from parasitoids. Females supplied with an abundance of seeds in the
laboratory did in fact lay only one egg per seed (also Foxet al., 1996).
A final potential influence of natural enemies is via effects on larval
competitiveness, which, as we have seen, tends to coevolve with host
discrimination inCallosobruchusbeetles. After transferringC. chinensis
beetles from a large-seeded legume host to a smaller one, Tuda (1998)
noted an evolutionary shift from scramble-type competition among larvae
to contest-type competition. Interestingly, beetle populations exposed to
parasitoids evolved in this direction more slowly than did populations
without parasitoids. This delay in the evolution of highly competitive
larvae was predicted from population models in which parasitoids
contribute an additional source of mortality that lessens the severity of
competition (Tuda and Iwasa, 1998). By modifying larval competition
within seeds, chronic attack by parasitoids can affect which type of
egg-laying behaviour is advantageous. It is worth noting, however, that
transfer to a small host eventually increased the frequency of contest
competition even in the presence of parasitoids (Tuda, 1998).Conclusion
Herbivorous insects obviously use multiple cues and a variety of sensory
inputs in the hierarchical process of locating and consuming plants
(Bernays and Chapman, 1994). Seed parasites, which typically have
sessile larvae and narrow host ranges, illustrate the sometimes close
relationship between female egg-laying behaviour and offspring fitness.
We therefore expect parasite females to display a suite of non-random
behaviours that enhance the survival and reproduction of their progeny.
In one species that attacks its host in the flowering stage, the parasite
female may actually manipulate the host to ensure greater fruit set and
hence the amount of food available to larvae (Brody and Morita, 2000).
Host discrimination is a common behaviour of seed parasites because
of the importance of minimizing competition among larvae within seeds.
Yet we cannot ignore other determinants of host use, such as intrinsic
plant quality or mortality from natural enemies. Some of these factors
may in fact generate trade-offs between behaviour that reduces larval
competition and other components of offspring or maternal fitness. Differ-
ences in local host availability, host size and larval aggression are all
predicted to modify the strength of host discrimination. Variation in
host discrimination can be expressed either as genetic differences at the
population level or as behavioural plasticity within the lifetime of an
individual forager.
Seed beetles and host legumes have been particularly useful for
testing these predictions, in part because they are quite amenable to
laboratory manipulations and in part because several species are major
economic pests. Population crosses and breeding designs have revealedHost Discrimination by Seed Parasites 81