As a result of the laboratory experiments, it has been established that along the 4 generations in mixed populations of T. evanescens Westw. and T. pintoi Voeg., gradual substitution of T. evanescens by T. pintoi occurs. The mechanism of interspecific competition on grain moth eggs was established. In the field of cereal, technical and vegetable crops, T. evanescens is dominant specie (up to 95-99%). T. pintoi represents a laboratory population, which’s preferred host is Siotroga cerealella Ol.. In the presence of the two species of Trichogramma in the biotope, their quantitative ratio depends on the specific conditions of climate and resource of preferred host’s egg. Biological indices for T. pintoi are higher than ones of T. evanescens. Therefore, in the laboratory conditions we substitute T. evanescens by T. pintoi, but in the field it’s opposite.