TOPICAL AND ORAL ACTIVITIES OF IMIDACLOPRID AND CYFLUTHRIN AGAINST SUSCEPTIBLE LABORATORY STRAINS OF HELIOTHIS-VIRESCENS AND SPODOPTERA-LITTORALIS

Activity of the newly developed nitroguanidine insecticide imidacloprid against fourth-instar larvae of insecticide-susceptible strains of the tobacco budworm, Heliothis virescens F., and the Egyptian cotton leafworm, Spodoptera littoralis (Boisd.) was determined from topical application and oral ingestion tests. The activity of imidacloprid was compared to that of cyfluthrin. The LD50 at 48 h of topically applied imidacloprid was estimated as 7.7 and 36.7 mug larva-1 for H. virescens and S. littoralis, respectively. In oral tests, the LC50 at 48 h of imidacloprid incorporated into artificial diet was estimated at 821.0 and 17.7 mug (g food)-1 for H. virescens and S. littoralis larvae, respectively. Imidacloprid showed lower contact and oral toxicities to both species than cyfluthrin, but the difference was much less important in ingestion tests. Mortality caused by both insecticides occurred more slowly in dietary exposures than in topical applications, but imidacloprid acted much more rapidly than cyfluthrin in ingestion tests. When incorporated into the diet, imidacloprid did not produce feeding deterrence, in contrast to cyfluthrin which exhibited a slight repulsive/antifeedant effect. Though cyfluthrin was the more active compound against both H. virescens and S. littoralis larvae, imidacloprid acted much more rapidly in ingestion bioassays. However, the level of activity of the compound is not high enough to provide efficient control of cotton insect pest populations in the field.