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 imidaclopri d against fourth-instar larvae of insecticide-susceptible strains of t he tobacco budworm, Heliothis virescens F., and the Egyptian cotton le afworm, Spodoptera littoralis (Boisd.) was determined from topical app lication and oral ingestion tests. The activity of imidacloprid was co mpared to that of cyfluthrin. The LD50 at 48 h of topically applied im idacloprid 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 im idacloprid incorporated into artificial diet was estimated at 821.0 an d 17.7 mug (g food)-1 for H. virescens and S. littoralis larvae, respe ctively. Imidacloprid showed lower contact and oral toxicities to both species than cyfluthrin, but the difference was much less important i n ingestion tests. Mortality caused by both insecticides occurred more slowly in dietary exposures than in topical applications, but imidacl oprid acted much more rapidly than cyfluthrin in ingestion tests. When incorporated into the diet, imidacloprid did not produce feeding dete rrence, in contrast to cyfluthrin which exhibited a slight repulsive/a ntifeedant effect. Though cyfluthrin was the more active compound agai nst both H. virescens and S. littoralis larvae, imidacloprid acted muc h more rapidly in ingestion bioassays. However, the level of activity of the compound is not high enough to provide efficient control of cot ton insect pest populations in the field.