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Impact of water lettuce on water quality Plant morphology of water hyacinth

Water hyacinth and water lettuce in the Vaal River Barrage System Biological agent for water hyacinth

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

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