Patterns of relationship between Banana (Musa spp.) types and the Banana Weevil, Cosmopolites Sordidus (Germar) (Coleoptera: Curculionidae)

Abstract

Banana (Musa spp.) is an important food crop of the humid tropical lowland areas of the world. Banana production is beset by several problems, related to crop husbandry, selection of planting material, soils, diseases, and pests. Although more than 25 borers have been recorded on banana, the banana weevil Cosmopolites sordidus (Germar) (Curculionidae) is regarded as the major contributor to the general loss in plant vigour and yield. However, some banana cultivars have been found to show resistance/tolerance to the weevil's attack. Little is known about the mechanisms that render some cultivars resistant/tolerant. In this study the relationship between the banana plant and the banana weevil were investigated with the view of incorporating the results obtained into the existing IPM programmes on the control or management of the pest. The following experiments were conducted:(I)Banana cultivar characterization (II)Banana weevil orientation/arrest towards banana plant parts (III)Banana weevil arrest towards different cultivars (laboratory and field)(IV)Banana weevil movement between banana cultivars(V)Preference/non-preference of banana weevil for different cultivars, and (VI)Banana rhizome nutrition status. Six banana cultivars representing three genomes (AAA, AAB, and AB), and four traits (cooking sweet or desert, beer, and roasting) and adult weevils, 4-5 days old, were used in all the experiments. The resistant cultivar, soth (AAB), attracted less weevils than susceptible cultivars nakyetengu (AAA) and gonja (AAB), and the moderately resistant cultivar sukalindizi (AB). There was a differential response among the susceptible cultivars with nakyatengu (AAA) and gonja (AAB) being more attractive than mbidde (AAA) and lusumba (AAA). There was no significant difference between the moderately resistant cultivar sukalindizi (AB) and the susceptible cultivars (P<0.05). Significant differences were found in the number of weevils that moved off the infested resistant cultivar soth (AAB) to the susceptible cultivars. Further, males were attracted more to banana than females and gave credence to observations that females dispersed and wonder around more than males, and that males depended on an aggregation pheromone which they produce to attract the females back to the banana for mating and /or oviposition. The dual choice preference/non-preference experiment demonstrated that weevils used cues that are perceivable when the insect is in close contact with the banana plant. The weevils damaged (consumed) more of the susceptible cultivars than resistant check, but were indifferent when made to choose between susceptible cultivars. The method employed in this experiment could be used to evaluate large banana germplasm for banana weevil resistance/tolerance or susceptibility in breeding programmes.There were no significant differences (P<0.05) between the cultivars in as far as their nutritive contents were concerned. There was also no correlation between nutritive content, rhizome weight, and weevil counts. This showed that banana weevil population built-up or colonization of a banana cultivar was not influenced by the nutritive or density status of the rhizome.This study has shown that weevils do not depend on distant perceivable stimuli to select cultivars for colonization, but more on contact stimuli. They also do not depend on the nutritive status of the banana for them to successfully colonize a cultivar. It seems they mostly depend on secondary metabolites (especially feeding stimulants). The study has also shown that preference, antibiosis and tolerance are at play in the banana-banana weevil interaction.