Effect of crop sanitation on banana weevil Cosmopolites sordidus (Germar) populations and associated damage

This thesis was submitted to the Wageningen University and Research Centre, the Netherlands for the fulfilment of the award of Doctor of Philosophy (PhD), Production Ecology and Resource Conservation.The banana weevil, Cosmopolites sordidus (Germar) (Coleoptera: Curculionidae) is a serious pest of bananas. However, its ecology is not well elucidated especially in East Africa where plantations are up to 50 years old and are under various management and cropping systems. No single satisfactory control strategy has been found. Detailed information on C. sordidus biology and ecology is needed to explain the population dynamics of this pest in order to develop a comprehensive package for the small-scale farmer to alleviate the pest problem. Cultural control forms the first line of defence in pest control. For instance, removal of crop residues after harvest by chopping is a widely recommended cultural strategy for C. sordidus control. However, the actual effect of these practices on the insect's population dynamics is not clear. This study was aimed at investigating the effect of crop sanitation on population dynamics of C. sordidus and its associated damage. Laboratory and field studies on the biology and ecology of this pest on crop residues were conducted in Uganda. We found that corms are most attractive to the weevils than any other type of crop residues. Oviposition occurred on residues up to 120 days after harvest, but mainly on freshly harvested residues up to 30 days, implying that residues should be left up to a month before destroying them. In the study on survivorship of C. sordidus in crop residues, our results revealed that the pest successfully completes its life cycle within crop residues and emerging adults from different aged residues are equally fit. As crop residues can be a source of C. sordidus infestation to the standing crop, they need to be destroyed. There is a general belief that covering banana stumps after harvest helps reduce weevils in banana farms. However, there was no data available to prove that this practice is useful. Moreover, the real effect of covering stumps remained unknown. Our studies reveal that covering banana stumps after harvest reduces oviposition on them in the wet season, but encourages oviposition in the dry season. Therefore, farmers may cover the stumps in the wet but not in the dry season. Also, our studies suggest that covering all stumps in the wet season may encourage C. sordidus to oviposit on the crop. Therefore, some residues should be left in the inter-mat alleys to attract weevils away from the crop. The residues can then be destroyed after three to four weeks. Removal and chopping crop residues in farmers' fields helped to keep C. sordidus populations and damage lower than when the residues were left to accumulate. In comparison, removal of all residues in young closed banana plots reduced C. sordidus populations but increased damage on growing plants and reduced the levels of natural enemies. The implications of these results on the role of crop sanitation in the integrated management of C. sordidus are discussed