the development of only a few larvae. The expenditure of time or eggs
during oviposition on an infested host reduces a searching female’s ability
to exploit higher-quality hosts that she may encounter in the future
(Rosenheim, 1999). However, the consequences of female egg-laying
decisions depend on several factors, including a female’s age, experience,
egg load and survival probability. A particularly important factor is the
shape of the larval competition curve, which describes the combined
fitness of surviving larvae as a function of an increasing number of eggs on
a host (Smith and Lessells, 1985).
In many cases, the combined fitness or total productivity from a host
will increase with an increasing density of larvae, up to a maximum
determined by host size and quality (Fig. 4.2). This value has been called
the single-host maximum; any eggs beyond this number will only
decrease the combined fitness of all offspring. A key point is that, if per
capita fitness declines monotonically with each additional larva in a host,
then the total productivity per host (estimated as the number of surviving
larvae times per capita fitness) rises in a decelerating fashion up to the
single-host maximum (Fig. 4.2). In the chestnut weevil (Curculio elephas
(Gyll.)), for example, per capita fitness can be measured as a larva’s
survival probability times its potential fecundity (a function of its
eventual adult weight). Per capita fitness is highest when one larva
develops per seed, drops by 8% when two larvae share a host and drops
by 15% when four larvae share a host (Desouhantet al., 2000). Yet the
highest total productivity (the single-host maximum) occurs when seven
to eight larvae coexist in a single chestnut.72 F.J. Messina
Fig. 4.2. Relationship between larval density and the combined fitness of all
larvae in a seed (total productivity). Combined fitness is the product of larval density
and per capita fitness, which in turn equals the product of offspring survival and
fecundity. Curves a and b depict different ‘penalties’ associated with exceeding the
most productive number of larvae (the single-host maximum).