When forced by a suitable electric field, calamitic nematics can undergo electro-hydrodynamic instabilities and become turbulent. In contrast to isotropic fluid, they present a transition at high forcing from a turbulent regime (DSM1) to a regime characterized by high density of topological defects (DSM2). We study the transition threshold in nematic mixtures with some percent of 5CB in MBBA. The threshold decreases by increasing the 5CB concentration, an opposite behavior to what is expected by the increase of dielectric anisotropy. We suggest that the threshold control parameter is the biaxial coherence length, that is of the order of defect size.