Ecology and Biologically Based Management of Cereal Stemborers on Maize and Sorghum in the Amhara State of Ethiopia

Abstract

A series of studies were conducted in an attempt to establish baseline data on the importance and geographic distribution of stemborers and to determine key interactions among a whole range of factors, abiotic - i.e., soil fertility - and biotici. e., host plants, intercrops, planting time, borer species, and beneficial organisms, in the stemborer ecosystem in the Amhara State of Ethiopia. These data are prerequisites for devising environmentally sound sets of stemborer management technologies. Thus, distribution of stemborers and natural enemies, the effect of different levels of fertilization on stemborers and associated yield loss (by exclusion and inclusion of borers by using insecticide), effects of indigenous cropping systems, and the effect of planting time on the borer dynamics were studied from 2003 to 2004 in the coolhumid western and semi-arid eastern regions of the Amhara state. The mixed model and the general linear model procedures of SAS, stepwise regression, correlation, logistic regression, and dispersion analyses were employed in data analysis. Three lepidopterous borer species on maize and sorghum, i.e., Chilo partellus, Busseola fusca, Sesamia calamistis, and on sorghum one coleopterous borer, Rhynchaenus niger, were found. In eastern Amhara, the species composition was 91% C. partellus, 8%B.fusca and 1% S. calamistis. In western Amhara, only B. fusca was recorded on sorghum, whereas on maize 61% was B. fusca and 39% S. calamistis. Borer density generally increased with crop growth stage. C. partellus parasitism by Cotesiaflavipes, which occurred only in eastern Amhara, varied among districts ranging from a mean of 5% to 39%. In western Amhara, unidentified nematodes parasitised medium sized B. fusca larvae during the wet months. Borer damage was generally higher in eastern Amhara. Taylor's power law equations showed aggregated distribution for C. partellus and B.fusca larvae. Response to N was observed in western Amhara. Percentage leaf nitrogen content was positively related to N fertilizer dosage. In general, pest density, parasitism and plant growth and borer damage variables increased with crop growth stage. On sorghum, in western Amhara, increasing levels ofN fertilizer also tended to increase pest density, plant growth and damage variables, and in eastern Amhara, it increased larval parasitism by C. flavipes. Likewise, sorghum yields increased by up to 74% due to fertilization, and although grain losses due to borers across locations were between 19 and 49%, the losses decreased linearly from 49% at no fertilizer level to 36% at the highest N level. In maize, although the trend is similar, there were no discernable trends for pest infestation and yield losses due to the low borer densities. Sorghum yields were positively related to plant growth variables (stem diameter, plant height) and negatively to borer numbers and holes bored. In eastern Amhara, the response to fertilizer input of pest, damage and yield was low due to the higher soil fertility and probably low precipitation. Results of intercropping systems indicated that in western Amhara mustard (Brassica carinata Braun) (Cruciferae) had lower borer density (B.fusca) and damage levels. Borer density was higher at the vegetative than other growth stages. Yield was generally higher on the intercropped plots than in plots with sole maize, and was positively related to plant growth parameters and negatively to stem tunneling. In eastern Amhara, intercrops except sweet potatoes seemed to decrease C. partellus numbers and their damage. Results of planting time indicated that B. fusca density and damage decreased .with delay in planting time. It increased with growth stage until grain filling, but declined at harvest. R. niger borers were also observed. Parasitism of stemborers by nematodes was higher, though not significant on the mid-season planted plots. Grain yields were higher on the mid planted crop; this was partly due to bird attack on earlyplanted plots and lack of sufficient moisture on the late-planted crops. Stepwise regressions showed that yield was related positively to basal diameter, and negatively to borer density, number of holes, internode damage, peduncle damage, and tunneling. Thus, early planting may be practiced in areas of short rainy season, regardless of higher borer damage, while there is ample reason to stagger planting in areas of long rainy-season. However, studies specific to the major agro-ecologies is required for complete recommendation. In summary, the present study showed that the distribution, species compositiori and economic importance of stemborers vary with regions in the Amhara state, calling for specific strategy for future management. In western Amhara, on sorghum on which borer density and damage were high, increasing soil nutrients via fertilizer application leads to an increase in borer attack, but also to improved plant vigor, resulting eventually in a net benefit for the plant and the grain yield; while, mustard intercropping seems to be a dynamic tradition which helped to keep stemborers away from the otherwise would-be stemborer hot spot areas of western Amhara and, which can also be a potential component of stemborer management in areas where it is not practiced. The practical application aspects of the results and further researchable areas of intercropping systems are discussed.