Individual and combined treatments with imidacloprid and spinosad disrupt survival, life-history traits, and nutritional physiology of Spodoptera littoralis

Abstract

The Egyptian cotton leafworm Spodoptera littoralis (Boisduval) (Lepidoptera: Noctuidae) is a pervasive agricultural polyphagous insect pest. Because of the negative side-effects of conventional pesticides used in agricultural fields, safer alternatives for insect pest management are required. We evaluated here susceptibility, biological features, and nutritional indices of S. littoralis after treatment of 4th-instar larvae with the neonicotinoid imidacloprid and the spinosyn spinosad separately or in combination. Larvae were fed for three successive days on treated leaves of castor-bean Ricinus communis using leaf-dip technique (treatment period). In addition, in case of nutritional indices study, treated leaves were replaced by fresh untreated leaves for two successive days (recovery period). Spinosad was more toxic than imidacloprid, and their combination revealed additive effects based on the co-toxicity factor. Individual and combined treatments significantly decreased pupation rate, adult emergence rate, pupal weight, number of eggs laid per female, egg-hatch, and female longevity, compared to those of controls. Pupal and adult malformations were recorded. During the treatment period, nutritional indices were insecticide- and time-dependent. On the 2nd day of recovery, all nutritional indices of treated larvae were not significantly different, compared to those of controls. The results presented herein may help in developing more effective crop protection methodologies within integrated pest management of this insect.