Abstract.  1. Direct observations of Aphidius colemani foraging for Aphis glycines demonstrated that A. colemani attacks large aphid size classes selectively, in contrast to other Aphidiinae (Hymenoptera: Braconidae) species that generally prefer small or medium‐sized nymphs of different host species. 2. To determine the effect of this size preference on the potential ability of A. colemani to control A. glycines outbreaks, the stage‐dependent survival and fecundity of A. glycines were measured to parameterise a stage‐structured demographic model for the density‐independent population growth rate of the aphid. 3. Compared with hypothetical parasitoids that show either no size preference or preference for medium‐sized host nymphs, the preference of A. colemani for large hosts caused a greater reduction in the population growth rate of A. glycines. This occurs in the model because, by attacking reproductive adults, A. colemani kills those aphids that have the greatest immediate effect on the population growth rate. 4. The strong effect of size preference by A. colemani is diminished somewhat by the continued reproduction of A. glycines adults, which can reproduce for up to 3 days following parasitism. Nonetheless, reproduction by parasitised aphids is not enough to compensate for the stronger, negative effect of the preference of A. colemani for large aphids, which removes individuals with the greatest reproductive value from the A. glycines population.