Host Acceptance
When a host is found, the female parasitoid’s decision-making continues.
Will it accept the host for oviposition, will it use it for host feeding or will
it reject the host? If it accepts the host for oviposition, the number of eggs
and the sex of these eggs (which is under female control (see below)) need
to be decided.Host quality
Individual hosts differ in their suitability for offspring development or as
a resource for the adult female, i.e. they differ in quality. Although some
parasitoid species are limited to a single host species, many use a range of
species. Upon encountering a host, the parasitoid needs to assess the
species. For instance, some host species, such as the larvae of some
species ofDrosophilaand some scale insects, are able to encapsulate the
eggs after they have been parasitized (Blumberg and DeBach, 1981; Carton
and Kitiano, 1981). This reduces the likelihood that an egg will success-
fully develop to an adult, and hence the quality of the host. Within a host
species, quality of hosts can vary with their age, size or stage. This is
especially true for idiobionts, in which the host is killed at oviposition.
Koinobionts, in which hosts continue to grow after being parasitized, can
often develop on a much wider range of host sizes.
A final aspect of host quality is whether or not the host has previously
been parasitized. It could either contain eggs from a different parasitoid
species, from the same species or even from the female herself. Also, the
number of eggs present can vary. A specific term is used for the ability to
distinguish parasitized from unparasitized (sometimes called healthy)
hosts: host discrimination. Intraspecific host discrimination is common,
and there are species that are even able to discriminate between hosts
parasitized by themselves and those parasitized by conspecifics (van
Dijkenet al., 1992; Visser, 1993). Also, some species can distinguish
between hosts with one and with two eggs (‘counting’) (Bakkeret al.,
1990).Accept or reject?
Assessing host quality is thus of crucial importance for parasitoids.
However, even when parasitoids can assess host quality perfectly, low-
quality hosts are not always rejected for oviposition. There are conditions,
such as a very low density of high-quality hosts or under low survival
probabilities, when accepting low-quality hosts will yield more offspring
(higher fitness) than rejecting these hosts. There is a vast body of
literature, both theoretical and experimental, that deals with the question
of host acceptance (the field of optimal foraging or optimal dietFlexibility in Host-search and Patch-use Strategies 49