TOLERANCE LEVEL TO BACILLUS-THURINGIENSIS BERLINER IN THE COTTON LEAFWORM SPODOPTERA-LITTORALIS (BOISDUVAL) (LEP NOCTUIDAE)

A standard laboratory strain of the Egyptian cotton leafworm, Spodoptera littoralis (Boisduval) was treated with Bacillus thuringiensis var. kurstaki-HD1 (Dipel 2x) for eight successive generations. Selection with B. thuringiensis did not establish true resistance in the population. On the contrary, a latent toxicity was demonstrated. Slight delay in response to the toxic effect of the pathogen was developed after continuous selection for four successive generations. This may be attributed to the increase in vigor tolerance due to intrinsic or extrinsic factors rather than development of true resistance.