moths (Huth and Pellmyr, 1999) and seed wasps (Kouloussis and
Katsoyannos, 1991). Yet some seed parasites fail to discriminate between
occupied and unoccupied hosts, even though they use small hosts and
their juvenile stages are unable to move between hosts (Povey and Sibly,
1992). One explanation is that there may be a non-linear relationship
between larval density and fitness (sometimes called an Allée effect), such
that larval performance is optimal at intermediate numbers of larvae per
host. Allée effects can occur when the food source is improved by group
feeding or when mortality from natural enemies is inversely density-
dependent (Messina, 1998). Advantages to host discrimination also
depend on the degree to which egg-laying females are constrained by time
available for oviposition or resources available for egg production (see
‘Foraging Considerations’ below). Povey and Sibly (1992) have argued
that host discrimination is absent or weak in the rice weevil (Sitophilus
oryzae) because females are iteroparous (lay eggs in multiple bouts)
and use adult feeding to replenish resources needed for continual egg
production.
Host discrimination has been especially well studied among bruchid
beetles. These insects have been intimately associated with legume seeds
for much of the Cenozoic (Poinar, 1999). Throughout this chapter, I shall
focus on pest bruchids (especiallyCallosobruchus spp.), which have
infested human stores of grain legumes for thousands of years and are well
suited to laboratory manipulations. FemaleCallosobruchusbeetles lay
eggs singly on the surfaces of legume seeds and pods. The hatching larva
burrows into the underlying cotyledons, and must complete its develop-
ment within a single host seed. The cowpea seed beetle,Callosobruchus
maculatus(F.), has evolved multiple traits (including a dispersal poly-
morphism and a facultative diapause) that enable it to infest legume seeds
both in human stores and in the field (Utida, 1972; Messina, 1987).
Non-diapausing females infest hosts within hours after they emerge, so
that populations build up rapidly in storage. As a consequence, traits that
mediate intraspecific competition appear to be particularly important in
this species (Smith and Lessells, 1985).
Among Callosobruchus beetles, host discrimination has been
documented in several ways. Females have long been known to produce
non-random, uniform distributions of eggs among seeds (Utida, 1943;
Avidovet al., 1965). The fitness benefits associated with uniform egg
laying are easily quantified (Mitchell, 1975; Credland et al., 1986);
multigeneration studies have shown that competition within seeds
sharply reduces the survival of larvae and the body size, egg size,
fecundity and longevity of emerging adults (Messina, 1991a,b; Fox and
Savalli, 1998). Paired choice tests have demonstrated that egg-laying
females discriminate between seeds bearing different numbers of eggs, as
well as between seeds with or without eggs (Messina and Renwick, 1985;
Wilson, 1988; Horng, 1997). This quantitative response to egg density
maintains uniform egg distributions even after all seeds bear several eggs.
Finally, focal-animal observations have shown that the presence of an egg68 F.J. Messina