Plants play a pivotal role in interactions involving herbivores and their natural enemies. Variation in plant primary and secondary metabolites not only affects herbivores but, directly and indirectly, also their natural enemies. Here, we used a commercial NPK fertilizer to test the impact of three fertilizer, namely 50, 100, and 200 ppm nitrogen, and one control (i.e., water) treatments, on the weight of the nymphs of the whitefly Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae). Subsequently, the whitefly parasitoid Eretmocerus mundus Mercet (Hymenoptera: Aphelinidae) was reared on the different groups of whitefly nymphs and upon parasitoid emergence, the number of oocytes was determined as a measure of reproductive capacity.Trials were done on tomato and tobacco plants.The level of nitrogen concentration in tobacco leaves was directly correlated with the fertilizer applications, thus confirming the effect of our fertilizer treatments. Both in tomato and tobacco plants, healthy as well as parasitized whitefly nymphs, were heaviest in the 200 ppm nitrogen treatment.The highest number of oocytes per female parasitoid was recorded in the 200 ppm nitrogen treatment in tomato (31% more oocytes as compared with the control) and in the 100 and 200 ppm nitrogen treatments in tobacco (200% more oocytes). We suggest that the increase in oocytes was the result of the enhanced size (food quantity) and/or nutritional quality of the whitefly host.The practical implications of these results for the mass rearing of whitefly parasitoids and for biological pest control are discussed.