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for their nutrition. On the other hand, there are those species that do

engage in host-feeding, in addition to feeding on separate carbohydrate

sources. The two food sources usually cover separate requirements.

Whereas nectar or honeydew feeding primarily provides carbohydrates

to cover the parasitoid’s energetic needs, insect haemolymph usually

contains relatively low levels of carbohydrates (trehalose and glycogen).

Instead, host-feeding constitutes a primary source of protein for physio-

logical processes, such as egg maturation. As a result of the difference

in their nutritional composition, the two food sources are only partly

interchangeable.

In many parasitoid species, host-feeding and reproduction are

mutually exclusive, as host-feeding leaves the host unsuitable for larval

development. For host-feeding species, this may create conflict over

whether to use a host for current (oviposition) or future reproductive

success (host-feeding). The question of how parasitoids balance this dual

exploitation of their host resources has been the topic of optimization

models, as well as empirical studies (Ueno, 1999). While earlier models

assumed equal host suitability to address the effect of varying host

density, later work incorporated the effect of varying host quality. 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 (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 development.

Within a single species, 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 parasitization. In the latter case, parasitoids prefer-

entially feed on hosts that contain offspring of conspecifics (Ueno, 1999)

or heterospecifics, killing the resident parasitoid larvae.

Host search versus food foraging

In addition to the issue of host-feeding, parasitoids can also be divided

according to the spatial association between host and carbohydrate

sources. A first group includes those parasitoid species whose hosts are

closely linked to carbohydrate-rich food sources. This applies to species

whose hosts excrete suitable sugars, e.g. honeydew, or whose hosts occur

on sugar-rich substrates, such as fruits or nectar-bearing plant structures.

For these parasitoids, the task of locating hosts and carbohydrates is

linked. Parasitoids from this group may show few specific adaptations to

the exploitation of additional carbohydrate sources and little or no task

differentiation between food foraging and host search. The second group

is comprised of those parasitoids whose hosts are not reliably associated

Flexibility in Host-search and Patch-use Strategies 57