458 © 2008 The Authors. Journal compilation © 2008 OEPP/EPPO, Bulletin OEPP/EPPO Bulletin 38, 458–463 Blackwell Publishing Ltd Biological control of water hyacinth – the South African experience 1 J. A. Coetzee and M. P. Hill Department of Zoology and Entomology, Rhodes University, P.O. Box 94, Grahamstown, South Africa; e-mail: julie.coetzee@ru.ac.za, m.hill@ru.ac.za Water hyacinth remains one of the worst aquatic weeds worldwide, and its presence in South Africa since the early twentieth century prompted research into biological control options. The first control agent released in South Africa was the weevil Neochetina eichhorniae in 1974, but the project was terminated three years later, and resumed in 1985. Since then, five arthropod biocontrol agents have been released in South Africa, more than anywhere else in the world, including another weevil, N. bruchi, a moth, Niphograpta albiguttalis, a mite, Orthogalumna terebrantis and a mirid, Eccritotarsus catarinensis. To date, the success of biological control of water hyacinth in South Africa has been variable, and this has been ascribed to, amongst other factors, climatic incompatibility of control agents, and eutrophication of water hyacinth impoundments. Successful control is achieved in warm subtropical sites which are not affected by frost in winter, and in water bodies that are not heavily polluted with nitrates and phosphates. The biocontrol programme is the result of collaborative research with scientists from the USA, Australia and Argentina, and the experience obtained and lessons learned in these regions and South Africa can be used to initiate a biocontrol programme in Europe. Introduction Worldwide, the floating aquatic macrophyte, water hyacinth [Eichhornia crassipes (Mart.) Solms], is considered one of the worst aquatic weeds (Julien et al., 1996). In the absence of its original suite of natural enemies, it quickly becomes invasive, colonising still or slow moving waters that result in thick extensive mats (Edwards & Musil, 1975) which degrade aquatic ecosystems and limit their utilisation. The negative impacts of the dense impenetrable mats include restricted access to water, which affects fisheries and related commercial activities, the functioning of irrigation canals, efficient navigation and transport, hydroelectric programmes and tourism (Navarro & Phiri, 2000). Water hyacinth also alters the livelihoods of any community with high dependence on freshwater waterways for food (either subsistence or commercial), transport and clean water. Ecologically, benthic and littoral diversity is reduced (Masifwa et al., 2001; Toft et al., 2003; Midgley et al., 2006), while increases in populations of vectors of human and animal diseases, such as bilharzia, malaria and cholera, are associated with water hyacinth infestation as these plants interfere with pesticide application (Harley et al., 1996). Furthermore, water hyacinth is able to out- compete indigenous flora through efficient utilisation of available nutrients in the water, and by successfully competing for space and sunlight (Cilliers, 1991), effectively doubling in number in suitable habitat every 11 to 18 days (Edwards & Musil, 1975). Because the environmental, social and economic impacts of water hyacinth infestations are considerable, many attempts have been made to control this weed, using a variety of methods. Generally, the initial control options implemented against water hyacinth are manual removal and mechanical control, which are labour intensive and often expensive, unfortunately this usually only results in temporary control because of the weed’s rapid rate of increase from plants that were not removed. In poverty stricken rural areas, this is often the only method of control available (Julien et al., 1999). Chemical control using herbicides is widely used and has been effective for relatively small infestations, accessible by land, air or boat, as it is quick and temporarily effective, but is expensive and has to be regularly and frequently re-applied (Center et al., 1999). Biological control Biological control (biocontrol) of water hyacinth, using natural enemies from its country of origin, is the most economical and sustainable method of control because control persists with little ongoing cost and usually with no negative environmental impacts (Julien et al., 1996). The aim of biocontrol is to reduce weed populations to manageable levels through a balance between populations of the host plant and its natural enemies which are host specific, depending entirely upon their host plant for survival (DeBach, 1974). Biocontrol is the preferred method of control for large infestations of floating aquatic weeds because complete control of major aquatic weeds can be achieved through biological control alone, although the overall effectiveness is sometimes compromised by environmental factors such as floods and frost, and alternative management practices, such as reduction of nutrient enrichment and herbicidal control. 1 Paper presented at the EPPO/CoE Workshop – How to manage invasive alien plants? The case study of Eichornia crassipes. Mérida, Spain, 2008-06-02/04.