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Increasing likelihood of egg limitation
Utilise available host
Forego available host
Profit from finding new host
Figure 8 The effect of egg-limitation on the cost to utilising an available host; and,
therefore, foraging strategy. As there is a high likelihood of egg-limited species
encountering a host and not having an egg available to allocate to it, there is a greater
cost to optimal fitness associated with allocating each egg to a suboptimal host. We
suggest, therefore, that species with a higher likelihood of experiencing egg-limitation
are more likely to forego an available host than species that are less likely to run out of
eggs.
The ovigeny index is related to the importance of egg or time limitation
experienced by an individual. Synovigenic species have a reduced mature egg
load compared to pro-ovigenic species and have a greater risk of experiencing
egg-limitation at any given time ( Casas et al., 2000 ). Conversely, pro-ovigenic
species experience greatest fitness gains by maximising the number hosts in
which they successfully oviposit; as they have shorter lives and are more likely
to die before allocating all of their eggs ( Ellers et al., 2000; Jervis et al., 2008 ).
The effect of pro-ovigeny on foraging strategy is suggested in the results of a
study of the pro-ovigenic parasitoid Ibalia leucospoides, where the study
population exhibits strong adherence to the IFD ( Corley et al., 2010 ); this
corroborates the idea that species with a reduced likelihood of egg-limitation
must maximise host encounter rate in order to forage optimally. We could
suggest, therefore, that the ovigeny index is related to the realised niche of a
parasitoid, where foragers with a high likelihood of running out of eggs before
they die, that is, synovigenic species with a index score close to zero, preferen-
tially utilise host species of greater quality (i.e. a greater body size), resulting in
networks structured by host characteristics and not abundance ( Figure 4 B).
This could explain the lack of a relationship between host density and egg load
in the synovigenic parasitoid, Aphytis aonidiae, reported in a previous study
( Heimpel and Rosenheim, 1998 ).
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