Identification of drought tolerant trap plants (grasses)
Brachiaria and Guinea grass were highly ranked by smallholder farmers as having potential as fodder while vetiver was considered of no fodder value. In replicated two-choice tests, all the three grasses were significantly preferred to sorghum and maize for oviposition by the stemborer moths (Chilo partellus). In replicated multiple-choice tests all plants were accepted for oviposition by C. partellus moths but the grasses were preferred to maize and sorghum. However, survival of C. partellus larvae on Brachiaria was minimal, <20%) confirming the suitability of Bracharia as a trap crop.
In addition, studies showed that oviposition on Brachiaria lead to suppression of key volatile organic compounds, principally (Z)-3-hexenyl acetate altering the profile of the volatile organic compound blend (known as herbivore induced volatiles-HIVs) released and making it attractive to both egg and larval parasitoids. Additionally, Brachiaria exposed to oviposition by C. partellus induced defense responses in the neighbouring ‘smart’ maize (See below), making the maize highly attractive to both egg and larval parasitoids. This secondary response however involves increased emission of the key HIVs, notably (E)-caryophyllene, (E)-4,8-dimethyl-1,3,7-nonatriene, (E)-β-farnesene and (E,E)-4,8,-trimethyl-1,3,7-tridecatetraene. Thus combining Brachiaria, with an appropriate landrace maize provides a greatly enhanced Push-pull strategy.
‘ Smart’ maize for drier agro-ecologies:
Farmers in drier agro-ecologies grow drought tolerant cereals, mainly sorghum and millets. But there are also maize varieties with drought-tolerance traits, and farmers in harsh environments tend to grow their own open-pollinated varieties (OPVs) as these perform better than conventional hybrids under drought conditions. Cereal crops such as maize emit HIPVs upon feeding by stemborer larvae. The project aims to select crop varieties that posses superior defence traits that can effectively recruit natural enemies and improve the overall effectiveness of the adapted Push-pull technology. Studies with landraces from South America (Haiti, Brazil and Cuba) indicate that in contrast to commercial hybrids, which have gone through formal breeding processes, these landraces maintain diverse traits of great importance for crop improvement. In dual-choice olfactometer bioassays the odours from maize landraces were significantly more attractive to the egg (Trichogramma bournieri) and larval parasitoids (Cotesia sesamiae) compared to unexposed plants. Analysis of headspace samples by gas chromatography (GC) and GC coupled to mass spectrometry (GC-MS) revealed marked increases in emission of HIPVs including (E)-caryophyllene, (E)-4,8-dimethyl-1,3,7-nonatriene, (E)-β-farnesene and (E,E)-4,8,-trimethyl-1,3,7-tridecatetraene, in the volatile profile of landrace plants when exposed to C. partellus eggs. More recently, the project has established that smallholder farmers’ varieties in western Kenya have a similar trait where oviposition by C. partellus moths results in increased emission of HIPVs. These findings suggest that these indirect plant defence traits, which may have become lost during crop breeding, could be valuable in new resistance breeding for sustainable agriculture in addition to being used to increase indirect defence against attack by stemborers.