Zeyaur R. Khan

Ecological management of cereal stemborers in African smallholder agriculture through behavioural manipulation

1. Africa faces serious challenges in feeding its rapidly growing human population owing to the poor productivity of maize and sorghum, the most important staple crops for millions of smallholder farmers in the continent, with yields being among the lowest in the world.

Responses of Parasitoids to Volatiles Induced by Chilo partellus Oviposition on Teosinte, a Wild Ancestor of Maize

Maize, a genetically diverse crop, is the domesticated descendent of its wild ancestor, teosinte. Recently, we have shown that certain maize landraces possess a valuable indirect defense trait not present in commercial hybrids. Plants of these landraces release herbivore-induced plant volatiles (HIPVs) that attract both egg [Trichogramma bournieri Pintureau & Babault (Hymenoptera: Trichogrammatidae)] and larval [Cotesia sesamiae Cameron (Hymenoptera: Braconidae)] parasitoids in response to stemborer egg deposition. In this study, we tested whether this trait also exists in the germplasm of wild Zea species. Headspace samples were collected from plants exposed to egg deposition by Chilo partellus Swinhoe (Lepidoptera: Crambidae) moths and unexposed control plants. Four-arm olfactometer bioassays with parasitic wasps, T. bournieri and C. sesamiae, indicated that both egg and larval parasitoids preferred HIPVs from plants with eggs in four of the five teosinte species sampled. Headspace samples from oviposited plants released higher amounts of EAG-active compounds such as (E)-4,8-dimethyl-1,3,7-nonatriene. In oviposition choice bioassays, plants without eggs were significantly preferred for subsequent oviposition by moths compared to plants with prior oviposition. These results suggest that this induced indirect defence trait is not limited to landraces but occurs in wild Zea species and appears to be an ancestral trait. Hence, these species possess a valuable trait that could be introgressed into domesticated maize lines to provide indirect defense mechanisms against stemborers.

Behaviour and biology of Chilo partellus on maize landraces

Plants defend themselves against herbivores through activation of both constitutive and induced defences. Previous studies reported that egg deposition on maize landraces induces the release of volatiles that attract parasitoids, but little is known on the effect of these volatiles on subsequent herbivore oviposition. In addition, larval preference and development on these maize landraces is unknown. We evaluated six landraces and one hybrid maize variety for their resistance to Chilo partellus (Swinhoe) (Lepidoptera: Crambidae). Larval orientation, settling, arrest and dispersal, feeding, development, survival, and subsequent oviposition of moths were determined for individuals reared on each of these varieties under laboratory and screen house conditions. For oviposition preference studies, all treatments were initially exposed to egg deposition whereas the control treatments were not. Larval preference was generally higher for hybrid maize, compared to the landraces. Similarly, first‐instar feeding on maize leaves was more intense in hybrid maize than in four of the six landraces. The amount of food consumed and assimilated by third instars over a 24‐h period was, however, not different among the maize varieties. Larval survival was significantly lower in maize landraces (32%) compared to hybrid maize (54%). However, there was no difference in the larval development period between any of the treatments. Two‐choice oviposition assays showed that moths preferred non‐exposed maize landraces for subsequent oviposition, whereas in the hybrid, there was no difference in oviposition preference between exposed and non‐exposed plants. Although the mechanism of larval suppression was beyond the scope of this study, it was inferred that the landraces showed some resistance to C. partellus feeding and that initial egg deposition on these landraces deters further colonisation by the herbivore.

An Indirect Defence Trait Mediated through Egg-Induced Maize Volatiles from Neighbouring Plants

Attack of plants by herbivorous arthropods may result in considerable changes to the plant’s chemical phenotype with respect to emission of herbivore-induced plant volatiles (HIPVs). These HIPVs have been shown to act as repellents to the attacking insects as well as attractants for the insects antagonistic to these herbivores. Plants can also respond to HIPV signals from other plants that warn them of impending attack. Recent investigations have shown that certain maize varieties are able to emit volatiles following stemborer egg deposition. These volatiles attract the herbivore’s parasitoids and directly deter further oviposition. However, it was not known whether these oviposition-induced maize (Zea mays, L.) volatiles can mediate chemical phenotypic changes in neighbouring unattacked maize plants. Therefore, this study sought to investigate the effect of oviposition-induced maize volatiles on intact neighbouring maize plants in ‘Nyamula’, a landrace known to respond to oviposition, and a standard commercial hybrid, HB515, that did not. Headspace volatile samples were collected from maize plants exposed to Chilo partellus (Swinhoe) (Lepidoptera: Crambidae) egg deposition and unoviposited neighbouring plants as well as from control plants kept away from the volatile emitting ones. Behavioural bioassays were carried out in a four-arm olfactometer using egg (Trichogramma bournieri Pintureau & Babault (Hymenoptera: Trichogrammatidae)) and larval (Cotesia sesamiae Cameron (Hymenoptera: Braconidae)) parasitoids. Coupled Gas Chromatography-Mass Spectrometry (GC-MS) was used for volatile analysis. For the ‘Nyamula’ landrace, GC-MS analysis revealed HIPV production not only in the oviposited plants but also in neighbouring plants not exposed to insect eggs. Higher amounts of EAG-active biogenic volatiles such as (E)-4,8-dimethyl-1,3,7-nonatriene were emitted from these plants compared to control plants. Subsequent behavioural assays with female T. bournieri and C. sesamiae parasitic wasps indicated that these parasitoids preferred volatiles from oviposited and neighbouring landrace plants compared to those from the control plants. This effect was absent in the standard commercial hybrid we tested. There was no HIPV induction and no difference in parasitoid attraction in neighbouring and control hybrid maize plants. These results show plant-plant signalling: ‘Nyamula’ maize plants emitting oviposition-induced volatiles attractive to the herbivore’s natural enemies can induce this indirect defence trait in conspecific neighbouring undamaged maize plants. Maize plants growing in a field may thus benefit from this indirect defence through airborne signalling which may enhance the fitness of the volatile-emitting plant by increasing predation pressure on herbivores.

Climate-smart push-pull: a conservation agriculture technology for food security and environmental sustainability in Africa

This chapter describes the push-pull technological innovation developed by the International Centre of Insect Physiology and Ecology (ICIPE) in the UK and partners in East Africa, which addresses smallholder agricultural constraints, food insecurity, and environmental degradation and has the potential to equip farmers with the resilience and adaptability they need to deal with climate change. The push-pull technology fits conservation agriculture (CA) principles of minimum soil disturbance in its minimum soil tillage agronomic management, continuous soil cover with a perennial cover crop and plant residue, as well as a diversified cereal-legume-fodder intercropping strategy. The perennial intercrop provides live mulching, thus improving above-ground and below-ground arthropod abundance, agrobiodiveristy and the food web of natural enemies of stem borers, thus effectively controlling major insect pests of cereals. The field implementation of this technological innovation in Africa is discussed, as well as its various benefits.