COMPARATIVE TOXICITY OF 2 ECDYSTEROID AGONISTS, RH-2485 AND RH-5992, ON SUSCEPTIBLE AND PYRETHROID-RESISTANT STRAINS OF THE EGYPTIAN COTTON LEAFWORM, SPODOPTERA-LITTORALIS

The comparative toxicity of two non-steroidal ecdysteroid agonists, RH-2485 and RH-5992 (tebufenozide), on development stages, fecundity and egg viability of a susceptible laboratory strain and a pyrethroid-resistant field strain of Spodoptera littoralis (Boisduval) was evaluated. Taking the LC50s as the criterion, RH-2485 was 3-7-fold more potent than RH-5992 against the susceptible and 7-14-fold more against the field strain. The LC50 of RH-2485 in the 1st and 6th instars of the susceptible strain was 0.32 and 0.57 mg a.i./l, respectively. The field strain showed a mild cross-resistance of about threefold to both compounds in 1st instars and to a lesser extent in 6th instars. A considerable increase in fecundity (approximately 3-fold) and no effect on egg viability was observed when 6th instars were fed on cotton leaves treated with 0.25 mg a.i./l RH-2485 (approximately LC40). Our results indicate that both compounds are potentially potent insecticides for controlling S. littoralis larvae, being 10-60-fold more potent than a previous ecdysteroid agonist, RH-5849.