Given a dome-shaped or concave-down relationship between larval
density and the total productivity, Charnov and Skinner (1984) argued
that host acceptance and clutch size should depend on the costs of search-
ing. If hosts are plentiful (and search costs low), a female can maximize
her rate of fitness gain by rejecting infested hosts and by depositing only a
single egg on each unoccupied host. If hosts are scarce (and search costs
high), females should become more likely to accept an occupied host and
lay more eggs per clutch. Only when search costs are infinite (a female is
not expected to encounter another host) should a female deposit as many
eggs as the single-host maximum, and it is always maladaptive to lay more
eggs than this value. In the chestnut weevil example, females typically lay
only one or two eggs per host. Desouhantet al. (2000) suggested that
clutch sizes in this species are well below the single-host maximum
because the number of suitable chestnuts is usually high relative to the
number of egg-laying weevils.
In general, a female should reject a host or stop laying additional eggs
when the marginal rate of fitness gain drops to a point where she could
gain fitness more rapidly by exploiting another host (Wajnberget al.,
2000). Realistic models of egg-laying decisions need to incorporate effects
of a female’s physiological status, in addition to whether a host is already
infested or not (Parker and Begon, 1986; Wilson and Lessells, 1994).
Because some of these variables change continuously over a female’s
lifetime, dynamic optimization models have been used to show how
the degree of host discrimination is expected to change according to
a female’s physiological state or the rate of encountering new hosts
(Mangel, 1987).
Unlike seed predators (such as rodents), seed parasites do not quickly
consume entire seeds. After a parasite female has deposited an egg, the
seed remains available to other females. One might expect a female to lay
fewer eggs if there is a risk of later-arriving females adding eggs to the
same host. Ives (1989) demonstrated that the evolutionarily stable clutch
size again depends on the shape of the larval competition curve. Small
clutches are expected if per capita fitness declines linearly with an
increasing number of larvae per host. In this case, there will be a steep
decline in the total productivity once the number of larvae exceeds the
single-host maximum (curve a in Fig. 4.2). If the first-arriving female lays
a number of eggs that is close to the single-host maximum, any eggs added
by subsequent females will greatly decrease offspring fitness. But, if
increasing larval density causes a non-linear, decelerating drop in per
capita fitness, then exceeding the single-host maximum causes only a
gradual decrease in total productivity (curve b in Fig. 4.2). In this case, the
optimal clutch size is largely independent of the number of females that
oviposit on the same host.
The above discussion assumes that female egg-laying decisions
depend on reducing competition among offspring. A different scenario
occurs when seed parasites also act as facultative or obligate pollinators
of their host plants. In the yucca moth (Tegeticula yuccasella(Riley)),Host Discrimination by Seed Parasites 73