Page 37 - Waterfall City Issue 5 May 2024

up mechanical removal will then be
conducted to ensure that these do
not further add to the nutrient load of
the system, as they submerge in the
water post-spraying. In the longer-
Impact of water lettuce on water quality Plant morphology of water hyacinth term, biocontrol agents will also be
released to aid in the control of these
large mats, particularly in tributaries
and slow-moving parts of the river,
creating a population of weevils that
will then actively control the VRBR,
biologically.

Quick facts about the VRBR &
IAP infestation
Water hyacinth and water lettuce in the Biological agent for water hyacinth 1. The area under management for
Vaal River Barrage System
IAPs is approximately 930 hectares
How can IAPs be controlled? • Biological in extent, with numerous reed beds
There are four methods that can Biological control refers to the use of and other smaller channels.
be used to control and/or avert host-specific natural enemies of the 2. Poor water quality from numerous
infestations of IAPs. These are IAPs to reduce the population(s) to upstream pollutants and overflows
mechanical, chemical, biological and an acceptable/aesthetically pleasing from sewers have contributed to the
an integrated control. level. Host-specific biocontrol agents reduced water quality, acting as a
do not interfere with the indigenous feeder for these IAPs.
• Mechanical flora and fauna, as these feed 3. Rand Water, in collaboration with
Mechanical control refers to the use of predominantly on the IAPs, where industry-leading scientists from the
machinery to physically remove IAPs the disappearance of the said IAPs Centre for Biological Control (CBC),
from a system. For example, in an aquatic would entail the disappearance of the uses host-specific biocontrol agents
environment, IAPs such as Pistia stratiotes biocontrol agent. An example of this that have been approved by the
and Eichhornia crassipes are controlled by is the weevils that have been released Department of Forestry, Fisheries
raking, mowing or hauling. Disturbance at the Vaal River Barrage Reservoir to and Environment; these will not alter
stemming from the physical removal can counteract the exponential growth or disturb the natural/indigenous
cause the seed bank to proliferate, and it of water lettuce via herbivory. ecosystems at the VRBR.
is therefore essential to have continuous The weevils damage the plant 4. Rand Water’s water supply will not be
follow-up action in order to improve material, inhibiting further growth impacted by the integrated control
efficiency. The disadvantages of this and reproduction of the plant, and being implemented at the VRBR
method, when used solely, is that the rate reducing the population therein. because:
of growth of the invasive plants usually • Rand Water does not extract water
exceeds the control efforts, resulting in • Integrated for purification and subsequent
inefficiency in control. It is also labour An integrated control method, as dispersion from the VRBR.
intensive. employed by Rand Water at the VRBR, • The water that is purified and
encompasses a combination of the dispersed by Rand Water undergoes
• Chemical three methods as a strategy to control a stringent testing process that is
Chemical control refers to the and reduce the plant populations in ascribed to SANS 241; it cannot cause
application of registered and approved the system. In essence, mechanical cancer as a result of the herbicides
herbicides directly to the IAPs, with the removal will be employed to remove used at the VRBR.
aim of suppressing and subsequently dense mats of these weeds in the mid • Scientists and studies from credible
killing them. For the process to be most to lower stream of the river, whilst institutions reiterate that following
effective, the choice of herbicides, upstream areas, covered by dense an integrated control method is best
correct application method, dosage, mats will be sprayed with registered to reduce IAP populations and limit
time of application and follow-up and approved herbicides (by the future infestations in the long-term.
actions are important. Improper use relevant departments), following a
can result in the persistence of these stringent method which aims to limit Information supplied by K. English, Rhodes
University Biocontrol Unit and compiled by L.
herbicides in the water, leading to the contact of the herbicide to the Mhlongo, Rand Water. Photos supplied by Rand
further ecosystem degradation. plants, and not the water. Follow- Water.

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