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 for oviposition to adult eclosion of parasitoids varied with host age at parasitization, and thus suggested differences in host quality. We proposed 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.

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SummaryAdult size (in terms of dry weight; DW) and development time (Tp) of thc solitary parasitoid Aphidius ervi Varied when reared in different nymphal instars of its host, apterous virginoparae of the pea aphid (Acyrthosiphon pisurn). Parasitoid DW increased with an increase in the DW of the host at parasitization, from the first to the third aphid instar. Female wasps gained 1.1 times more in DW than their male counterparts in all four host classes, but Tp did not significantly differ between the sexes. Parasitoid DW was consistently more variable than Tp. The two traits covaried positively with an increase in host size from the first to the third instar, but they varied independently in parasitoids from fourth-instar hosts. The host size (and stage) at the time of parasitization imposes constraints on the growth and development of immature A. ervi that are reflected in the pattern of covariation between DW and Tp. When growing in aphids below a certain size threshold, parasitoids can maximize fitness by a trade-off between DW and Tp. Consequently, the assumption implicit in host-size models of parasitoid oviposition decisions -that females incur a relatively greater reduction in size (used as an index of fecundity) than males when developing in poor quality hosts -can be falsified.

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Changes in the weekly abundance in two natural populations of the Turkeyoak aphid, Myzocallis boerneri, from 1975 to 1992, were analyzed using autoregression methodology to determine the nature of dynamic processes. Seasonality forms the basis of aphid population dynamics. Analysis of time series of weekly and monthly data indicate statistically detectable seasonality in density changes. The monthly data statistically adjusted for seasonal effects (deseasoned) show that density fluctuates around a seasonally changing equilibrium value. Bulmer's test of density dependence applied to the seasonally adjusted monthly data reveals density dependence. The partial autocorrelation function (PACF) plots of smoothed deseasoned monthly density indicate time-lagged density-dependent processes operating between seasons and possibly on a shorter time scale of between 2 wk and 2 mo. The PACFs of smoothed deseasoned data provide statistical evidence for the existence of a ''see-saw'' relationship between density in spring, autumn, and the following spring. Analysis of annual abundances calculated from the weekly data suggests that overall abundance in any year is influenced by abundance the previous year. The pattern of changes in abundance between years is most likely the consequence of the see-saw effect operating between seasons. It is argued that aphid population density is regulated by means of density-dependent processes acting within years, which is reflected in the year-to-year changes in overall abundance.