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