Nabis stenoferusis a zoophytophagous predator that lives in grasslands around agricultural fields. It is a candidate biological control agent for use via augmentation or conservation. To find a suitable food source for mass-rearing and to better understand this predator’s biology, we compared the life history characteristics ofN. stenoferusunder the three different diets: aphids only (Myzus persicae), moth eggs only (Ephestia kuehniella), or a mixed diet of aphids and moth eggs. Interestingly, when only aphids were supplied,N. stenoferusdeveloped to the adult stage but lacked normal levels of fecundity. There was a significant synergism of the mixed diet onN. stenoferusfitness in both the immature and adult stages, i.e., a 13% reduction in the nymphal developmental period and an 87.3-fold increase in fecundity, compared to aphid-only diet. Furthermore, the intrinsic rate of increase was significantly higher for the mixed diet (0.139) than either aphids only (0.022) or moth eggs only (0.097). These results show thatM. persicaealone is not a complete diet for the mass-rearing ofN. stenoferus,whereas this aphid can be a supplementary food when combined withE. kuehniellaeggs. Implications and applications of these findings for biological control are discussed.