Abstract: Prevalent experiments to investigate the host-stage selection of parasitoids manipulate host cohorts at the optimal temperature (≈25℃). To explore the oviposition strategy and fitness-related performances of parasitoids in different host instars, the black bean aphid, Aphis fabae Scopli was cultured at 30℃ to obtain cohorts of different instars and adults and then exposed to parasitism of Lysiphlebus ambiguous (Haliday). The parasitism and fitness-related performances were recorded including parasitization rate, emergence rate, sex ratio, body size, and development time. The results indicated that the aphid parasitoid attacked more young aphids than the old, even though the 1st through the 4th instar nymphs and eclosing adults were all susceptible to parasitism. The parasitism rate on the adult aphid (28.0%) was significantly lower than on the 1st instar nymph (40.0%) and the 2nd instar nymph (42.8%). The proportion of female progeny and emerging adult size of the offspring parasitoid declined with the increase of aphid instars (and body sizes) at parasitism, but the development time remained constant, which contrasted with fitness performances of the parasitoid attacking the host reared at the optimal temperature. It was argued that L. ambiguous may not or may not always base the host value on body size and instar of the host, and other clues (e.g. behavioral, chemical etc.) associated with dynamics of endosymbiotic bacteria in the host aphid should be considered.

Key words: Lysiphlebus ambiguus, Aphis fabae, host-stage selection, fitness, Buchnera, endosymbiosis