Water hyacinth (Eichhornia crassipes) (Pontederiaceae)

The weed

Water hyacinth is frequently branded as the world’s worst aquatic weed due to its invasive potential, negative impact on aquatic ecosystems, and the cost it necessitates to control it. Originally a native of the Amazon basin, the weed has spread throughout tropical, subtropical and some warmer temperate regions of the world since the late 1800s. Water hyacinth is typified as an herbaceous, free-floating aquatic plant with erect aerial leaves, lilac flowers and submerged roots. Free-floating individual plants develop short bulbous petioles which are spongy, enabling the plant to float on the water’s surface. Once growth is sufficient to cause crowding of individual plants, these petioles elongate and interweave, forming dense self-supporting mats that can cover the entire surfaces of dams and slow flowing rivers.

Water hyacinth was first recorded in South Africa on the Cape flats in the early 1900s and since then, has spread throughout the country. This extensive distribution, as well as the resilience of the weed, is attributed to the highly eutrophic, or nutrient enriched, state of South Africa’s waters, and has led to the severe degradation of a number or aquatic ecosystems. Negative effects associated with water hyacinth infestation include the suppression of local aquatic biodiversity, the obstruction of river flows which may aggravate flooding and promote siltation, interference with water utilization for activities like recreation or irrigation, and increased rates of evapotranspiration from water storages. Infestation also poses a potential health risk in that the plant has been implicated in the creation of breeding habitats for malaria carrying mosquito larvae as well as other disease vectors such as the bilharzia snail. Due to these effects, coupled with the limited water resources available in South Africa, E. crassipes has been declared a category 1b weed in terms of the National Environmental Management: Biodiversity Act (10/2004): Alien and Invasive Species Regulations, 2014, which necessitates its control or eradication where possible.

Water hyacinth infestation

A single plant

Water hyacinth mat in bloom

Control

Combating water hyacinth infestations has drawn upon various management techniques designed to reduce both the weed's spread and biomass. These include physical removal, the application of herbicides, utilisation for commercial and subsistence purposes, and the importation and release of biocontrol agents. In most cases however, the use of any of these techniques in isolation has had limited success, and in terms of physical or chemical intervention, proven both costly and unsustainable in the long-term. This is especially true in South Africa where management of the weed has been complicated by the eutrophic state of the country's waters, and in terms of biological control, the highly seasonal and predominantly temperate climate. Whilst biological control remains the most cost-effective and environmentally friendly technique for the sustainable control of water hyacinth, recent research has sought how best to integrate it with other management strategies in order to achieve the best results.

The biological control of E. crassipes in South Africa currently relies on six established agents (information on these species is available to download below under ‘More information’), and two newly released insects, the planthopper Megamelus scutellaris, and the grasshopper Cornops aquaticum. Years of research on water hyacinth and its management has shown that, in addition to developing effective integrated management, additional biocontrol agents that have the potential to be damaging to the plant in eutrophic environments, and to complement the existing biocontrol agents, are needed to better control the weed. This prompted consideration of C. aquaticum, and the grasshopper was first introduced into quarantine in South Africa for screening in 1995.

Both the adults and juvenile stages of C. aquaticum are extremely damaging to water hyacinth, causing significant reductions in both growth rates and the reproductive potential of the weed. Research by the Agricultural Research Council’s Plant Protection Research Institute has shown the grasshopper to be damaging to water hyacinth growing in eutrophic environments where increased plant quality was found to positively affect both survival and reproduction of the insect. Other studies, which investigated interactions between C. aquaticum and two of the already established biocontrol agents, also found a positive interaction between the grasshopper and the weevil Neochetina eichhorniae, the most effective and widely distributed of the current biocontrol agents in South Africa. In combination, C. aquaticum and N. eichhorniae had the greatest impact on the growth and productivity of the plant, higher than that of other combinations of agents, or of agents in isolation. Cornops aquaticum was first released in January 2011 but despite the presence of perceivably favourable conditions and suitable habitat, the grasshopper has failed to establish at any of the release localities to date. A recent renewal of funding from the Working for Water Programme (Department of Environmental Affairs: Natural Resource Management Programmes), together with collaboration with the South African Sugarcane Research Institute, is allowing for the development of a tailor made release strategy for the grasshopper. Current research is therefore focusing on identifying possible factors that could be hampering the establishment of the grasshopper in order to develop an effective release methodology which ultimately leads to its successful and widespread establishment.