A beetle biocontrol agent of rice-field weeds recognizes its host plants by surface wax long-chain alkanes and free fatty acids

Abstract

The importance of long-chain alkanes and free fatty acids present in leaf surface waxes of two Commelinaceae rice-field weeds, Commelina benghalensis L. and Murdannia nudiflora (L.) Brenan, was evaluated as short-range attractant and oviposition stimulant in the Lema praeusta (Fab.) (Coleoptera: Chrysomelidae). Surface waxes were extracted by dipping leaves in n-hexane for 1 min at 27 ± 1 °C. Thin-layer chromatography, gas chromatography–mass spectrometry, and gas chromatography–flame ionization detection analyses of n-hexane extracts revealed 20 n-alkanes from C14 to C36 and 13 free fatty acids from C12:0 to C22:0. Pentacosane and palmitoleic acid were predominant among n-alkanes and free fatty acids, respectively. Females showed attraction to one leaf equivalent surface wax of both weeds against the control solvent (petroleum ether) in Y-tube olfactometer bioassays. However, the insect could not discriminate between one leaf equivalent surface waxes of two weeds, suggesting that both weeds were equally attractive to females. Among all identified alkanes and fatty acids, females showed attraction towards individual docosane, tricosane, pentacosane and heptacosane, and tridecanoic acid, palmitoleic acid, linoleic acid, and arachidic acid, resembling in amounts as present in one leaf equivalent surface wax of C. benghalensis and M. nudiflora, respectively. A synthetic blend of either docosane, tricosane, pentacosane, and heptacosane, resembling in amounts as present in one leaf equivalent surface wax of C. benghalensis, or tridecanoic acid, palmitoleic acid, linoleic acid, and arachidic acid, resembling in amounts as present in one leaf equivalent surface wax of M. nudiflora, served as short-range attractant and oviposition stimulant in L. praeusta.