and reproductive parts often show high concentrations of secondary
metabolites (Hamiltonet al., 2001). Some legume seeds, for example,
contain chemicals in the seed-coat or endosperm that are highly toxic
to non-adapted herbivores, but they are frequently attacked by specialist
parasites (Johnson, 1990; Povey and Holloway, 1992; Siemenset al.,
1992).
A second common feature of seed-parasite life histories is a high
frequency of intra- and interspecific competition within seeds. Many
hosts support the development of only one or two parasite larvae per seed
(Messina, 1991a). Because of the narrow host ranges of most parasites,
intraspecific competition is inevitable when parasite populations become
large relative to local seed availability (Delgadoet al., 1997). The capacity
of some seed parasites to infest nearly an entire seed crop has led to their
importation for the biological control of exotic weeds (e.g. Briese, 2000;
Radfordet al., 2001). Several behavioural, physiological and life-history
traits of seed parasites have been interpreted as evolutionary responses
to frequent intraspecific competition (Smith and Lessells, 1985). In this
chapter, I focus on host-discrimination behaviour as a means by which
egg-laying females reduce competition among their offspring within
seeds.
In much of the parasite literature, host discrimination simply refers to
a parasite’s ability to recognize and exploit a host. A more restricted
meaning, applied especially to insect parasitoids, is the tendency of
an egg-laying female to avoid hosts that already bear conspecific eggs
or larvae (Roitberg and Prokopy, 1987). Host discrimination can be
expressed either as a lower rate of acceptance of infested hosts or as
smaller clutch sizes on such hosts (Messina and Fox, 2001). By reducing
the degree of competition experienced by offspring, discriminating
females will under most circumstances have higher fitness than females
that do not respond to previous infestation. Yet we might expect variation
in the degree of host discrimination when there is concomitant variation
in the associated gain in fitness (Nufio and Papaj, 2001). Host discrimin-
ation thus provides an opportunity to examine the adaptive modification
of parasite behaviour in response to host traits.
I shall first review evidence for host discrimination and mechanisms
by which females distinguish between occupied and unoccupied hosts.
I then consider host-discrimination behaviour in the context of general
foraging models. Essential to this objective is identifying the genetic
and environmental causes of variation in the trait. Studies of seed beetles
demonstrate how variation in seed size can shape the egg-laying behav-
iour of associated parasites. I conclude by examining host discrimination
in a broader ecological context. A potential cost of host discrimination is
that cues mediating the behaviour can be exploited by the seed parasite’s
own natural enemies. On the other hand, the action of natural enemies
can make host discrimination advantageous even in the absence of
intraspecific competition.66 F.J. Messina