Abstract:
In a previous study using three plant species (maize, sorghum and Napier grass), naïve gravid B fusca females significantly preferred sorghum and maize over Napier grass for oviposition a phenomenon, which was partly related to plant physical characteristics such as leaf surface texture and sheath toughness. The purpose of this study was therefore to determine the importance of plant chemical characteristics (volatiles as well as surface compounds) in host selection for oviposition using the same plants species as used in previous works. Differences in the reproductive behaviour (oviposition) between the artificially reared and wild naive B. fusca females in relation to their host plant specificity were compared in laboratory bioassays. Unlike wild insects, artificially-reared insects tends to lose their host plant specificity for oviposition after several generations. These insects accepted oviposition supports totally outside their original host plants such as paper surrogate stem and showed no preference for oviposition on artificial stems baited with stimulatory plant extracts in contrast to the wild population. In addition few laboratory-reared insects exhibited an oriented flight behaviour towards maize plants in wind tunnel conditions in contrast to the wild population. However, the females after being reared artificially conserved the same antennal sensitivity towards host plant volatiles as the wild ones. All these results indicate that laboratory-reared B. fusca insects differ from natural population in the host plant specificity and this limits their representativeness of the species in the wild. Therefore it is important to use wild insects in future related studies involving the influence of plant chemistry in host plant selection for oviposition. The volatiles emitted by maize, sorghum and Napier grass were analysed by GC and identified by GC-MS. The blend of volatiles emitted by these plants were in trace quantities though comparable and comprised mainly the green leaf volatile, (Z)-3-hexenyl acetate and the isoprenoids (Z)-4,8-dimethyl-1,3,7-nonatriene and caryophyllene. Comparatively, larger odor quantities were emitted by Napier grass than sorghum or maize. No correlation was evident between the amount of volatiles emitted and insect orientation to the host plants according to the wind tunnel results obtained in another study. Water-soluble compounds from the surface of maize, sorghum and Napier grass did not stimulate B. fusca oviposition. Gas chromatographic analyses of the water extracts and comparison with retention times of authentic standards indicated the presence of simple sugars, amino acids and organic acids in the extract in similar composition among the plants species analysed, confirming the non-involvement of such compounds in oviposition stimulation of B. fusca. In contrast, chloroform-soluble compounds from the surface of maize and particularly sorghum were stimulatory to oviposition but no stimulation was observed when using Napier grass extracts. Preliminary analyses of these extracts by HPLC-MS revealed significant difference in the number of compounds present in these chloroform extracts. Apart from the common compounds present in all the three extracts, sorghum had three more compounds with two eluting at low and another at a higher acetonitrile concentration. The compound in sorghum extract that eluted at high acetonitrile concentration was tentatively identified as a high molecular weight of intermediate polarity. These results suggest that oviposition stimulation in B. fusca is mostly influenced by chloroform-soluble compounds from the surface with low or medium polarity.