Kariba Weed management on the surface of the Garden Route Dam is going well and remains ongoing.
George Municipality Superintendent Parks and Recreation Priscilla Burgoyne said chemical and biological treatment since February was very successful. “The weed at the Kat River continues to die off. This takes time, so the visual presence of dying weed gives the incorrect impression that it is not under control. We are satisfied with the control of this section for now.
“Our integrated Kariba Weed management plan should be completed this month. The plan is currently being developed with the help of aquatic specialist Dr Jackie Dabrowski and will comprise biological control, manual removal and spraying when necessary. We will be connecting with other departments in the hope of securing funding and assistance for measures including the installation of a floating boom at the Kat River entrance into the dam to further curtail spread.
“There are plans for distribution of another set of weevils this month at the Swart River entrance to the dam where an increase of live plants has been spotted. The weevils had proven highly successful where they had been introduced in the Kat River earlier this year and we continue to work with the researchers from the Aquatic Weeds Programme at the Rhodes University Centre for Biological Control (CBC) in this regard. A site has been identified at the Garden Route Botanical Gardens for the establishment of a weevil biocontrol station, funded by the CBC, where students will be keeping weevils for the purpose of ongoing treatment of Kariba Weed in the Garden Route dam where necessary. Work on the construction of the biocontrol station will start next month (May 2021).
The municipality will pursue as many controlling measures as possible and takes into consideration contributing factors such as inevitable contamination from nearby built-up areas and human habitation. “We are hopeful that general pollution and associated phosphate levels will remain minimal and Kariba Weed will decrease to negligible pockets that are easily managed,” said Ms Burgoyne.
The municipality reported in March that weevils of the species Cyrtobagous salviniae were released mid-February 2021 at the Kat River inlet into the dam and near the dam wall where most of the Kariba weed occurred. Upon follow-up investigation, studies confirmed a 91% infection uptake. Since the weevils eat the plant, a significant reduction in the amount of dead plant material is expected once the weevils have established. During a visit to the infection sites at the time, the CBC indicated that the infestation was not a concern as the surface coverage was proportionally small compared to overall size of the dam and could be effectively addressed with a few interventions.
“The municipality reiterates that this matter is well in hand, is being addressed by learned experts in their fields, and that there is no current threat of oxygen depletion or eutrophication as had been speculated,” said Ms Burgoyne.