EN
The aim of present study was to determine in vitro susceptibility of the yellow mealworm larvae (Tenebrio molitor, Coleoptera: Tenebrionidae) on some environmental isolates of Bacillus thuringiensis and define the degree of change in the midgut epithelial cells of T. molitor larvae. A study of ultrastructural changes in the midgut of the yellow mealworm larvae treated with B. thuringiensis spores and crystals mixture was carried out. Under laboratory conditions four B. thuringiensis environmental isolates were tested for their insecticidal activity against T. molitor larvae. The tested insects pests showed little susceptibility to bacterial suspensions of spores and crystals (13.3%). Despite the relative pathogenicity the microscopic observation of structure of epithelium cells of T. molitor showed changes in the midgut of the treated larvae.