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 (Priceet 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; Craiget 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 off-
spring 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 para-
sitized concealed hosts almost always favoured size over development
time.
Host Defences and Parasitoid Counterstrategies
The most significant challenge confronting parasitoid larvae is survival
in the host. The first line of defence available to hosts is to avoid
being parasitized at all by either hiding or fending off oviposition by the
parasitoid adult. Once oviposition occurs, however, the host’s immune
system serves as the main defence against the parasitoid egg and larva.
Encapsulation
Insect blood contains cells called haemocytes, which have many different
functions in immunity. The main immune response to large, multi-
cellular invaders, such as parasitoids, is encapsulation. Although some
Diptera form melanotic capsules without the apparent participation of
haemocytes (Carton and Nappi, 1997), haemocytes are responsible for
encapsulation in most other insects (Strand and Pech, 1995). The insect
134 M.R. Strand