Abstract:
The Lepidopteran stemborer Chilo partellus is a key constraint to cereal production in most resource-poor farming systems as it causes damage to the crop, accruing up to 80%. When plants are damaged by herbivorous insects, they produce herbivore-induced plant volatiles (HIPVs) which can act as repellants for these insect pests and as attractants for organisms antagonistic to these pests (parasitic wasps and predators). Plants can also produce signals that warn other plants of impending attack. Thus, HIPVs potentially have a practical role in crop protection by directly deterring further colonization in already infested and neighbouring plants and indirectly by attracting natural enemies of the pests, particularly parasitoids. The study sought to evaluate the effect of herbivore-induced plant volatiles on maize plants exposed to neighbouring Brachiaria brizantha (A.Rich) Stapf (Poaceae) grass oviposited by the spotted stemborer Chilo partellus (Swinhoe) (Lepidoptera: Crambidae). This was done using six maize varieties; four local varieties and two hybrids. Treatments comprised of maize seedlings exposed to B. brizantha with C. partellus eggs versus non-exposed ones and or maize seedlings exposed to B. brizantha without C. partellus eggs. Responses of stemborers and larval parasitoids to HIPVs collected from maize plants exposed to B. brizantha with and without stemborer eggs was determined by olfactometer and oviposition bioassays respectively. Qualitative and quantitative analyses of these HIPVs was determined through gas chromatography (GC) and GC-Mass spectrometry (GC-MS) to identify electrophysiologically active compounds. The results of this study demonstrated that exposing farmers‟ local maize varieties to Brachiaria brizantha with Chilo partellus eggs induced the plants to emit volatile blends that are attractive to the parasitoid Cotessia sesamiae Cameron (Hymenoptera: Braconidae) parasitoid. Also the same volatile blends were repellent to Chilo partellus. On the other hand the hybrids exposed to Brachiaria brizantha with Chilo partellus eggs were not induced. The biologically active chemical compounds responsible for the behaviour of Cotessia sesamiae and Chilo partellus obtained from local maize exposed to B. brizantha with eggs were (E)-4, 8-Dimethyl-1, 3, 7-nonatriene, Decanal, (E)-Caryophyllene, Linalool, linalool (plus Nananal), E-β-fernesene, Methyl salicylate and (3E, 7E)-4, 8, 12-trimethyl-1, 3, 7, 11-tri-decatetraene. Hybrids did not produce any biologically active chemical compounds. Attraction of larval parasitoids implies that natural enemies can be recruited to attack the newly hatched stemborer larvae before they cause damage to the crop. In addition, this grass can be used as a trap crop to ward off Chilo partellus from the maize crop in the field. The results reported in this study are of great significance for the management of Chilo partellus stemborers as they provide insights into possible exploitation of the signalling of defence responses in the smart maize varieties using an equally smart plant. In addition, it also would open up opportunities for a more efficient exploitation of natural enemies as they would be recruited earliest before larvae of the pest hatch from eggs to cause damage to maize plants. It is recommended that resource poor farmers adopt the findings of this research to increase food on the table and cash in the pocket.