A second, more complex function is that precocious larvae in some
species also kill male siblings, which significantly distorts the sex ratio of
adult wasps that emerge from the host (Grbic’et al., 1992). Like most
Hymenoptera, polyembryonic wasps are haplodiploid, with male off-
spring developing from unfertilized eggs and female offspring developing
from fertilized eggs. Many species of polyembryonic wasps lay only a
single egg per host and, as a result, produce broods comprised exclusively
of one sex or the other. In these species, precocious larvae presumably
develop from both male and female eggs and function exclusively as
soldiers that defend their reproductive siblings from competitors. Other
species, includingC. floridanum,usually lay two eggs per host (one
male and one female) and produce broods comprised of both sexes. In
C. floridanum, precocious larvae develop almost exclusively from female
eggs, such that, in most broods, soldiers are fully related to their sisters
but not to their brothers. Female precocious larvae attack their brothers
while still embryos, but not their sisters, resulting in most of the adult
wasps (> 95%) that emerge from the host being females. These females
then mate with the surviving male wasps before dispersing to find new
hosts. This interaction may have arisen as a consequence of genetic
conflict between siblings. Female-biased sex ratios are predicted in a
situation called local mate competition, where female wasps mate with
their brothers at the site of emergence and mating opportunities for males
are insignificant away from the brood. Under such conditions no genetic
conflict exists between parent, sons and daughters, and sex ratios are
predicted to be strongly female-biased. However, if males can obtain a
significant number of matings away from the natal brood, the male clone
would favour a more male-biased sex ratio to capitalize on opportunities
to mate with other females, but the female clone would favour a
more female-biased sex ratio to reduce competition for the limited food
resources of the host. Any difference in evolutionary optima would be
increased by the genetic asymmetries between males and females due to
haplodiploidy. Field data suggest thatC. floridanum males do obtain
additional matings away from the natal host and that the resulting conflict
between siblings has been resolved in favour of sisters by precocious
larvae.Summary
Adult parasitoids are free-living and mobile, while immature stages are
largely immobile, obligate parasites of other organisms. This combination
of traits provides tremendous opportunity for dispersal to novel habitats
by winged adults, while simultaneously favouring larval specialization to
exploit particular types of hosts. These traits are also probably responsible
for the high species diversity of parasitoids and the evolution of
diverse taxa that parasitize almost all groups of terrestrial arthropods
and molluscs. As emphasized in this chapter, the behaviour of larvalInteractions between Larval Parasitoids and Their Hosts 145