Distribution and Relative Importance of Cereal Stem Borers and their Natural Enemies in the Semi-Arid and Cool-Wet Ecozones of the Amhara State of Ethiopia.

Abstract

The distribution and relative importance of lepidopteran and coleopteran stem borers and their natural enemies on maize and sorghum were studied in cereal growing zones of the Amhara State of Ethiopia from 2003 to 2004. Sorghum is the major crop in semi-arid eastern and maize in the cool-wet western zones of the Amhara state. Four administrative zones, 10 districts and 88 localities in the semi-arid ecozone (SAE) and four zones, 19 districts and 71 localities in the cool-wet ecozone (CWE) were chosen for the study. In SAE, the species composition was 91% Chilo partellus (Swinhoe) (Lepidoptera: Crambidae), 8% Busseola fusca (Fuller) (Lepidoptera: Noctuidae) and 1% Sesamia calamistis Hampson (Lepidoptera: Noctuidae). In the CWE, maize and sorghum are grown in different ecozones and thus B. fusca was the dominant species on sorghum, whereas 61% B. fusca and 39% S. calamistis were recorded on maize. Borer density generally increased with crop growth stage. C. partellus parasitism by C. flavipes Cameron (Hymenoptera: Braconidae), which occurred only in SAE, varied among districts ranging from 5% to 39%. In the CWE, unidentified nematodes parasitized medium-sized B. fusca larvae during the wet months. Population of native parasitoids was very low. The coleopteran borer, Rhynchaenus niger (Horn) (Coleoptera: Rhynchophoridae), attacked sorghum plants in both regions. Sorghum yields were negatively related to plant damage variables and positively to larval parasitism and plant growth variables. On maize, plant damage was too low to affect yields. Taylor’s power law indicated aggregated distribution for C. partellus and B. fusca larvae and pupae combined.