Spodoptera littoralis (Lepidoptera: Noctuidae) resistance to CryIC and cross-resistance to other Bacillus thuringiensis crystal toxins

Laboratory selection of a strain of Spodoptera littoralis Boisduval with spore-crystal preparations of the Bacillus thuringiensis CryIC toxin for 14 generations resulted in resistance ratios from 10- to 500-fold resistance by using 2 different methods of selection. Partial cross-resistance was exhibited to CryID, CryIE, CryIAb toxins and to the parental strain B. thuringiensis aizawai 7.29. No cross-resistance occurred to CryIF. Resistance declined from 500 times the resistance ratio to 74 times after 1 generation without selection pressure, remained approximately 100 times after 3-6 generations and fell to 11 times after 8 generations. Sex ratio remained the same over selection period. Pupal weights of the selected strain varied, but we observed no trend in these differences. Developmental time increased from 5 to 6 d for treated, resistant insects