The occurrence and extent of water hyacinth in Rwanda and the selection of appropriate biological control

Mukarugwiro, Jeanne d’Arc
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The notorious weed water hyacinth is an invasive aquatic plant that has been associated with major negative economic and ecological impacts in water systems worldwide including Rwanda, since the weed’s establishment in the country in the 1960s. Despite efforts to control the invasion, water hyacinth continues to overwhelm Rwandan fresh water systems. Therefore, it is crucial to understand the distribution and dynamics of water hyacinth in order to quantify its abundance, to identify factors constraining biological control of the weed, and to select appropriate biocontrol agents. Specifically, the study sought to (1) determine the occurrence of water hyacinth and its distribution in the major water bodies of Rwanda using Landsat 8 imageries, (2) investigate spatial and temporal distribution of water hyacinth using Landsat TM and Landsat 8, (3) Assess water quality indicators in Rwandan water system using Landsat 8 imagaries, (4) investigate the effect of water turbidity on the pupation of water hyacinth biocontrol agent Neochetina weevils and (5) to assess the physiological suitability of two additional biocontrol agents (Cornops aquaticum and Megamelus scutellaris) for control of water hyacinth infestations in Rwanda. Remotely sensed data with Random Forest and Support Vector Machine, advanve machine learning algorithms were used to map water hyacinths and its surrounding landcover types. Remotely sensed imageries were also used to map water quality indicators in Rwandan water systems. Furthermore, filed survey, and laboratory trials were conducted to investigate the feasibility of biological control program in Rwanda. Remotely sensed data revealed that most Rwandan fresh water bodies are invaded by water hyacinth. Four main rivers such as the Mukungwa River, the Akanyaru River, the Nyabarongo River and the Akagera River are confirmed to be the source and distributors of water hyacinth into the wetlands, dams and lakes through which they pass. Coverage of the weed ranges from 3 to 40%, with an average of 17%; this will increase in the absence of sustainable control measures. Biological control, using the natural enemies of the weed, is safe and sustainable. However, it can be limited by water parameters such as temperature, nutrients and turbidity levels. The results showed that warm temperatures and lower eutrophication should not constrain the efficiency of biocontrol of water hyacinth. However, the high turbidity observed in Rwanda water bodies, especially during the rainy season, was found to negatively affect the development, establishment and performance of Neochetina weevils used for biocontrol. Climate matching analysis revealed that among the biocontrol agents tested Cornops aquaticum and Megamelus scutellaris should be able to establish and perform well under climatic conditions. In addition, these two species will not be affected by high turbidity because all stages of their life cycle occur on the emergent parts of the plant (petioles and leaves). Therefore, these two insects are recommended for biocontrol of water hyacinth in Rwanda. The study concludes that remote sensing is a useful tool for mapping and monitoring the spread of water hyacinth. It should be used to evaluate pre- and post-management interventions to control water hyacinth
A thesis submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Doctor of Philosophy, 2020