Distribution and incidence of the oil palm weevil Rhynchophorus Phoenicis (Fabricius, 1801) (Coleoptera: Curculionidae) in selected districts of Uganda

Abstract

The weevil Rhynchophorus (Herbst) (Coleoptera: Curculionidae) is a key pest of oil palm worldwide, but information on its infestation of crops in Uganda is eleven decades old, therefore, outdated. The current study was carried out to establish distribution and incidence of Rhynchophorus spp at three oil palm production and farming systems, that comprised large scale plantations, adaptive trials and small-scale farms in seven major districts of Uganda where the crop is grown. The study was carried out to establish whether there is more than one species of Rhynchophorus in the selected districts. Further, it also intended to establish their distribution in all the selected districts. Lastly, the study investigated effects of seasonality, weather and distance from natural vegetation on incidence of Rhynchophorus spp. Two surveys were conducted in 2018 in Kalangala, Masaka, Mayuge, Bugiri and Mukono, Kibaale and Bundibugyo districts. The first survey was conducted during the dry season, and the second survey was carried out during the wet season. Weevils were manually collected from infested oil palm trees and identified morphologically. Oil palm trees in each study site were scored as infested (1) or not infested (0) with Rhynchophorus species per study site to determine incidence of the pest (proportion of infested trees). Incidence was compared across factor levels using generalized linear modelling. Only one species of Rhynchophorus namely Rhynchophorus phoenicis was detected. Mean incidence of R. phoenicis was higher in the wet (2.3%) than the dry (1%) season. The highest mean incidences of R. phoenicis (15% and 30% of infested oil palm trees in the dry and wet seasons respectively) were recorded in Masaka district; whereas the pest was not detected in Mayuge and Bugiri districts. The incidence of the pest was higher in oil palm fields farther from natural vegetation than those that were nearer. Mean incidence was 1.8% and 1.2% for field farther from and near the natural vegetation respectively in large scale plantations. In small scale plantations, mean incidence was 3.2% and 0% for fields farther from and near the natural vegetation respectively. Mean incidence of R. phoenicis did not correlate with any of the weather variables considered. These findings suggest that natural vegetation, location and season are critical in designing strategies for managing R. phoenicis in Uganda.