Page 21 - IFV Issue 4_ 2024

Estate News

do not further add to the nutrient load
of the system, as they submerge in the
water post-spraying. In the longer-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
Impact of water lettuce on water quality Plant morphology of water hyacinth
VRBR, biologically.

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

The Villager • Issue 4 2024 • 19