hydrosoil following clearing, therefore requiring repeated applications. Integrated
control, combining biological control with limited herbicide applications can reduce
plant coverage and collateral damage to native vegetation (e.g. Jadhav et al. 2008).
Herbicidal control is not recommended for the floating species under effective
or complete biological control (i.e., P. stratiotes, S. molesta, M. aquaticum and
A. filiculoides). Newly-identified Category 1a aquatic invaders (see Box 1.1 in van
Wilgen et al. 2020, Chap. 1, for a definition of categories), such as I. pseudacorus
and S. platyphylla, are targeted for eradication by the South African National
Biodiversity Institute’s Biological Invasions Directorate (SANBI’s BID), and these
species require both mechanical and herbicidal control. Herbicides are registered for
use against some of these new invaders, but should be seen as short-term solutions
because their distribution has developed beyond the lag phase of invasion, and
eradication is no longer possible.
Large populations of floating macrophytes can be controlled effectively through
biological control, which is both economically and environmentally sustainable
(Hill et al. 2020). Floating macrophytes are particularly susceptible to biological
control with a number of successful cases throughout the world, and in South Africa.
For example, P. stratiotes, S. molesta, M. aquaticum and A. filiculoides have all been
brought under complete biological control by a single agent in as little as 2 years, to a
point where they no longer threaten aquatic ecosystems (Hill 2003). In contrast,
biological control of Water Hyacinth has been variable, depending on water nutrient
quality, cold winter temperatures and interference from herbicide operations
(Coetzee et al. 2011a). In systems such as New Year’s Dam near Alicedale in the
Eastern Cape, where the water is oligotrophic, the biological control of Water
Hyacinth has been highly successful (Hill and Coetzee 2017). Ultimately, the
long-term success of floating macrophyte control requires the integration of a variety
of methods, with the most emphasis on reducing nitrate and phosphate pollution into
aquatic environments (Hill 2003).
Utilisation of the excessive biomass of floating aquatic plant invasions, particularly in poorer rural areas, is often encouraged as a management option, where local
communities are perceived to benefit from their use (Coetzee et al. 2009). Unfortunately, this is rarely effective due to the effort required to remove significant amounts
of high water content biomass, and may even promote their spread. Water Hyacinth,
for example, is nearly 95% water, and to gain 1 tons of dry material, 9 tons of fresh
material is required, decreasing the commercial viability of such harvesting operations (Julien et al. 1999).
While South Africa has decades of experience in controlling floating aquatic
plants, the initiation of biological control programmes against new aquatic invaders
is in its early stages. The most recent release of an aquatic plant biological control
agent was made in early October, 2018, when a leaf-mining fly, Hydrellia egeriae
Rodrigues (Diptera: Ephydridae), was released on the Nahoon River, East London,
Eastern Cape, for the control of the submerged Brazilian Waterweed, E. densa
(Box 4.2).
4 Invasive Alien Aquatic Plants in South African Freshwater Ecosystems
107
control, combining biological control with limited herbicide applications can reduce
plant coverage and collateral damage to native vegetation (e.g. Jadhav et al. 2008).
Herbicidal control is not recommended for the floating species under effective
or complete biological control (i.e., P. stratiotes, S. molesta, M. aquaticum and
A. filiculoides). Newly-identified Category 1a aquatic invaders (see Box 1.1 in van
Wilgen et al. 2020, Chap. 1, for a definition of categories), such as I. pseudacorus
and S. platyphylla, are targeted for eradication by the South African National
Biodiversity Institute’s Biological Invasions Directorate (SANBI’s BID), and these
species require both mechanical and herbicidal control. Herbicides are registered for
use against some of these new invaders, but should be seen as short-term solutions
because their distribution has developed beyond the lag phase of invasion, and
eradication is no longer possible.
Large populations of floating macrophytes can be controlled effectively through
biological control, which is both economically and environmentally sustainable
(Hill et al. 2020). Floating macrophytes are particularly susceptible to biological
control with a number of successful cases throughout the world, and in South Africa.
For example, P. stratiotes, S. molesta, M. aquaticum and A. filiculoides have all been
brought under complete biological control by a single agent in as little as 2 years, to a
point where they no longer threaten aquatic ecosystems (Hill 2003). In contrast,
biological control of Water Hyacinth has been variable, depending on water nutrient
quality, cold winter temperatures and interference from herbicide operations
(Coetzee et al. 2011a). In systems such as New Year’s Dam near Alicedale in the
Eastern Cape, where the water is oligotrophic, the biological control of Water
Hyacinth has been highly successful (Hill and Coetzee 2017). Ultimately, the
long-term success of floating macrophyte control requires the integration of a variety
of methods, with the most emphasis on reducing nitrate and phosphate pollution into
aquatic environments (Hill 2003).
Utilisation of the excessive biomass of floating aquatic plant invasions, particularly in poorer rural areas, is often encouraged as a management option, where local
communities are perceived to benefit from their use (Coetzee et al. 2009). Unfortunately, this is rarely effective due to the effort required to remove significant amounts
of high water content biomass, and may even promote their spread. Water Hyacinth,
for example, is nearly 95% water, and to gain 1 tons of dry material, 9 tons of fresh
material is required, decreasing the commercial viability of such harvesting operations (Julien et al. 1999).
While South Africa has decades of experience in controlling floating aquatic
plants, the initiation of biological control programmes against new aquatic invaders
is in its early stages. The most recent release of an aquatic plant biological control
agent was made in early October, 2018, when a leaf-mining fly, Hydrellia egeriae
Rodrigues (Diptera: Ephydridae), was released on the Nahoon River, East London,
Eastern Cape, for the control of the submerged Brazilian Waterweed, E. densa
(Box 4.2).
4 Invasive Alien Aquatic Plants in South African Freshwater Ecosystems
107