Mass-rearing of natural enemies for biological control faces many challenges. Genetic deterioration can be a serious problem. When highly diverse founding populations are allowed to inbreed and adapt to artificial laboratory or insectary conditions, characteristics important to field fitness can be lost. In this study, we use genetically marked isofemale lines of Trichogramma pretiosum to test the hypothesis that genetic diversity and therefore field fitness can be preserved by maintaining many inbred lines, rather than large mixed cultures, until just before release.