Insecticidal and antifeedant activities of clerodane diterpenoids isolated from the Indian bhant tree, Clerodendron infortunatum, against the cotton bollworm, Helicoverpa armigera.

Gholamreza Abbaszadeh,Suresh Walia,Chitra Srivastava,James Ottea

doi:10.1093/jis/14.1.29

Abstract

The Indian bhant tree, Clerodendron infortunatum L. (Lamialus: Lamiaceae), is a well-known medicinal plant, but little information about its bioefficacy against agricultural pests exists. This scarcity was addressed in the present study, in which dried leaves of C. infortunatum were subjected to extraction with hexane and methanol and then partitioned using different solvents of varying polarity. In a preliminary bioassay, the antifeedant effects of the crude extracts and fractions were tested on a highly polyphagous pest, the cotton bollworm, Helicoverpa armigera Hübner (Lepidoptera: Noctuidae), using the no-choice test method with cabbage leaf discs. The methanol fraction resulted in maximum antifeedant activity. This fraction was further subjected to crystallization and column chromatography in order to isolate the compounds responsible for the activity. Three pure compounds were isolated and identified as clerodin (CL), 15-methoxy-14, 15-dihydroclerodin (MD), and 15-hydroxy-14, 15-dihyroclerodin (HD). The antifeedant activity of these compounds was studied using a choice as well as a no-choice test method with 24 and 48 hr observation periods. Insecticidal activity was measured using the topical application method at 0.5, 1, 1.5, 2, 2.5, and 3% concentrations, and data were recorded 24, 48, and 72 hr after treatment. In the no-choice test conditions, compounds CL and MD showed significantly higher antifeedant activity compared to the key ingredient in many commercial pesticides, azadirachtin, at its highest concentration. Compound HD also showed very good antifeedant activity, which did not differ significantly from that of azadirachtin. In the choice test conditions, all three compounds and azadirachtin showed 100% antifeedant activity at the highest concentration. Antifeedant Index (AI50) values of CL, MD, and HD were 6, 6, and 8 ppm in choice tests, and increased to 8, 9, and 11 ppm in the no-choice tests, respectively. Insecticidal activity of the isolated compounds was not significant compared to the control condition, even at the highest con-concentrations of the compounds. These results suggest that extracts of C. infortunatum have very good antifeedant effects against H. armigera due to the presence of specific compounds. These compounds could be utilized in the development of new biopesticides.

Highlights

Indiscriminate use of synthetic insecticides has led to problems such as the resurgence of primary pests, secondary pest outbreaks, resistance development, insecticide residue, health hazards, environmental contamination, and increased costs of insect control (Roy and Mukhopadhyay 2010)
The purity of the compound was examined via thin layer chromatography (TLC) (Rf values of 0.71, solvent system ratio of ethyl acetate to hexane was 60:40) and high-performance liquid chromatography (HPLC) (RP-18 column and acetonitrile-water as the solvent system with a ratio of 70:30) at a flow rate of 0.75 mL/min that showed an Rt value of 5.683 min
When these pure compounds were subjected to nuclear magnetic resonance spectrometry (NMR) and mass spectrometry (MS) analyses, they exhibited molecular ion peaks at 466 and 452, which corresponded to the molecular mass of the compounds (Figures 4 and 5)

Summary

Introduction

Indiscriminate use of synthetic insecticides has led to problems such as the resurgence of primary pests, secondary pest outbreaks, resistance development, insecticide residue, health hazards, environmental contamination, and increased costs of insect control (Roy and Mukhopadhyay 2010). Plants are the most efficient producers of phytochemicals in the environment, including secondary metabolites that are used by the plant in defense against phytophagous insects (Ahmad 2007) These secondary metabolites include tannins, alkaloids, polyphenols, terpenoids, polyacetylenes, flavonoids, unusual amino acids, sugars, phenylpropanoids, quinines, essential oils, etc., that have a wide range of anti-insect properties, including insecticidal, repellent, antifeedant, and insect growth inhibitory activities (Ahmad 2007; Dhaliwal and Koul 2011). These phytochemicals and plant extracts have been investigated intensively for the past 30 years in an effort to develop alternatives to conventional insecticides (Isman 2006). The goal of research on plant secondary metabolites is to find new environmentfriendly, biodegradable, and biologicallyactive natural products with low mammalian toxicity to avoid the deleterious effects of synthetic chemicals on the environment and nontarget animals (Kubo 1997)

Methods

Results

Conclusion