ECOLOGY OF THE AFRICAN MAIZE STALK BORER, Busseola fusca(Fuller) (LEPIDOPTERA: NOCTUIDAE) ON MAIZE (Zea maysL.) IN SOUTHERN ETHIOPIA

Abstract

Maize (Zea mays L.) is an important crop in sub-Saharan Africa (SSA) and the developing world. In Ethiopia, smallholderfarmers almost in allregions of the country dominantly produce maize. In sub- Saharan Africa, maize suffers from various biotic and abiotic constraints resulting in considerable yield loss. Among the biotic factors, insect pests particularly fall armyworm and stem borers are currently the most important pests of maize. Stem borers attack maize from seeding up to maturity. The sampling number and unit are important features of any empirical study in which the goal is to make inferences about a population. The number of sampling number and unit could vary with insects being sampled, their distribution patterns purpose of sampling, infestation pattern and economic considerations. Diapausing B. fusca larvae live in maize stem during unfavorable conditions. Environmental factors such as temperature influence the duration of B. fuscalarvae development, intensity and time of pest out break varied between localities. Information on pest incidence, injury and yield loss in the different agro-ecologies will facilitate strategic decisions with regard to selection of management practices. This study provides information on B. fusca, spatial distribution pattern, sampling size, phenology, population density and termination of diapauses larvae, crop injury, and yields losses in maize in southern Ethiopia. xxii During the study periods, B. fusca was present and injurious to maize in all study areas. The spatial distribution pattern of B. fuscawas determined by using four distribution indices namely, Taylor’s power law, Iwao’s mean crowding regression, Lloyd's mean crowding, and index of dispersion. At the mid-whorl stage of maize, the distribution of B. fusca infestation was aggregated and uniform at both silking and maturity stages. At silking as well as mid-whorl stage of maize, B. fuscalarvae exhibited an aggregated distribution pattern but, larvae at maturity stage and pupae in both silking and maturity stages of maize found randomly distributed. To set control options for B. fusca at its economic threshold level in maize farm, required sampling units was determined using four precision levels and Iwao’s patchiness regression. The required sample units to estimate 5 to 30% mean infestation of maize by B. fuscaranged from, 101 - 73, 45 - 32, 25-18 and 16 to 12 in 2015 and 104-76, 42-30, 26-19 and 17 to 12 in 2016, for 10%, 15% , 20% and 25% precision, respectively. For 10% infestation, which is considered as action threshold level for stem borers management on maize, 22 sampling units (660 plants) per hectare at the precision of 20% are required. The phenology of B. fusca was studied at Hawassa and Wolaita Sodo using sowing date and pheromone traps.Three generations of B. fusca per year were observed in southern Ethiopia between May and September. In Hawassa, the first moth flight which emerged from diapause larvae was started at mid-April in 2015 and mid of March, 2016. In Wolaita moth flight started at early May and April in 2015 and 2016, respectively. The potential of crop residues as sources of carry-over populations of the insect was also assessed. Flight period and population density of adult B. fuscafrom diapause state were significantly varied between seasons. Access to water did not enhance pupation during the diapause. Planting of maize at the beginning of April and May xxiii in Hawassa and Wolaita Sodo, respectively could be no longer suitable for oviposition and consumption by second-generation B. larvae. Population density of B. fuscalarvae and pupae were significantly different among years, locations and crop stages. Early and highest infestation of B. fuscaon maize was observed in Gurage followed by Wolaita as compared in Sidama Zone. Higher population density, injury variables and grain yield losses due to B. fuscaon maize were recorded from chemical treated plots as compared with untreated. Tunnel length, hole number, bored internode, larvae and pupae population had a negative effect on grain yield of maize. Increase in altitude positively affected the population density of B. fuscaand the resultant infestation and yield loss. Cultural practice such as crop diversification; disposal of crop residues after maturity can reduce initial establishment of stem borer infestation. Chemical control is one of the components of integrated pest management and should be used only when necessary combined with other management practices. This study provides information on the distribution, biology, pest status and yields losses due to B.fuscaon maize in southern Ethiopia.