Identification of host kairomones from maize, Zea mays, for the maize weevil, Sitophilus zeamais.

The maize weevil, Sitophilus zeamais, is an economically important pest of stored grains in tropical and subtropical regions of the world. The behavioral responses of adult S. zeamais to volatile organic compounds (VOCs) from maize seeds, Zea mays, were studied to identify semiochemicals used in host location and provide potential tools for managing this pest. VOCs released directly from crude seed extracts, vacuum distilled extracts, hexane and diethyl ether fractions from silica gel chromatography of the vacuum distillates, air entrainment samples, and identified volatile compounds were assayed using a Perspex four-arm olfactometer. Weevils spent significantly more time, and made a higher number of visits, to the region of the olfactometer where Z. mays volatiles were present than in control regions comprising solvent only. When white and yellow Z. mays VOCs were compared in a choice test, the mean time spent in the two olfactometer treatment arms was significantly greater than the mean time spent in the control arms. However, weevils did not show any preference for either of the two treatments, thus demonstrating that both varieties of maize have similar activity. Gas chromatography (GC), coupled gas chromatography-mass spectrometry (GC-MS), GC peak enhancement and electroantennography (EAG) identified hexanal, (E)-2-heptenal, and octanal as biologically active compounds in air entrainment samples and diethyl ether fractions of vacuum distillates. A 3-component synthetic blend of the identified compounds was significantly attractive to both sexes of the weevil. These host kairomones could be deployed in semiochemical based monitoring and management of S. zeamais in the tropics.

Behavioural responses of the maize weevil, Sitophilus zeamais, to host (stored-grain) and non-host plant volatiles.

BACKGROUND: Four-arm olfactometer bioassays were conducted to assess the behavioural responses of the adult maize weevil, Sitophilus zeamais (Motschulsky) (Coleoptera: Curculionidae), to harvested seeds of host plants, i.e. white maize, yellow maize (Zea mays L.) and winter wheat (Triticum aestivum L.) (Poaceae), and non-host plant materials, i.e. alligator pepper, Aframomum melegueta (Rosk) K. Schum (Zingiberaceae), rhizomes of ginger, Zingiber officinale (Roscoe) (Zingiberaceae), and West African black pepper, Piper guineense Thonn and Schum (Piperaceae). Additional bioassays with host plant volatiles were conducted in the presence of three doses of non-host plant materials. RESULTS: Both sexes of the weevil showed strong attraction to maize and wheat seed volatiles, but were significantly repelled (P < 0.001) by odours from A. melegueta, Z. officinale and P. guineense. Furthermore, S. zeamais avoided maize and wheat seeds presented in combination with the non-host plant material at 10% (w/w) and 33% (w/w) levels. CONCLUSIONS: A. melegueta, Z. officinale and P. guineense have the potential for use in the protection of stored grains by resource-poor farmers with local access to these plants.

Repellent activity of alligator pepper, Aframomum melegueta, and ginger, Zingiber officinale, against the maize weevil, Sitophilus zeamais.

The repellent activity of alligator pepper, Aframomum melegueta, and ginger, Zingiber officinale (Zingiberaceae), against the maize weevil, Sitophilus zeamais (Coleoptera: Curculionidae), was investigated in four-way olfactometer bioassays. Results showed that vacuum distilled A. melegueta and Z. officinale extracts were repellent towards adult S. zeamais both in the absence and the presence of maize, Zea mays, grains. Bioassay-guided liquid chromatographic fractionation of the distillates showed that fractions containing oxygenated compounds accounted for the repellent activity. Coupled gas chromatography-mass spectrometry (GC-MS), followed by GC peak enhancement and enantioselective GC using authentic compounds, identified 3 major compounds in the behaviourally active fractions of A. melegueta and Z. officinale to be (S)-2-heptanol, (S)-2-heptyl acetate and (R)-linalool in a ratio of 1:6:3, and 1,8-cineole, neral and geranial in a ratio of 5.48:1:2.13, respectively. The identification of these behaviourally active compounds provides the scientific basis for the observed repellent properties of A. melegueta and Z. officinale, and demonstrates the potential for their use in stored-product protection at the small-scale farmer level in Africa.