Page 35 - Intra Muros May 2024

Development stage of Megamelus feeding on Biological agent for water hyacinth Plant morphology of water hyacinth
water lettuce water hyacinth

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

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