Body Size, Life History and the Structure of Host–Parasitoid Networks - Advances in Ecological Research - page 160

cannot increase further, resulting in host density independent parasitism
rates ( Heimpel and Casas, 2008 ). Conversely, where parasitoids are primarily
searching time limited, resulting in a directly proportional relationship be-
tween host encounter rate and lifetime reproductive success, parasitoids
should maximise host encounter rate and forage in a host-density-dependent
manner ( Heimpel and Casas, 2008; Wajnberg, 2006 ); that is, construct a
realised niche according to host abundance ( Figure 4 A).
E. Handling Time and Life History in Parasitoids
As the allometric structuring of food webs is determined by predator body
size altering the handling times of resources, larger predators should extract
greater profits from large, more nutritious resources (in absence of social
behaviour such as pack hunting) ( Brose et al., 2006; Petchey et al., 2008 ).
However, size restrictions to trophic level are not evident in host-parasitoid
networks, and fundamental niche in parasitoids is determined by host
phylogeny and parasitoid life history, although there is some evidence that
parasitoid body size and host handling times are related, but this is deter-
mined by host characteristics ( Henry et al., 2009 ). For example, for para-
sitoids that attack sessile host stages, such as egg parasitoids or idiobionts
that parasitize cocoons or pupae, handling time costs do not scale positively
with host size or potential fitness gain ( Gross, 1993 ). Conversely, some hosts
are capable of aggressive defensive behaviours, such as aphids kicking or
caterpillars rolling, the effectiveness of which have been suggested to be
positively correlated with host body size, suggesting that for these aggressive
types of host, those individuals that provide greater offspring fitness incur
greater time costs of parasitisation ( Allen, 1990; Firlej et al., 2010; Gross,
1993; Henry et al., 2009 ).
In parasitoids that parasitize aggressive host types, it is thought that
larger adult parasitoids are less affected by defensive host behaviour,
providing a mechanism by which host handling time correlates negatively
with adult parasitoid body size, analogously to the pattern observed in
food webs ( Gross, 1993; Henry et al., 2009; Lykouressis et al., 2009 ). In a
study of the primary parasitoid A. colemani, larger female parasitoids
were capable of parasitizing larger hosts of the same species, while small
females were limited to small and early stage hosts ( Lykouressis et al.,
2009 ). Differences in handling time due to parasitoid size have also been
reported for host species of different size; larger parasitoid wasps of the
species Pachycrepoideus vindemiae spent less time handling larger host
species in choice tests and, as a result, exhibited greater preference for
the larger host species than smaller parasitoids ( Morris and Fellowes,
2002 ). Conversely, studies of the parasitoid Mastrus ridibundus,which