Allelochemicals from sorghum bicolor that stimulate feeding by the larvae of the stem borer chilo partellus

Abstract

Feeding bioassays with cellulose acetate discs impregnated with the hexane, ethyl acetate and methanol extracts of the leaf-whorls of field grown plants of sorghum cultivars IS 18363 (susceptible) and IS 2205 {LdSistant) showed that the methanol extracts were most stimulatory to the feeding of the third-instar larvae of Chilo partellus. Ethyl acetate extracts were intermediate in stimulatory activity whilst hexane extracts were the least stimulatory. Extracts of the more susceptible cultivar were more stimulatory than those of the more resistant cultivar and those of the whorls of the 3 week old plants were more stimulatory to larvae than those of the 6 week old plants. The phagostimulatory compounds in the ethyl acetate extracts were phenolic, p-hydroxybenzaldehyde and phydroxybenzoic acid being the major components and ferulic and caffeic acids being in minor amounts. p-Coumaric acid was also present in minor amounts but was non-stimulatory at all the doses tested. p-Hydroxybenzaldehyde was a more potent feeding stimulant for the larvae relative to some ofits possible theoretical biogenetic analogues. Limited structure-activity studies with some hydroxybenzoic acids and their corresponding cinnamic acids showed that the former were more stimulatory to the feeding of the larvae than the latter and that oxygen substitu~ion i n the benzene ring was crucial for activity. The phagostimulatory compounds in the methanol extracts were phenolic, identical to those in the ethyl acetate extracts, and sugars. The sugars which were identified in the extracts comprised sucrose, fructose, glucose and xylose. The feeding response of larvae to these sugars followed the order sucrose >> glucose ~fructose; xylose was non-stimulatory. Comparison of the activities of sucrose with mixtures of glucose and fructose showed that the high activity of the disaccharide is due to its total structure and not to a summation of its monosaccharide moieties. Sugars synergised with phenolics to give enhanced feeding response of the third-instar larvae. Chromatographic analyses of the extracts showed that stimulatory and non-stimulatory components in the extracts differed quantitatively rather than qualitatively in the whorls of the two cultivars at the two growth stages. This may have implication in resistance screening and breeding programmes.