We investigated whether tolerance can be induced after continuous application of the nonsteroidal ecdysteroid mimic tebufenozide at about LC30 over 5 generations of the cotton leafworm Spodoptera littoralis (Boisduval). After 2 and 4 generations, toxicity data showed that such continuous treatment did not result in a shift of insect susceptibility even after 4 generations in S. littoralis, indicating the difficulties in achieving resistance in this pest to tebufenozide. Intoxicated larvae showed signs of premature and lethal molting within 24 h of treatment. In addition, tebufenozide inhibited oviposition which resulted in loss of the specifically treated strain after 5 generations. In a 2nd series of experiments, the potency of the ecdysteroid agonists tebufenozide and RH-5849 against 3rd instars of a laboratory and multiresistant field strain of S. littoralis was compared with the activity of 2 commonly used pyrethroid, organophosphorus, carbamate and benzoylurea insecticides against Lepidoptera. According to LC50s, teflubenzuron was the most potent compound, followed by cypermethrin and deltametrin, and tebufenozide. The remaining compounds were 2—100 less effective than tebufenozide. Our results indicate that resistance with tebufenozide is difficult to achieve. however, further research on tolerance and cross-resistance is required before definitive conclusions can be drawn.
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Smagghe et al. (1997) studied this question.