NUTRITIONAL ECOLOGY OF AN INSECT HOST PARASITOID ASSOCIATION - THE PEA APHID APHIDIUS-ERVI SYSTEM

Evolutionary host-size models assume that host quality for parasitoid growth and development is a linear function of host size. To test this assumption, we compared the growth patterns of the hymenopteran parasitoid Aphidius ervi when it developed in different nymphal instars of apterous pea aphids, Acyrthosiphon pisum. At daily intervals, unparasitized aphids and aphids that were parasitized at age 24, 48, 72, and 120 h (corresponding to nymphal instars L1-L4) were dissected. We weighed parasitoid larvae and host remains to determine changes in their relative growth rates, based on wet mass (WM) and dry mass (DM). The growth in DM of unparasitized aphids followed a sigmoid curve. Aphids that were parasitized by A. ervi continued to grow for 5-7 d before their DM started to decline. Trajectories of the parasitoid's growth during its larval and pupal stages could be described by nonlinear equations. The maximum larval DM and the time from oviposition to adult eclosion of parasitoids varied with host age at parasitization, and thus suggested differences in host quality. We propose that, for parasitoids developing in growing and feeding stages of the host, host quality is not a linear function of host size at parasitization but reflects the degree of integration of the two systems.