| Abstract
| - The biological activity of 6β-hydroxygedunin isolated from Azadirachta indica A. Juss. was assessedusing the gram pod borer, Helicoverpa armigera (Hubner), and Asian armyworm, Spodoptera litura(Fabricius) (Lepidoptera: Noctuidae), alone and in combination with other limonoids, gedunin, salannin,nimbinene, and azadirachtin. The compound exhibited growth inhibitory activity in artificial dietbioassays, with 24.2 and 21.5 ppm, respectively, inhibiting growth by 50%. This efficacy was higherin comparison to gedunin (EC50 = 50.8 and 40.4 ppm), salannin (EC50 = 74.5 and 72.0 ppm), andnimbinene (EC50 = 391.4 and 404.5 ppm). Azadirachtin, however, remained the most active neemallelochemical against both insect species. Nutritional assays clearly demonstrated that, though relativeconsumption and growth rates of fourth instar larvae were reduced, gedunin-type compounds inducedphysiological toxicity, evident by reduced efficiency of conversion of ingested food (ECI) in feedingexperiments. Salannin and nimbinene, on the contrary, induced concentration-dependent feedingdeterrence only. In feeding experiments, combinations of the compounds revealed that whenazadirachtin was present in a mixture, EC50 values did not deviate from the individual efficacy ofazadirachtin (0.26 and 0.21 ppm, respectively) against H. armigera and S. litura larvae. However, acombination without azadirachtin did show a potentiation effect with potent EC50 values amongstructurally different molecules, i.e., when salannin or nimbinene was combined with 6β-hydroxygedunin or gedunin rather than structurally similar salannin + nimbinene or 6β-hydroxygedunin +gedunin. Obviously, azadirachtin being the most active compound in neem is not synergized orinfluenced by any other limonoid, but other non-azadirachtin limonoids were more potent in specificcombinations vis-à-vis the structural chemistry of the compound. It is obvious from the present studythat potentiation among non-azadirachtin limonoids having explicitly two different modes of action,such as feeding deterrence and physiological toxicity, may be playing a significant role in thepotentiation effect. Keywords: Azadirachta indica; gedunin; 6β-hydroxygedunin; salannin; nimbinene; azadirachtin; growthinhibitors; antifeedants; synergism; Helicoverpa armigera; Spodoptera litura
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