Activity of some nonazadirachtin limonoids from Azadirachta indica against lepidopteran larvae

Abstract

The biological activity of azadirachtin, nimbocinol, azadiradione and salannin isolated from Azadirachta indica A. Juss. (neem) was assessed alone and in combination against the cotton bollworm, Helicoverpa armigera (Hübner) and cluster caterpillar, Spodoptera litura (F) (Lepidoptera: Noctuidae). Nimbocinol exhibited growth inhibitory activity in artificial diet bioassays with 82.4 and 92.2 mg kg-1 concentrations inhibiting growth by 50%, respectively, in the two species. This efficacy was almost comparative to azadiradione (EC50 = 109.6 and 102.1 mg kg-1) and salannin (EC50 = 72.2 and 70.2 mg kg-1). Azadirachtin was the most active neem allelochemical against both insect species. In nutritional analysis, only nimbocinol and azadiradione reduced the efficiency of conversion of ingested food (ECI) in feeding experiments, indicating toxic rather than antifeedant effects. In a combination, when azadirachtin was present in a mixture, it always dominated in its efficacy and EC50 values did not deviate much from the individual efficacy of azadirachtin (0.23 and 0.21 mg kg-1, against H. armigera and S. litura larvae, respectively). However, enhanced activity among structurally variable molecules was observed, i.e., when salannin combines with nimbocinol or azadiradione, rather than structurally similar molecules like nimbocinol with azadiradione. The activity among nonazadirachtin limonoids in specific combination as opposed to the structural chemistry having explicitly two different modes of action, like feeding deterrence and chronic toxicity, may be playing a significant role in the multicomponent system.