The reproductive success of female parasitoids is dependent on their ability to accurately assess the suitability of a host for larval development. For generalist parasitoids, which utilize a broad range of species and instars as hosts, a set of assessment criteria determines whether a host is accepted or rejected. The suitability of a host, however, can only be imperfectly assessed by the female parasitoid, which can result in the selection of lesser quality hosts for oviposition. In this study we explored the disparity between host quality and host preference using the generalist koinobiotic parasitoid Aphidius ervi Haliday (Hymenoptera: Aphidiidae) and the host Aulacorthum solani (Harris) (Homoptera: Aphididae), the foxglove aphid. The second instar hosts produced the highest level of reproductive success, while third and fourth instars resulted in a substantially reduced reproductive performance. When given a choice of host instars, parasitoids preferred the older hosts for oviposition disregarding their reduced suitability for larval development. Results are discussed in context of mechanisms involved in A. ervi host selection and biases in the criteria used to assess hosts that may arise when parasitoids transfer host species between generations. Aphidius ervi (Aphidiidae) : Parasite Aulacorthum solani (Aphidoidea) : Host Reproduction; Reproductive behaviour; Parasites diseases and disorders; Parasites; Insect parasites; Hosts; Insect hosts Aphidius ervi; Egg laying; Oviposition preference; Reproductive productivity; Hemipteran hosts; Parasite oviposition preference effect on reproductive performance; Aulacorthum solani Aulacorthum solani; Hymenopteran parasites; Parasite oviposition preference effect on reproductive performance; Aphidius ervi none