The diamondback moth (DBM), Plutella xylostella L., is the most destructive pest affecting vegetable production in Kenya and around the world. Parasitoids have shown promising results in lowering the pest populations and damage caused by DBM. However, variations in host plant quality have been reported to have bottom-up effects up to the third and fourth trophic levels. We assessed the effects of two cultivated Brassica varieties (cabbage, Brassica oleracea var. capitata L. cultivar ‘Gloria F1′ and kale, B. oleracea var. acephala L. cultivar ‘Thousand headed’) on the development and performance of the specialist pest P. xylostella and two exotic parasitoids Diadegma semiclausum (Hellen) and Cotesia vestalis (Haliday). The exposed larval period of DBM took about 1.5 d longer on kale than cabbage and the total immature development time of both females and males was significantly longer on kale than cabbage. Higher pupal weight and higher fecundity were recorded on DBM fed on kale. Development time of D. semiclausum and C. vestalis was not affected by the host crop as was the parasitism rate of D. semiclausum. Heavier male pupae and larger adults of D. semiclausum, as well as more fecund adults of C. vestalis, were obtained from hosts fed on cabbage. Larger adults of C. vestalis were obtained from herbivores fed on kale. These results show potentially positive effects of host plant allelochemicals that are detrimental to herbivores while promoting parasitoid development and performance, which can be harnessed for the control of DBM.