no-choice tests) are combined with ablation experiments and electro-
physiological recordings, so that both stimuli and receptors can be
identified. Attempts to identify mechanisms of host discrimination
should also not overlook the roles of non-chemical cues, including tactile
or visual cues. InCallosobruchusbeetles, females laid fewer eggs on seeds
bearing egg-shaped models than they did on seeds bearing a flat dab of the
material used to construct models (Messina and Renwick, 1985). Tactile
cues are known to be important in the responses ofC. maculatusfemales
to different legume species and cultivars. A final consideration is that the
use of gustatory cues after contact with a host does not preclude long-
distance olfactory cues that could permit females to avoid entire patches
of highly infested hosts (Ignacimuthuet al., 2000).Foraging Considerations
Female seed parasites must distribute their eggs among many scattered
hosts. Because a search for suitable oviposition sites resembles a search
for suitable food, foraging theory has been used to model the costs and
benefits of host discrimination. These models predict circumstances
under which a female should accept or reject an occupied host. For
insects that lay clutches of eggs rather than single eggs, foraging theory
has also been used to predict the optimal number of eggs to deposit on
infested vs. uninfested hosts (Charnov and Skinner, 1984; Iwasaet al.,
1984).
At first glance, it might seem that a female should always reject a host
that already bears conspecific eggs, especially when a seed can supportHost Discrimination by Seed Parasites 71
Fig. 4.1. Effect of sense-organ ablation on the egg distributions of females of
Callosobruchus maculatus.Dispersion was measured as the variance-to-mean
ratio: 1 = random, < 1 = uniform, and > 1 = aggregated.