This project focused on improving the in vitro rearing methods of Toxoneuron nigriceps, an important biological control agent of Heliothis virescens, by supplementing the rearing media with the host hemolymph. T. nigriceps larvae of different ages were incubated in the artificial rearing media. The growth (increase in length and width), development (molting), and survival of the incubated larvae were observed. Changes in osmotic pressure of the rearing media before and after incubation were evaluated. The protein profiles of day 1, 3 and 5 hemolymph samples of the parasitized and unparasitized H. virescens fifth instar larvae, used in the artificial rearing media, were determined. Host hemolymph improved the growth and development of T. nigriceps larvae in the artificial rearing media except in the case of new and early first instar larvae. Osmotic pressure of all media showed a significant decrease after the incubation of the larvae except media incubated with the new first instar larvae indicating that the older larvae absorbed and utilized more of the nutrients from the rearing media than the younger larvae. Growth and development was higher in the semisolid media than in the liquid media. In the artificial rearing media, the late first instar T. nigriceps larvae molted to second instars and a few of them molted to third instars. The early and late second instar larvae incubated in the artificial media also grew well and molted to third instars. Some of the in vitro reared third instar larvae demonstrated behavioral changes that could be expressed as the preparation for cocoon formation or pupation, however neither a cocoon nor pupation occurred. No qualitative differences in the protein titers were detected between hemolymph of the parasitized and unparasitized fifth instar H. virescens larvae. There was a significant increase in the protein concentration in both unparasitized and parasitized H. virescens larvae as the age increased. This increase in the protein concentration showed a positive effect on the growth and development of T. nigriceps larvae indicating that availability of nutrients was an important factor for the growth of T. nigriceps larvae in vitro.