The Impact of Host Quality on Fitness
Host quality influences three main components of parasitoid fitness:
survival to the adult stage, size and development time (Waage and
Godfray, 1985; Godfray, 1994). Size and age are usually considered the
most important measures of host quality, although other factors, such as
the diet a host feeds upon, are also important. Studies with a diversity of
parasitoids indicate that host size strongly influences offspring survival
and adult size, and that parasitoid size is positively correlated with other
measures of lifetime reproductive success, such as fecundity, mating
success and longevity. The relationship between host and parasitoid size
is most direct for idiobionts, such as egg or pupal parasitoids, whose hosts
are closed resources that do not change in size after parasitism. For
solitary species, offspring fitness will be determined primarily by host
size alone, while, for gregarious species, offspring fitness will be affected
by both host size and the total number of other progeny in the host.
Idiobionts are predicted to grow at a constant rate and to maximize adult
size per unit of host resource consumed (Mackauer and Sequeira, 1993).
Wasps will take longer to develop on larger hosts than on smaller hosts,
because it takes longer to consume the resources available. However,
adult size and presumably fitness will be correspondingly greater. Several
empirical studies with idiobiont parasitoids demonstrate that adult
females accurately assess host size before ovipositing, and that gregarious
species accurately adjust clutch sizes in relation to available host
resources (Wylie, 1967; Strand and Vinson, 1983; Schmidt and Smith,
1985; Takagi, 1986; Hardyet al., 1992; Mayhew, 1998).
The relationship between host and parasitoid size is less direct for
koinobionts such as larval endoparasitoids, because hosts continue to
grow after parasitism. Koinobionts can either consume hosts rapidly or
delay their development, allowing the host to increase in mass before
consumption, to yield a larger parasitoid. If size is the primary target
of selection, koinobiont offspring that are oviposited into a low-quality
(i.e. small) host should exhibit a lag phase in development to allow the
host to increase in size before consumption. In contrast, a koinobiont in a
high-quality (i.e. large) host should grow at a constant rate. Parasitoid
development time in turn should vary with host quality at oviposition,
and selection should act on larvae to complete development coincident
with when the host attains an optimal size. Several studies lend support
for these predictions (Mackauer and Sequeira, 1993; Strand, 2000). For
example, Harveyet al. (2000a) found that development time of the solitary
braconidApanteles carpatusis much longer in small hosts than in large
hosts, because first-instar larvae delay their first moult until the host
moults to its final instar. Because of this delay, however, the average size
of wasp progeny produced from small hosts does not significantly differ
from those produced from large hosts.Interactions between Larval Parasitoids and Their Hosts 133