Trichogramma are used for the biological control of numerous pests, For Trichogramma, as for other insects, the specificity of matings is ensured by several barriers that prevent copulation attempts between insects from different species, We have recently shown that insecticides may totally suppress species recognition that occurs from pheromonal communications between two Trichogramma species, a sublethal effect that will increase mating attempts between two different species, In this work, we have assessed the fitness cost of such interspecific matings and demonstrate that they are very costly for females, After an interspecific mating, females can generate only males because fertilized eggs degenerate (Trichogramma are haplo-diploid species; males are issued from unfertilized eggs and females from fertilized eggs), The resulting offspring are reduced in number by more than half, corresponding to the missing progeny from fertilized eggs. After an interspecific mating, the fecundity of females cannot be restored even if females subsequently mate intraspecifically. These results highlight the strong fitness cost of any event that would decrease the specificity of matings in Trichogramma. Because Trichogramma are key species regulating insect populations, these effects must be considered in the context of sustainable development.