may create the conflict of whether to use a
host for current (oviposition) or future repro-
ductive success (host-feeding) (Heimpel and
Rosenheim, 1995). The question of how para-
sitoids balance this dual exploitation of their
host resources has been the topic of optimiza-
tion models (Jervis and Kidd, 1986; Kidd and
Jervis, 1991), as well as empirical studies
(Rosenheim and Rosen, 1992; Ueno, 1999).
While earlier models assumed equal host suit-
ability to address the effect of varying host
density, later work incorporated the effect of
varying host quality (Jervis and Kidd, 1986;
Rosenheim and Rosen, 1992; Ueno, 1999). In
the latter (more realistic) scenario, models
predict that parasitoids should selectively use
low-quality hosts for feeding and restrict
oviposition to high-quality hosts (Jervis and
Kidd, 1986; Kidd and Jervis, 1991).
Empirical studies have demonstrated that
parasitoids do indeed selectively exploit their
hosts according to various quality parameters.
When given a choice between different host
species, parasitoids tend to feed on the species
that is the poorer host for parasitoid develop-
ment. Parasitoids can discriminate by size,
using the smaller hosts for host-feeding
(Rosenheim and Rosen, 1992). Parasitoids can
also use information on host developmental
stage (Kidd and Jervis, 1991) or previous para-
sitization. In the latter case, parasitoids prefer-
entially feed on hosts that contain offspring by
conspecifics (Ueno, 1999) or heterospecifics,
killing the resident parasitoid larvae.
Host search versus food foragingA second conflict may arise from the spatial
distribution of host and carbohydrate
sources. This problem need not occur for
those parasitoid species whose hosts are
closely linked to carbohydrate-rich food
sources. Examples of this category are species
whose hosts excrete suitable sugars, e.g. hon-
eydew (England and Evans, 1997; but see
Wäckers, 2000), or whose hosts occur on
sugar-rich substrates, such as fruits or sugar-
excreting plant structures (Illingworth, 1921;
Morales-Ramos et al., 1996). For these para-
sitoids the task of locating hosts and carbohy-
drates is interlinked. Parasitoids from this
group may show specific adaptations to the
exploitation of additional carbohydrate
sources (F.L. Wäckers, L. Obrist and W. Völkl,
unpublished) and little or no task differentia-
tion between food foraging and host search.
The conflict of spatial dissociation between
host and carbohydrate sources is mainly acute
for those parasitoids whose hosts are not reli-
ably associated with a suitable carbohydrate
source. These parasitoids have to alternate
their search for hosts (reproduction) with
bouts of food foraging, which requires a clear
task differentiation. Consequently, parasitoids
face the issue of whether to stay in a host
patch, thereby optimizing short-term repro-
ductive success, or to leave the host patch in
search of food sources, a strategy that could
optimize reproduction in the long term.66 F.L. Wäckers
Fig. 5.1.Feeding–reproduction trade-offs in hymenopteran parasitoids. Dotted arrows indicate trade-offs
between sugar foraging and host search (all parasitoid species), or between host-feeding and oviposition
(host-feeding species only). Letters indicate costs and benefits to longevity (L), egg supply (E) and achieved
fecundity (F).