BIOLOGICAL-CONTROL OF WATER LETTUCE IN VARIOUS IMPOUNDMENTS OF ZIMBABWE

An initial colony of 150 Neohydronomus affinis (Hustache) was imported from Australia in September, 1987. Between 1988 and 1991, about 8,000 N. affinis were released in five localities in northern Zimbabwe. Mon itoring of water impoundments revealed that N. affinis caused severe d amage and death of water lettuce plants through feeding scars caused b y adults on growing shoots and mines by larvae on leaves. In less than ten months of initial releases, the populations of N. affinis reached 18 insects per plant and the reduction in water lettuce permitted wat er hyacinth and other macrophytes to fill the vacant niche, particular ly on the Manyame River system. However, in four other localities, whe re only water lettuce was a problem, more than 85% of the water area b ecame free from water lettuce. N. affinis populations decreased from 1 8 to 2 per plant and at the same time water lettuce populations and pl ant size were reduced.