Crossing incompatibility between Trichogramma minutum and T-platneri (Hymenoptera : Trichogrammatidae): Implications for application in biological control

The two egg parasitoid species Trichogramma minutum Riley and T. platneri N agarkatti are closely related. No morphological or molecular characters are known to distinguish them. Their treatment as distinct species rests on th e absence of female offspring in between-species crosses. The current speci es definitions, which must be considered tentative, assume their natural ra nges to differ, with T. minutum occurring east of the Rocky Mountains and T . platneri occurring to the west. Both species are used throughout North Am erica 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 he identified after crosses with known cultures, it is likely that producers often cannot be sure of t he identity of their wasps. Here we studied the causes of the lack of femal e offspring in the between-species crosses and what happened when one speci es is released for biological control in the native area of the other. Our results show that females in between-species crosses are inseminated and us e the sperm to fertilize their eggs, but that these fertilized eggs die. In addition, females do not preferentially mate with males of their own speci es when exposed to conspecific and nonconspecific males. These results are used in a model to predict the effect of releasing the non-native species i n the native area of the other species. This model shows that such introduc tions can result ina prolonged and substantial reduction of intended biolog ical control.