Geoscience Reference
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Time limitation
Egg limitation
Cost to using time
Figure 7 Diagram indicating how the likelihood of a forager experiencing egg or
time limitation affects the cost associated with allocating an egg or a unit of time.
Where whichever of the two resources is more likely to be a limitation on reproductive
success is the resource that should be allocated optimally.
forager is more likely to encounter a better host but not have any eggs
available ( Minkenberg et al., 1992; Rosenheim et al., 2008 ). By having a
low likelihood of dying before allocating their entire egg complement, an egg-
limited individual should maximise its individual fitness not by optimising its
host encounter rate but by foraging according to host characteristics, thus
maximising offspring fitness. This means that parasitoids with a high likeli-
hood of experiencing egg-limitation should be more likely to forego an
available host if it is of low quality than those with a low probability of
egg-limitation ( Figure 8 ). The reduced importance of host encounter rate in
egg-limited foragers is evident in the results of a study of the dynamics of
PAT rules in the cereal aphid parasitoid, Aphidius rhopalosiphi, where for-
agers switched from an incremental to a decremental PAT pattern as they
used up their eggs ( Outreman et al., 2005 ). Further, an optimal foraging
model suggested that parasitoids that are more likely to be egg-limited best
optimise their fitness by preferentially allocating their eggs to hosts of higher
quality, when host quality was considered in terms of whether the host had
already been parasitized or not ( Outreman and Pierre, 2005 ). Species that
forage in a manner where they are unlikely to utilise a suboptimal host in
favour of finding a more suitable host are highly elective. Electivity has been
related to relative egg-limitation in the synovigenic parasitoid Aphytis meli-
nus, where foragers became more elective as they used up their daily egg load
( Casas et al., 2000 ); although this pattern is not applicable to all parasitoid
species ( Javois and Tammaru, 2006 ).
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