Geoscience Reference
In-Depth Information
explains how different life-history characteristics can be used to predict
electivity through their relation to the limited resources that must be opti-
mally allocated.
VI. PARASITOID LIFE HISTORY AND HOST
ELECTIVITY
A. Time and Egg-Limitation
It has been suggested that adult parasitoids may be limited in the number of
offspring they can produce by either the number of eggs available for
oviposition or the number of hosts they encounter during their lifetime;
therefore, eggs and time can be considered currency that must be optimally
allocated in order to maximise parasitoid reproductive output (
Cook and
Hubbard, 1977; Hubbard and Cook, 1978; Rosenheim et al., 2008;
Wajnberg, 2006
). Ideally, a parasitoid will produce exactly the same number
of eggs as the number of viable hosts it encounters, while allocating resources
in such a way as to maximise its realised fecundity (
Rosenheim et al., 2008
).
However, this realisation is unlikely due to the stochasticity of population
dynamics, and foraging individuals most likely either encounter viable hosts
without available eggs (egg-limitation) or die before laying all of their mature
eggs (time limitation). As has been shown above, there are two aspects of
optimal foraging: investment in quantity of offspring or investment in the
quality of offspring. Which of these two methods best optimises individual
fitness is dependent on the relative levels of egg or time limitation that an
individual experiences; as the relative costs and benefits to future reproduc-
tion associated with finding a host and utilising it can be explained by how
foragers must allocate their time or their eggs in order to maximise individual
fitness (
Rosenheim et al., 2008, 2010
).
B. The Cost of Egg or Time Limitation
Egg and time limitation has played an important role in theories regarding
the evolution of different life histories in parasitoids; however, there has been
some debate as to which has been the more important driving force (
Jervis
et al., 2008; Rosenheim et al., 2008
). As has been suggested in the Lepidop-
tera (
Jervis et al., 2007
) and Coleoptera (
Tatar et al., 1993
), there exists in
parasitoid Hymenoptera a trade-off between reproduction and survival
(
Ellers, 1996
). This is described by the ovigeny index, which quantifies the
relative allocation of an individual's reproductive capacity towards early or
