Biological control of Salvinia molesta in some areas of Moremi Game Reserve, Botswana

Abstract

The Curculionid weevil, Cyrtobagous salviniae Calder & Sands, has been established on Salvinia molesta (salvinia) in Botswana. The study determined the intensity of weevil activity on the weed at the margin and the centre of four selected sites on the Khwai system of the Okavango Delta. Random samples of salvinia were collected from each site at monthly intervals in 1998 to extract weevils and to demonstrate the effect of weevil on the weed. The rate at which the weed was controlled at different sites varied with mat and weevil density. The biological control at Paradise Pools was moderate without much increase in the weed biomass per m2, while fluctuations in both weevil and plant populations were noticed in the Khwai streams. Weevil numbers remained low at Dombo for the first nine months of the year, during which time the mat density increased. With the onset of higher temperatures, weevil numbers increased to 155 and 457/kg fresh weight of salvinia at the margin and centre respectively in Dombo Pool in early summer. A significant control in MGR 6 HATAB pool between mat biomass and the weevil number resulted in the disappearance of the weed in three months. This study shows that C. salviniae is an effective biological control agent of S. molesta in semiarid areas.