Understanding agronomic and phytochemical properties of Brachiaria for management of cereal stemborers in East Africa
Grasses in the genus Brachiaria, commonly known as brachiaria are grown as a fodder crop in sub-Saharan Africa, with some genotypes being used in management of the spotted stemborer Chilo partellus (Swinhoe) (Lepidoptera: Crambidae) through a habitat management strategy. Stemborer is a major insect pest of sorghum (Sorghum bicolor L. Moench) and maize (Zea mays L.) in Africa. However, utilization of brachiaria in cereal-livestock based farming systems in the region faces several biotic and abiotic challenges. Increasing drought conditions limit productivity of this grass species as fodder and its value in pest management. Further, spider mite, Oligonychus trichardti Meyer (Acari: Tetranychidae), has recently been reported as a major pest of Brachiaria spp. in the region. The study aimed at evaluation and identification of drought tolerant, spider mite resistant and adaptable brachiaria genotypes. Potential candidates were further tested for their suitability for use as trap plants in management of C. partellusand their roles in tritrophic interactions with the pest’s natural enemies. Morphological and physiological characters of 18 brachiaria genotypes were studied under simulated drought conditions, well-watered (control) plants were watered every 48 h to 100% field capacity while treatments were allocated by suspending watering for 14 and 28 days, representing moderate and severe drought, respectively. Shoot length, leaf length and width (leaf area), number of tillers, leaf relative water content, chlorophyll content, and above ground biomass were studied. Based on the drought stress index (DSI) values for the measured parameters and PCA (Principal Component of Analysis) biplots, Xaraes, Piata, Marandu, CIAT 679, Mulato II, and Mulato displayed tolerance to severe drought conditions. The same genotypes were further tested for resistance to O. trichardti under controlled conditions in a screenhouse while adaptability to different environments and field resistance to mites was evaluated in three locations for two cropping seasons in 2016 and 2017 under farmers’ conditions. The parameters evaluated as indicators of resistance to pest damage included leaf damage, chlorophyll content reduction, plant height, leaf area, number of tillers and shoot biomass. Significant correlations between parameters were only observed between leaf damage and yield (r = -0.50, P < 0.05), and leaf damage and chlorophyll loss (r = 0.84, P < 0.01). The cultivar superiority index (Pi) ranked Xaraes, Piata, ILRI 12991 and ILRI 13810 as reliable genotypes that combined moderate resistance to the mite (Pi ≤ 48.0) and high biomass yield (Pi ≤ 8.0). Seven putative candidates of the studied genotypes were assessed for oviposition preference by C. partellus moths and subsequent larval performance. In two-choice tests with an open-pollinated maize variety (cv. Nyamula), significantly higher numbers of eggs were deposited on brachiaria genotypes Marandu, Piata, and Xaraes than on maize, whereas fewer eggs were recorded on plants of Mulato II, Mulato, and Cayman. There was a significant and negative correlation between the trichome density on plant leaves and C. partellus oviposition preference for the different brachiaria varieties. First instar larvae did not consume leaf tissues of brachiaria plants but consumed those of maize, which also suffered more stem damage than brachiaria plants. No larvae survived on brachiaria plant tissue for longer than five days, whereas 79.2% of the larvae survived on maize. Higher percentages of eggs were laid on previously oviposited plants of Piata and Xaraes varieties (P < 0.05), while non-oviposited plants of Mulato II was significantly (P < 0.05) preferred to previously oviposited plants. Female Cotesia sesamiae Cameron (Hymenoptera: Braconidae) spent significantly more time attending to volatiles from previously oviposited than non-oviposited plants of all varieties except Marandu. This study proposes brachiaria genotypes that could be of value in improvement of cereal livestock-based livestock productivity in sub-Saharan Africa in the current scenarios of increasing aridification and attacks by spider mites. Among the proposed genotypes in each category, Xaraes and Piata combined drought tolerance, spider mite resistance and adaptability to different environments. Furthermore, they are both suitable for use as "dead-end" trap plants to C. partellus while their head space volatiles are attractive to the parasitoid C. sesamiae. Attractiveness of headspace volatiles, both oviposition induced and constitutive, from most of the genotypes highlights the value of these grasses in stemborer management strategies that exploits tritrophic interactions with the pest’s natural enemies.