The two egg parasitoid species Trichogramma minutum Riley and T. platneri Nagarkatti are closely related. No morphological or molecular characters are known to distinguish them. Their treatment as distinct species rests on the absence of female offspring in between-species crosses. The current species definitions, which must be considered tentative, assume their natural ranges to differ, with T. minutum occurring east of the Rocky Mountains and T. platneri occurring to the west. Both species are used throughout North America for the biological control of moths in orchards and forests. They are available to growers and researchers through a large number of biological control suppliers. Because the species can only be identified after crosses with known cultures, it is likely that producers often cannot be sure of the identity of their wasps. Here we studied the causes of the lack of female offspring in the between-species crosses and what happened when one species is released for biological control in the native area of the other. Our results show that females in between-species crosses are inseminated and use the sperm to fertilize their eggs, but that these fertilized eggs die. In addition, females do not preferentially mate with males of their own species when exposed to conspecific and nonconspecific males. These results are used in a model to predict the effect of releasing the non-native species in the native area of the other species. This model shows that such introductions can result in a prolonged and substantial reduction of intended biological control.