Secondary Metabolites of Colocasia esculenta Extract as Green Insecticide Against the Cotton Leaf Worm, Spodoptera littoralis (Boisd.)

Abstract

A petroleum ether extract of Colocasia esculenta leaves yielded two sterols which were identified as; β- sitosterol І, Stigmasterol ІІ, these secondary metabolites showed insecticidal, antifeeding and insect growth regulatory activities against the 4th instar larvae of Spodoptera littoralis. So, the efficacy of this extract and two insect growth regulators belonging to different groups; methoxyfenozide (an ecdysone agonist) and lufenuron (chitin synthesis inhibitor) was assayed against the 4th instar larvae of S. littoralis under laboratory conditions. C. esculenta extract, methoxyfenozide and lufenuron have sub-lethal concentrations LC25 (17.8 gm/liter, 5.07 mg/liter and 0.80 mg/liter), respectively. It was observed that in case of both C. esculenta and methoxyfenozide treatments, the larvae undergo a precocious molting where promoted apolysis process but die due to failure in completing the molting process, at time shorter than the control ones. Ecdysis and sclerotization were incomplete and the pupa that emerged showed many deformities. The joint action of mixing C. esculenta extract at two concentrations (LC10 and LC25) with methoxyfenozide and lufenuron at two concentrations (LC10 and LC25) can be evaluated. All the mixtures of C. esculenta and methoxyfenozide have synergistic activity against treated larvae, while all the mixtures of C. esculenta and lufenuron have antagonistic activity. Biochemical characterization of enzyme activities related to molting process and cuticle synthesis showed that C. esculenta extract treatment caused significant disturbance in enzyme balances either decrease or increase comparing to control ones, the reason behind deformations and mortality may be as a result of change in chitinase, phenoloxidase and protease enzymes. These plant sterols І, ІІ may be considered to be efficient insect growth regulators, as well as having activity similar to phytosteroids, as was evidenced by their significant inhibition of molting processes.