Optimization of Neochetina eichhorniae Warner and Neochetina bruchi Hustache for the management of water hyacinth, Eichhornia crassipes (Mart.) Solms in the Central Rift Valley lakes through the use of different nutrient levels in Ethiopia

Abstract

Water hyacinth, Eichhornia crassipes (Mart.) Solms, is an alien invasive weed in Ethiopia that has been established in freshwater bodies. Eichhornia crassipes negatively affects water transportation, fishing and biodiversity among others. Hence, implementation of eco-friendly management option such as biological control is a mandatory. The weevils, Neochetina eichhorniae Warner and N. bruchi Hustache, are the principal biocontrol agents against E. crassipes in their native ranges and have been introduced to Ethiopia for the management of the weed. The weevils and their host are affected by the nitrate and phosphate contents of the water body. Thus, this study was conducted to investigate the effect of N. bruchi and N. eichhorniae in relation to water nutrient contents in the management of E. crassipes. The experiment was conducted in a randomized complete block design with three replications in a 4x4x3 factorial arrangement. Eight weeks after the treatments’ application, the population of the weevils and E. crassipes growth parameters were recorded. The larval population was significantly (p < 0.05) affected only by the interaction effect of N. bruchi and the nutrient levels. Among the treatments, the highest mean number of larvae was found on the plants given high and medium nutrient levels. There was also a significant interaction (p < 0.05) between the two weevils adult density. The mean number of emerged adult density of the weevils increased with increasing nutrient levels. The nutrients and weevils exhibited a significant interaction effect on E. crassipes leaf scarring and petiole tunnel. Leaf scarring at the highest weevil density and nutrient level was sixfold greater than that at the lower nutrient level and weevil density. The tunnel length of the weed was threefold higher at the highest nutrient level regardless of the weevil density. The growth parameters of E. crassipes were found to be greater at high nutrient level, except for the mean number of flowers. Conversely, the growth parameters showed a non-uniform trend with the increasing density of the weevils. The current result suggests reduction in E. crassipes could be achieved by lowering the water nutrient levels and use of N. bruchi and N. eichhorniae.