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. Monitoring of water impoundments revealed that N. affinis caused severe damage and death of water lettuce plants through feeding scars caused by 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 water hyacinth and other macrophytes to fill the vacant niche, particularly on the Manyame River system. However, in four other localities, where only water lettuce was a problem, more than 85% of the water area became free from water lettuce. N. affinis populations decreased from 18 to 2 per plant and at the same time water lettuce populations and plant size were reduced.