COMPARATIVE TOXICITY AND TOLERANCE FOR THE ECDYSTEROID MIMIC TEBUFENOZIDE IN A LABORATORY AND FIELD STRAIN OF COTTON LEAFWORM (LEPIDOPTERA,NOCTUIDAE)

We investigated whether tolerance can be induced after continuous appl ication of the nonsteroidal ecdysteroid mimic tebufenozide at about LC 30 over 5 generations of the cotton leafworm Spodoptera littoralis (Bo isduval). After 2 and 4 generations, toxicity data showed that such co ntinuous 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 lar vae showed signs of premature and lethal molting within 24 h of treatm ent. 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 tebufe nozide and RH-5849 against 3rd instars of a laboratory and multiresist ant field strain of S. littoralis was compared with the activity of 2 commonly used pyrethroid, organophosphorus, carbamate and benzoylurea insecticides against Lepidoptera. According to LC(50)s, 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 tebufenoz ide is difficult to achieve; however, further research on tolerance an d cross-resistance is required before definitive conclusions can be dr awn.