SEASONAL-VARIATION IN BODY-SIZE AND OFFSPRING SEX-RATIO IN-FIELD POPULATIONS OF THE PARASITOID WASP, APHIDIUS-ERVI (HYMENOPTERA, APHIDIIDAE)

We evaluated seasonal variations in body mass and sex ratio in field p opulations of Aphidius ervi Haliday (Hymenoptera: Aphidiidae), a solit ary parasitoid of the pea aphid, Acyrthosiphon pisum (Harris) (Homopte ra: Aphididae). Parasitoid dry mass was sexually dimorphic, with femal es being larger than males. Sexual size dimorphism was independent of host instar at parasitization. Parasitoid mass varied seasonally, bein g highest early and late in the season. The population sex ratio was f emale-biased, with a seasonal mean of 65.7% females (95% confidence in terval, 63.8%-67.6%). As predicted by evolutionary host-size models, a greater proportion of female wasps emerged from large than small host s early (but not late) in the season. We propose that the pattern of h ost utilization and sex ratio variation in A. ervi is determined by se asonal differences in the availability of the preferred aphid stages.