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can be controlled by an adult female, who can decide whether to fertilise an
egg before oviposition or not (
Heimpel and de Boer, 2008
). It has been clearly
shown in laboratory experiments that the allocated sex of the offspring is
influenced by host quality, where unfertilized eggs are laid in hosts of lower
quality (
Charnov et al., 1981; Morris and Fellowes, 2002; Ode et al., 2005
).
This differential host utilisation is based on the idea that the relationship
between adult characteristics and reproductive output is stronger for females
than for males; therefore, allocating females to more suitable hosts will
increase individual fitness more than if a male were oviposited to the same
host (
Charnov et al., 1981; Heinz, 1991; Sullivan and Volkl, 1999
).
B. Allocation Strategies and Parasitoid Life History
We would expect then that proportionally more female parasitoids eclose
from larger, more suitable hosts than males, and there is some evidence for
this in the field (
Bukovinszky et al., 2008; Mackauer, 1996
). However, sexual
size dimorphism has been suggested to be related to parasitoid life history.
For example, koinobiont parasitoids, which are less capable of discerning
host size for offspring use because the size of the host at time of oviposition is
not the same as at the end of the embryonic diapause, will be less likely to
differentially allocate offspring sex according to host size (
Mackauer et al.,
1996
). Parasitoids in which female fitness is less related to size are also
predicted to exhibit reduced sexual size dimorphism; for example, species
that oviposit in sessile host stages do not incur increased reproductive success
from the size-based relaxation of handling-time costs (
Mackauer, 1996
).
Further, the differential use of host species regarding sex ratio is related to
host availability. Early in the season, more female A. ervi were reported to
eclose from large hosts and more males from small hosts, but as favourable
host stages became less available, adults did not exhibit such strong sex
allocation preferences (
Sequeira and Mackauer, 1992
).
C. Sex Allocation and Network Structure
Sex ratio allocation decisions can have strong impacts upon the structure of
host parasitoid networks. Relative size differences between available hosts
have been shown to be important, a female exposed to only larger hosts will
produce a more male biased sex ratio than those exposed to a mixture of large
and small hosts (
Chow and Heinz, 2005
). A female-biased sex ratio is an
important aspect of biological control, especially inundation biocontrol,
where only females control host populations and a female-biased sex ratio
