Augmentative biological control is a pest management strategy relying on the release of large numbers of natural enemies. It has been proved to be a successful alternative for chemical control especially in some areas of agriculture such as greenhouses. Providing high quality mass-reared natural enemies is essential for their use to combat pests. In this thesis, I focused on the study of diets for Orius majusculus, a predator commonly used in Europe for thrips control. As with many other heteropteran predators, the production of O. majusculus relies on the use of the eggs of the Mediterranean flower moth Ephestia kuehniella as a rearing diet. One of the main drawbacks is the high price of the eggs, ultimately increasing the price to pay for the growers. The use of artificial diets would reduce the complexity of the system by eliminating the need to rear a second species (i.e. the E. kuehniella moths). For this reason, six artificial diets varying in their macronutrient composition were tested as alternatives to E. kuehniella eggs. Results showed that nymphal survival and developmental time were acceptable in most artificial diets but female body weight and oviposition were reduced when compared to a diet of E. kuehniella eggs, which would prevent the artificial diets to be used commercially. When it comes to the different composition of the diets, offering the diets in ad libitum quantities and the lack of some essential nutrients potentially prevented bigger differences to appear between artificial diets. Finding alternative factitious prey to E. kuehniella eggs that are cheaper and simpler to produce could be a faster way to find alternative diets that reduce the production costs of O. majusculus. For this purpose, a study testing the use of frozen adults of Drosophila melanogaster of two different body compositions (a lipid rich and a protein rich) as alternative rearing diets was carried out. In the study, both the possibility of using D. melanogaster flies as nymphal diet and throughout the whole development was tested. Results show that protein-rich D. melanogaster appears to be a suitable nymphal diet for O. majusculus, with results, in terms of fitness and reproduction, comparable to E. kuehniella eggs. Since adult O. majusculus are released in the field for pest control, the sole use of D. melanogaster as a nymphal diet would certainly reduce the dependence on E. kuehniella eggs and therefore rearing costs. Finally, biological control agents needed to be efficient in controlling pests in the field. Therefore we tested the potential pest-supressing effect that O. majusculus reared on E. kuehniella, D. melanogaster (the potential alternative diet) and Aphisfabae (natural prey) would have on a population of A. fabae. For this study we used a purpose-built mesocosm with faba beans (Vicia faba) simulating a close to nature set-up. After eight days, O. majusculus was notable to affect the population of aphids. However, based on fecundity and oviposition, we speculate that due to a more numerous second generation, predators reared as nymphs on E. kuehniella will have a higher impact on pest-suppression, followed by those reared on D. melanogaster and A. fabaein that order. The findings and results presented in this thesis deepen our understanding of the effects of nutrition on insect predators, and have the potential to help improve predatory insect production systems.