Diamondback moth, Plutella xylostella (L.) (Lepidoptera: Plutellidae), larvae wriggle violently backwards when disturbed. As they vary greatly in size from 1.3mm in length of L1 instar to 12 mm of fully grown L4 instar, the 2.1 mm long female Cotesia vestalis (Haliday) (Hymenoptera: Braconidae) has to gain control of each host to insert ovipositor and lay an egg. This study tested the hypotheses that parasitism rates of L2 to L4 instars correspond with 1) ability of female wasps to subdue them, and 2) their suitability for parasitism. In a choice test involving direct observations, C. vestalis gained control of L2 and L3 instars until ovipositor removal, but it was unable to subdue L4 instar, which limited ovipositor insertions. As parasitoids increasingly avoided wrestling L4 instar in subsequent encounters, it was worth investigating if limited capability to subdue these hosts deprives C. vestalis access to a high quality resource. In both choice and no-choice tests, parasitism rates of L2 and L3 instars were similar, and they were significantly higher than of L4 instar. Although parasitoid developmental time was shorter on L4 than on both L2 and L3 instars, a significantly higher proportion of parasitoids and daughters emerged from L2 and L3 instars than those parasitised at L4 instar in both choice and no-choice tests. These results imply that L4 instar is a lower quality resource for intrinsic rate of increase of C. vestalis, and its limited capability to subdue this instar leads to higher parasitism of themoresuitable L2 and L3 instars.