parasitoids cannot be separated from that of adult parasitoids or the hosts
they attack. In most instances, the adult parasitoid locates hosts, assesses
their quality and decides how many and what sex of eggs to lay. The
importance of the adult parasitoid to offspring also extends beyond
oviposition, as maternal factors, such as polydnaviruses, are essential for
successful development. However, parasitoid larvae are by no means
passive entities totally dependent on their parent. They too influence the
development and behaviour of hosts and are also able to effectively
compete for resources.
Among the most important areas for future study of larval parasitoids
are: (i) understanding the molecular mechanisms that underlie the
developmental and behavioural changes in hosts; and (ii) determining
whether these changes have any functional significance for the parasitoid.
In the area of endocrine physiology, for example, it is relatively easy
to measure changes in hormone titres, but it is very difficult to
unambiguously determine whether a given alteration is due to a specific
parasitoid factor or is an indirect consequence of parasitoid feeding
or tissue damage. Likewise, while many studies report changes in the
behaviour of hosts after parasitism, far fewer studies have experimentally
tested whether these alterations have any adaptive relevance to the repro-
ductive success of the parasitoid. Identification of additional virulence
genes and development ofin vivoandin vitromethods for conducting
manipulative experiments in parasitized hosts are strongly needed
to enhance our understanding of physiological interactions. Greater
attention to conducting manipulative field experiments is also needed to
address the importance of behavioural changes in hosts for parasitoid
fitness. Another area of importance is understanding the molecular basis
for immunological resistance by hosts and how parasitoid virulence
factors overcome host resistance mechanisms. The coevolution of
resistance and virulence, their costs to hosts and parasitoids and how
variability in these traits are maintained in natural populations are all
issues of broad significance to the field of parasitology that parasitoids are
well suited to address.Acknowledgements
I would like to thank Jeff Harvey, Laura Corley, Mark Lavine and Markus
Beck for discussions about parasitoid–host interactions, and Jena Johnson
for creating Figs 7.1 and 7.2.References
Adamo, S.A., Robert, D. and Hoy, R.R. (1995) The effect of a tachinid parasitoid,
Ormia ochacea, on the behaviour and reproduction of its host, the field
cricket.Journal of Insect Physiology41, 269–277.146 M.R. Strand