Study on the biology and some ecological aspects of Chilo partellus (Swinhoe) (Lepidoptera: Crambidae) in relation to its altitudinal expansion

Survey was carried out during 2004 main cropping season to investigate the abundance and distribution of stem borers, with emphasis on Chilo partellus in central and eastern Ethiopia. Three species of stem borers, Chilo partellus (Swinhoe), Busseola fusca (Fuller) and sesamia calamistis (Hampson) were recorded. C. partellus was dominant and widely distributed species recorded in 96% of the surveyed sites. On the other hand, B. fusca was mainly recorded in highland areas while S. calamistis was found at all elevations but in small number. Though studies were not conducted so far on the increasing importance of Chilo partellus, the current survey revealed the expansion of the pest to high elevated areas as high as 2088 masl, where it was not recorded earlier in the country. Observations were also made during the survey on the effect of cropping system, wild host and weediness on per cent infestation, stem borer composition and density. The result showed higher level of infestation on monocrops and on fields with wild hosts than intercrop and wild host free fields. Significant variation in level of infestation between crop types was observed. Chi square test revealed strong relationship between cropping systems, crop type, presence of absence or wild host and species composition at intermediate and highland. Borer density was highest at low altitude as compared to intermediate and high altitude. Laboratory study was conducted to observe the combined effect of relative humidity and temperature on the biological features of Chilo partellus. Temperature levels tested were 22 deg C, 26 deg C and 30 deg C while the relative humidity were 40%, 60%, and 80%. The result obtained showed variation on potential and realized fecundity, the developmental time of immatures, and adult longevity due to differences in temperature levels, relative humidity and their interaction. Mean duration of C. partellus life cycle was 70.2 days at 22 deg C and 80; relative humidity whereas it took only 26.5 days to complete its life cycle at 30 deg C and 40; relative humidity. Average life span adult C. partellus ranged 6.9-11.1 days at 22 deg C and 2.3-7.2 days at 30 deg C for the different levels of relative humidity tested. Although developmental period of immatures and adult longevity was longest at lowest temperature level, C. partellus successfully completed its life cycle suggesting reason for its expansion to high elevated areas.