6β-Hydroxygedunin from Azadirachta indica. Its Potentiation Effects with Some Non-azadirachtin Limonoids in Neem against Lepidopteran Larvae

Abstract

The biological activity of 6beta-hydroxygedunin isolated from Azadirachta indica A. Juss. was assessed using 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 diet bioassays, with 24.2 and 21.5 ppm, respectively, inhibiting growth by 50%. This efficacy was higher in comparison to gedunin (EC(50) = 50.8 and 40.4 ppm), salannin (EC(50) = 74.5 and 72.0 ppm), and nimbinene (EC(50) = 391.4 and 404.5 ppm). Azadirachtin, however, remained the most active neem allelochemical against both insect species. Nutritional assays clearly demonstrated that, though relative consumption and growth rates of fourth instar larvae were reduced, gedunin-type compounds induced physiological toxicity, evident by reduced efficiency of conversion of ingested food (ECI) in feeding experiments. Salannin and nimbinene, on the contrary, induced concentration-dependent feeding deterrence only. In feeding experiments, combinations of the compounds revealed that when azadirachtin was present in a mixture, EC(50) values did not deviate from the individual efficacy of azadirachtin (0.26 and 0.21 ppm, respectively) against H. armigera and S. litura larvae. However, a combination without azadirachtin did show a potentiation effect with potent EC(50) values among structurally different molecules, i.e., when salannin or nimbinene was combined with 6beta-hydroxygedunin or gedunin rather than structurally similar salannin + nimbinene or 6beta-hydroxygedunin + gedunin. Obviously, azadirachtin being the most active compound in neem is not synergized or influenced by any other limonoid, but other non-azadirachtin limonoids were more potent in specific combinations vis-à-vis the structural chemistry of the compound. It is obvious from the present study that 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 the potentiation effect.

Limonoids: Overview of Significant Bioactive Triterpenes Distributed in Plants Kingdom

“…3,23,24) Azadirachtin and related limonoids from neem tree have shown strong antifeedant and growth regulating activity and at the same time these compounds are highly bio-degradable, are very slightly toxic towards non-target organisms, while they are non-toxic towards humans and mammals in general, with very low persistence in systemic action, 21,25) this is the reason why an array of insecticides and pesticides comprising of azadirachtin as principal constituent and based on neem extract are available today in the market. Koul et al 26,27) have further shown that azadirachtin being the most active compound in neem is not synergized or influenced by any other limonoid, but other non-azadirachtin limonoids show synergism in specific combination, which may be due to their different modes of action.…”

Bioefficacy of Couroupita guianensis (Aubl) against Helicoverpa armigera (Hub.) (Lepidoptera: Noctuidae) larvae

“…Phytochemicals have shown different kinds of actions that can modify specific physiological processes in insects (Liu et al, 2007). Koul et al (2003) noticed that plant chemicals' defense against insect herbivory almost never depends on a single compound, but instead several compounds interact with pests, individually or in unison. Application of biopesticides has been reported to have positive impacts on bollworm population management (Ge and Ding, 1996).…”

Azadirachta indica

“…Epoxyazadiradione, 17-epiazadiradione, and nimbin were not active. 6β-Hydroxygedunin, isolated from Azadira chta indica , exhibited growth inhibitory activity in feeding bioassays against gram pod borer, Helicoverpa armigera , and Asian armyworm, Spodoptera litura , with EC 50 of 24.2 and 21.5 ppm, respectively (Koul et al 2003 ). Its efficacy was higher in comparison to gedunin (EC 50 = 50.8 and 40.4 ppm), salannin (EC 50 = 74.5 and 72.0 ppm) and nimbinene (EC 50 = 391.4 and 404.5 ppm).…”