The systematics of the Trichogramma minutum species complex (Hymenoptera: Trichogrammatidae), a group of important North American biological control agents: the evidence from reproductive compatibility and allozymes

Abstract

Two North American Trichogramma, Trichogramma minutum Riley, and Trichogramma platneri Nagarkatti (the T. minutum complex), cannot be distinguished morphologically and their species status has been questioned. Both are commercially available for biological control and are important parasitoids of the eggs of Lepidoptera in agricultural ecosystems. Results are reported of an extensive survey of reproductive compatibility and allozymes at the phosphoglucomutase (Pgm) locus in collections representing 85 localities throughout much of the geographical range of this species complex. In total, 103 reproductive crosses were performed and 161 collections were analyzed electrophoretically. Results support the recognition of T. minutum and T. platneri as distinct species. The two are distinguishable electrophoretically at the Pgm locus. High levels of reproductive incompatibility are correlated with the allozymic differences. Although T. platneri is western in distribution (W of 115° longitude) and T. minutum is primarily eastern, the two are sympatric in the Pacific Northwest. A low level of interspecific hybridization in laboratory studies notwithstanding, there is no evidence that introgression occurs in the zone of sympatry. This study continues to stress the importance of species recognition and the inadvisability of using T. minutum and T. platneri interchangeably in biological control.