Significance of penetration and metabolism on topical toxicity of diflubenzuron in Spodoptera littoralis and Spodoptera exigua

Toxicity assays indicated that when diflubenzuron (DFB) was topically applied on larvae of the Egyptian cotton leafworm, Spodoptera littoralis (Boisduval), and the beet armyworm, Spodoptera exigua (Hübner), estimated LD₅₀ values were similar, reaching 0.47 and 0.44 ng/larva, respectively. This study evaluated the importance of the rate of uptake and excretion, and of enzymatic metabolism in building up an insecticidal toxicity after topical application on the insect cuticle in both species. In general, penetration of DFB in S. littoralis was about 2-fold higher than in S. exigua, whereas metabolic breakdown was of minor importance in S. littoralis as compared with S. exigua. The current data indicate the importance of cuticle penetration and discuss that extrapolation from oral feeding toxicity results to topical toxicity data should be evaluated with caution, especially for insect growth regulators used in integrated pest management.