a patch. Not only does the likelihood
that an encountered parasitized host has been parasitized by the female
itself decrease with increasing competition, but also the likelihood that
another female will superparasitize hosts the female itself has parasitized
increases. It therefore starts to become profitable to ‘defend’ the hosts by
self-superparasitism, even though only one parasitoid larva can complete
its development in the host. And, indeed, experiments confirm the effect
of competition on host-acceptance decisions. When twoL. heterotoma
females search a patch, more superparasitism is found than when a single
females searches a patch (half the size and with half the number of
hosts) (Visseret al., 1990). Even more striking is the result that females
that were kept together in a tube before being introduced singly on a
patch with only unparasitized hosts self-superparasitized more frequently
than females kept alone before the experiment (Visseret al., 1990), thus
‘anticipating’ the likelihood of future superparasitism by conspecifics.Clutch-size and sex-ratio decisions
When a host is accepted for oviposition, additional decisions need to be
made: how many eggs to lay (clutch size) and what sex ratio to produce.
Since parasitoids are haplodiploid (unfertilized, i.e. haploid, eggs result50 L.E.M. Vetet al.