The effect of Plant Resistance and Cultural practices on Population Densities of Banana Weevil Cosmopoliates Sordidus (Germer) and on Banana Yield

Abstract

In the present study field and open screen house experiments were conducted to investigate the effect of intercropping and mulching on the population densities of banana weevil, C. sorditus, its developmental responses in selected banana cultivars with varying susceptibility and to determine crop yield loss due to the banana weevil. The tests were carried out at Agricultural Research Institute(ARI) Maruku-in Bukoba, Tanzania during the two growing seasons of 1990 and 1991. A randomized complete block design was used in both the field and screen house experiments. To determine the impact of intercropping and mulching on population densities of C. sordidus; maize, bean and sweet potatoes with their respective mulches replicated four times were used. Nyoya and Nshakara cultivars (cooking types), Kijoge and Bokoboko cultivars (multipurpose types) were used in the developmental responses. Four replicates were used, the ovipositional responses studies. Nyoya cultivar suckers artificially infested at two different timings and replicated eight times on a virgin land was used for crop loss studies. All the data collected in percentages were transformed into square arcsine ?% before statistical analysis. The results obtained revealed that there was no significant difference (P > 0.05) on population growth of C. sordidus between the treatments. This implied that no effective change was shown by the intercrops or mulches within the experimental period. However, lowest reproduction indices of C. sordidus were recorded in the sweet potato and maize intercrops. Adverse effects by sweet potatoes and maize due to high mineral nutrient competition and shading on the banana plant growth parameters, flowering and maturity and on yield were expressed. This suggested that intercropping such plants in banana fields should be discouraged. Mulches showed higher population growth of C. sordidus despite of best results in plant growth parameters, crop maturity and on yield. However yields and yield attributes showed no significant difference (P > 0.05). With a modified microhabitat, natural enemies of C. sordidus were encouraged and conserved; thus regulated the pest population within economic threshold. High frequency of brown ants mainly Pheidole sp. Camponotus sp. and Dorylus sp. and earwigs Forficula sp. and some tennebroinids were encountered in the mulch treatments as compared to the control which had no soil cover. Significantly (P < 0.05) low Percentage Coefficient of Infestation was observed with sweet potatoes as compared to the rest of the other treatments. Significantly reduced hatchability of C. sordidus eggs was recorded in Kijobe cultivar. Results obtained from this study implied that the latex or the corm tissues of the cultivar had toxic effect on the eggs. Tunneling and feeding was significantly (P < 0.05) higher in Nyoya and in Nshakara cultivars as compared to Kijoge and Bokoboko cultivars. The fabric and compactness in Kijoge and the hardness in Bokoboko significantly reduced larval feeding. This resulted in reduced weights of the larvae feeding on the test cultivars thus suggesting that antibiosis was the mechanism responsible for the resistance found in Kijoge and in Bokoboko cultivars. Reduced larval emergence to pupal growth stage was expressed in the later test cultivars. Ovipositional results showed higher colonization in Nyoya and in Nshakara cultivars compared to Kijoge and Bokoboko cultivars. The trend was consistently the same with settling of the adults, total number of eggs laid in each cultivar and eggs laid per C. sordidus female. These results suggested antixenosis as a mechanism in the colonization of the four cultivars by C. sordidus. Overall results indicated that secondary plant metabolites and some volatiles were involved in the whole activity of C. sordidus colonization and development on the four cultivars. Both biochemical and biophysical factors were involved in identifying Nyoya