Abstract:
Studies were carried out to establish the role and significance of rice stem borers as pests of rice crop in irrigated low land rice ecosystems in Tanzania; and to evaluate the effectiveness of some management options against the pest. A survey by means of structured questionnaire was init ially used to assess the farmer’s perception on rice stem borers and their management practices in three wards of Kilombero district in Morogoro Region, namely, Mkula, Sanje and Signali. This was followed by diagnostic survey to assess the incidences and population dynamics of rice stem borer species and the naturally existing parasitoids in farmers’ fields. Farmers reported several insect pests such as stem borers, white flies, leaf rollers and grasshoppers as common insect pests infesting rice crop of which stem borers was reported as most injurious. Most farmers (78.2%) appliedchemical insecticides susch as Kung’fu, Karate and Amekan against rice insect pests particularly stem-borers. Only a small proportion (18.5%) of farmers used cultural methods which include burning of crop residues, removal and bury of infested straws, and split application of nitrogen ferliizer in combating stem borer problem. Two stem borer species; Chillo sp and Sesamia calamistis were identified to exist in Kilombero valley with Chillo sp being most abundant (79.24 - 92.05%) compared toS. calamistis (7.97 - 20.77%). One parasitoid, Cotesia flavipes of the family Braconidae was found parasitizing the stem borer larvae in the study area. The tested management options against rice stem bore included cultural methods (adjustment of spacing and nitrogen fertilizer), chemical methods (synthetic insecticides) and biological control methods (fungi based biopesticides and botanicals). The efficacy of biological and chemical control methods were tested under controlled environment (laboratory and in screen house) at Sokoine University of Agriculture by artificial infestation using first instar larvae of Chilo partellus. The experiments involved six treatments which included two commercial fungi based biopesticides (Beauveria bassianaand Metarhizium anisopliae), two botanical extracts (Neorautanenia mitis andDerris eliptica), one of the currently recommended synthetic insecticide, Amekan 344EC (Cypermethrin 144g/l + Imidacloprid 200 g/l), and untreated control (distilled water). The use of fungi based biopesticides and botanical extracts led to slightly lower stem borer mortality that ranged from 58% to 78% compared to 94% due to Amekan 344EC (positive control) under laboratory conditions. Results from screen house experiments (when compared with the negative control) indicated a significant (F = 11.94; df = 5, 15; P < 0.001) reduction of C. partellus damage incidences by 60.62% and 70.70% for dead heart and white head respectively when M. anisopliae was used while 61.86% and 51.17% dead hearts and white head, respectively were attained with B. bassiana.Neorautanenia mitisreduced dead heart and white heads by 53.14% and 49.28% respectively whereas 45% and 42.01% reduction in dead heart and white head, respectively were attained when D. elliptica was applied. Comparison of rice yield attained using Amekan 344EC (treated control) to control stem borer suggested percentage reduction of yield losses by 89.31% using M. anisopliae; 80.3% using B. bassiana; 72.42% using N. mitis and 67.77% using D. elliptica. Grain yield of 4.85 t ha-1 and 5.16 t ha-1 recorded in botanicals, 5.73 t ha-1 and 6.39 t ha-1 in fungi based biopesticides treated plots was higher than 2.84 t ha-1 recorded in un-treated (negative) control plots. The tested cultural control methods which involved the use of nitrogen fertilizer and spacing against rice stem borers in field experiments at Signali, Mkula and Sanje wards suggested a significant (P < 0.05) increase in rice stem borer’s damage incidences with increase in nitrogen quantity and decrease in plant population. Nitrogen significantly increased dead hearts by 4.8%, white heads by 2.8% and stem borer larvae density from 0 to 5.6 larvae/m2. Decreasing the planting density significantly increased dead hearts by 4.7%, white heads by 2.7% and stem borer larvae density from 0 to 5.4 larvae/m2. Rice yield increased with increase in nitrogen dosages to attain the optimum level at 80 Kg N ha-1 at plant population of 250 000 beyond which no further increase in yields was recorded. Conclusively, this study revealed two rice stem borer species, Chilosp and Sesamia calamistis as major problem in Kilombero valley. One parasitoid, Cotesia flavipes was found to exist in all study areas, which is an important bio logical control of the two stem borer species, and therefore need to be preserved.The two botanicals, D. elliptica and N. mitis at the rate of 10mil/l have been proved to be effective in the control of stem borers. Therefore efforts should be made to explore standardization and packaging for farmers to get them at easy. The two fungi based biopesticides; M. anisopliae and B. bassiana at the rate of 1ml/l have proved to be effective against stem borer problem. Preliminary of use should be by exploring the locally available biopesticides and of low cost. Application of nitrogen fertilizer at the rate of 80 Kg N ha-1and spacing of 20 cm x 20 cm were optimum for obtaining high rice yield with minimal stem borer infestation, therefore should be encouraged to be used for management of rice stem borers as alternative to synthetic insecticides.