COVARIANCE OF ADULT SIZE AND DEVELOPMENT TIME IN THE PARASITOID WASP APHIDIUS-ERVI IN RELATION TO THE SIZE OF ITS HOST, ACYRTHOSIPHON-PISUM

Adult size (in terms of dry weight; DW) and development time (T(p)) of the solitary parasitoid Aphidius ervi varied when reared in different nymphal instars of its host, apterous virginoparae of the pea aphid (Acyrthosiphon pisum). 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 T(p) did not significantly differ between the sexes. Parasitoid DW was consistently more variable than T(p). 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 T(p). When growing in aphids below a certain size threshold, parasitoids can maximize fitness by a trade-off between DW and T(p). 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.