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; Hardy et 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, Harvey et al. (2000a) found that development time of the solitary braconid Apanteles carpatus is 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.

Some conditions can diminish the importance of size for fitness. Among the most important of these are high predation risks, which could favour accelerated development time at the cost of reduced size (Cole, 1954; Curio, 1989). In insects, evidence for mortality increasing with development time derives primarily from studies on insect herbivores that develop in exposed locations (Price et al., 1980; Grossmueller and Lederhouse, 1985; Leather, 1985; Benrey and Denno, 1997). In contrast, prolonged development times appear to have lower costs for herbivores that feed in concealed sites, because risks of predation and parasitism are usually lower (Clancy and Price, 1987; Craig et al., 1990; Johnson and Gould, 1992). Studies of parasitoid species loads lend support for the importance of feeding niche in host mortality and intraguild competition among natural enemies. In their surveys of the literature, Hawkins and Lawton (1987) and Hawkins (1988) determined that external foliar feeders, leaf-rollers and leaf-miners had much higher parasitoid species loads than concealed species, such as gallers, borers and root feeders. Given that insect herbivores are the main hosts for parasitoids, these patterns suggest that rapid development time at the expense of size will also be favoured in parasitoid larvae whose hosts confront high risks of predation. The trade-off between development time and size should also be most apparent among koinobionts, who, as noted above, have the option to consume hosts rapidly or to delay their development until their larval host attains a larger size. Comparing development times and offspring sizes in large versus small hosts, Harvey and Strand (2002) found that ichneumonoid koinobionts of exposed foliar-feeding larvae usually favoured short development time over size. In contrast, species that parasitized concealed hosts almost always favoured size over development time.

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