Biological control initiatives against water hyacinth in South Africa: constraining factors, success and new courses of action

摘要

The success of biological control initiatives undertaken against water hyacinth in South Africa has been variable, despite the establishment of six natural enemy species (five arthropods and one pathogen) between 1974 and 1996. By contrast, successful biocontrol was achieved in a relatively short time frame (4 years) on Lake Victoria in Uganda and in Papua New Guinea, using only the two insect agents, Neochetina eichhorniae and N. bruchi. The variable results achieved in South Africa have so far been attributed to variable climatic conditions, eutrophication of the aquatic ecosystems and interference from integrated control operations. However, hydrological features, notably the size of the water body, and techniques for establishing agents, may also affect the degree of biocontrol. It is believed that bioc0ntrol is more successful in larger water bodies where wind and wave action increase the mortality of agent-stressed plants. These considerations have prompted several courses of action in South Africa, notably: (i) mass-rearing and re-releases of agents that failed to establish at specific sites; (ii) evaluation of the impact of the combinations of agents already established; (iii) development of management strategies in which biocontrol can be appropriately integrated with existing control operations; and (iv) search for additional agents that are effective under more temperate conditions. The success of these initiatives will ultimately rely on the extent to which water authorities and policy-makers become educated about, and come to accept, the principles of biological control.