Chemical ecology of plant-to-plant communication and opportunities for maize stemborer management in Africa

Abstract

Maize is the most widely grown cereal crop worldwide, and is the most important staple crop in sub-Sahara Africa. The spotted stemborer, Chilo partellus (Swinhoe) (Lepidoptera: Crambidae) is considered among the most important pests of maize and the damage it causes may result in yield losses of up to 88%. Previous studies showed that plants damaged by herbivores release huge amounts of volatile compounds, known as herbivore-induced plant volatiles (HIPVs) into the environment, which serve as attractant to natural enemies and repellent to herbivores. In addition, emitted HlPVs affect the defence responses of neighbouring plants. Previous studies reported that the non-host molasses grass, Melinis minutiflora P. Beauv. repels C. partellus moths and increases larval parasitism by Cotesia sesamiae Cameron (Hymenoptera: Braconidae) when intercropped with maize. However, the potential role of plant signalling between molasses grass and maize, and any subsequent effect on C. partellus and C. sesamiae behaviour remained unknown. Moreover, it was not known for how long the maize plant retained the information after removal from exposure to the emitter plant. Experiments were conducted by exposing plants of two maize landraces, "Nyamula" and "Jowi-red", and two hybrid maize varieties "WS505" and "PH4" to molasses grass volatiles for certain periods of time. In two-choice oviposition bioassays, gravid C. partellus moths preferred non-exposed maize landraces for oviposition compared to those exposed to molasses grass volatiles. Additionally, volatile samples collected from landrace maize plants were significantly more attractive to C. sesamiae compared to non-exposed maize plants in four-arm olfactometer bioassays. Similarly, maize plants previously exposed to molasses grass and removed for certain periods of time then infested by C. partellus larvae were not preferred for ovipostion by C. partellus moths, and headspace samples collected were more attractive to C. sesamiae comparared to non-exposed infested plants. Headspace samples were analysed using Gas Chromatography (GC), Coupled Gas Chromatography-Mass Spectrometry (GC-MS) and Gas Chromatography-Electroantennography (GC-EAG). GC-EAG analysis with attractive headspace samples from exposed maize landraces revealed that C. sesamiae was responsive to certain compounds, namely, myrcene, (Z)-3-hexen-1-ol acetate, (E)-ocimene, (ᵝ)-ocimene, (R)-linanool, (E)-4,8-dimethyl-l ,3, 7-nonatriene (DMNT), decanal, (E)-caryophyllene and (E,E)-4,8,12-trimethyl-1,3,7,11-tridecatetraene (TMTT). Notably, with the commercial maize hybrids, there was no significant difference in the number of eggs laid by C. partellus moths on exposed and non-exposed plants. Similarly, there was no significant difference in C. sesamiae attraction towards volatiles obtained from exposed and non exposed hybrid maize plants. These findings suggest that volatile organic compounds released by molasses grass have the ability to induce defence responses in neighbouring maize landraces, a trait that the commercial hybrid varieties seem to lack, and demonstrate the potential of plant signalling as a component of management approaches for stemborer pests in subsistence farming in Africa