Seed dynamics and control of Pistia stratiotes in two aquatic systems in Botswana

Abstract

The fruiting and seed dynamics of the alien invasive aquatic species Pistia stratiotes L. (water lettuce; Araceae) was investigated in the seasonally flooded Selinda Canal and Zibadianja Lake of the Kwando–Linyanti River system and in the perennial Chobe River, Botswana, in 1999–2003. The mean number of 30.3 seeds fruit^–1 in the Selinda Canal is the highest ever recorded. An artificial earthen barrier or dyke was constructed on the Selinda Canal to allow manipulation of water levels downstream. By drying and subsequently reflooding a region of the stream, seed germination was stimulated, followed by the manual removal of water lettuce seedlings before their seeds reached maturity, which resulted in a decline in seed germination in surface sediment samples from 63.5% in 2002 to 31.7% in 2003. Manipulation of flooding, followed by the physical removal of P. stratiotes at regular intervals prior to anthesis and seed maturity, is considered the most viable strategy for arresting further additions to the seed bank, and could lead to its eradication in seasonally flooded areas where the biocontrol weevil Neohydronomus affinis has become locally extinct since its introduction in 1987.