BIOLOGICAL-CONTROL OF WATER LETTUCE, PISTIA-STRATIOTES (ARACEAE), IN SOUTH-AFRICA

Water lettuce, Pistia stratiotes L., is a free-floating aquatic weed that is, as yet, of relatively minor importance in South Africa. The availability of agents that had proved successful in other countries prompted the importation of a weevil, Neohydronomus affinis (Hustache), from Brazil via Australia into South Africa for biological control of the weed. The introduction has been successful, particularly on motionless water bodies where rapid and extreme reductions in weed density have been achieved. The weevils have been less successful on fast-flowing rivers where plants with weevil larvae are continually washed downstream, thus diluting the populations of N. affinis, especially during floods. The recently initiated biological control programme against P. stratiotes in South Africa is reviewed.