Integrated pest management of the water hyacinth

Abstract

Water hyacinth, Eichhornia crassipes (Martius) Solms-Laubach (Pontederiaceae) is the most damaging water weed in South Africa. Biological control has had varied success and therefore attention has shifted toward integrated management, using insects and herbicides. The objective of this work was to find out how a sub-lethal dose of glyphosate herbicide can be used in conjunction with the Neochetina weevils in the control of water hyacinth in the field. Plants infested with the weevils, Neochetina eichhorniae and N. bruchi, were sprayed with a sub-lethal dose of herbicide (0.8% glyphosate concentration at 140 l/ha spray volume) at two sites, Delta Park and Farm Dam (Johannesburg). Plant parameters (plant biomass, number of leaves, and number of ramets) and insect parameters (reproduction, survival, and feeding) were compared between the sprayed plants and the unsprayed plants. Results showed that some aspects of plant growth (leaf production and biomass accumulation) were reduced, whereas the performance of the weevils was not impaired. Delta Park plants were found to be more susceptible to the herbicide compared to Farm Dam plants. The effect of glyphosate on water hyacinth nutritive quality was also analysed by testing N, C, and P contents of the plant. Generally the N content of the plant decreased resulting in an increased C:N ratio. In conclusion the combination of a sub-lethal dose of glyphosate and the Neochetina weevils is feasible in the field, however may not be an ideal control method for large infestations where radical reduction of water hyacinth mat is required.