An aerial view of a 7ha mat of hydrilla on the Pongolapoort Dam.
clockwise from top right: Part of the team battling hydrilla in the dam.
Photos: Lloyd Phillips
Farmer’s Weekly has previously highlighted the potential of northern KwaZulu-Natal’s massive Pongolapoort Dam and its immediate surrounds to become the SA equivalent of the world-renowned Kariba Dam system in terms of conservation and ecotourism. However, this jewel of the country is now facing possibly its greatest threat ever due to the discovery there of what has been dubbed “the perfect aquatic weed” – Hydrilla verticillata (hydrilla).
SA’s fight against invasive alien plants (IAPs) has, since 1995, been spearheaded by the Working for Water (WfW) programme. This programme is administered through the Department of Water Affairs and Forestry (DWAF), and works in partnership with local communities, to whom it provides jobs. It also works with government departments such as Environmental Affairs and Tourism, Agriculture, and Trade and Industry respectively, provincial departments of agriculture, conservation and environment, research foundations and private companies.
According to WfW national director Mandisa Mangqalasa, her programme has cleared more than 1 million hectares of IAPs since its inception. Jobs and training have been provided to approximately 20 000 people in that time, and there are currently 300 IAP control projects across SA.
“Prior to 2002, there were no known cases of hydrilla in any of South Africa’s freshwater systems. However, there were reports of this aquatic weed being found in ornamental dams on a few private properties,” explains Prudence Majozi, a biological control officer for WfW in the Zululand region. ”Then last year, a routine surveillance of Pongolapoort Dam revealed large pockets of hydrilla that had previously been overlooked and misidentified. There was immediate concern by the WfW programme, and concerted efforts have now been implemented to deal with this massive threat.”
A WfW report describes hydrilla as a submerged aquatic plant native to Asia. Ecological adaptability enables this species to become a competitive coloniser in freshwater habitats. The report adds that hydrilla’s physiological characteristics and reproductive abilities make it an aggressive invader with the potential to cause substantial economic hardships, interfere with various water uses, outcompete indigenous aquatic species, and adversely impact freshwater systems.
Hydrilla grows submerged in depths up to 12 metres, and is generally rooted to the bottom. However, fragments can break off and survive in free-floating mats. It can also reproduce through hybernacula, which are the dormant buds in leaf axils and the plant’s tubers. This species’ midrib is often red.
Prudence continues, “The plant grows under a wide range of conditions. It has the ability to colonise deeper water due to its low light requirements. Hydrilla adversely affects water bodies by forming dense canopies, providing poor habitats for fish populations, decreasing water quality, interfering with recreational activities and impacting power generation, irrigation and water delivery.”
The species is already causing problems with recreational activities on Pongolapoort Dam. The dam is well known as one of the very few water bodies in SA that offer much sought after tiger-fishing opportunities. Debbie Sharp, KZN’s WfW biocontrol officer, says there have been numerous complaints from boaters saying their propellers are being fouled by hydrilla, and that its spread on Pongolapoort is threatening prime tiger-fishing spots.
“After the presence of hydrilla was confirmed by WfW on Pongolapoort Dam last year, we were able to use previous records to determine that there was only a one hectare area of the weed on the dam in 2002. Aerial surveys in 2006 show that hydrilla has since spread rapidly to now cover 680ha in a mere four years. Unless this weed is controlled in the near future, we estimate that it has the potential to cover as much as 4 800ha of Pongolapoort Dam’s area,” explains Debbie.
Dr Julie Coetzee is the manager of the Aquatic Weed Programme of the provincial Department of Agriculture and Environmental Affairs. She says that in the US, three hydrilla control methods have been implemented with varying degrees of success. The most widespread method was chemical control, using the herbicide fluridone. At low concentrations of fluridone, the American hydrilla was selectively treated at relatively low cost. However, recent research showed that populations of hydrilla there became resistant to low fluridone concentrations.
Julie says that hydrilla can still be controlled using higher, sustained doses of fluridone, but these doses then impact on non-target native aquatic plants, and the cost of control becomes much greater.
She adds, “Despite its apparent success, herbicidal control provides only short-term relief, and must be regularly reapplied. The use of fluridone in South Africa is currently not an option because it has not been registered against the weed, but more importantly, because of the potential non-target impacts to our indigenous fauna and flora.”
Manual and mechanical removal of hydrilla can provide immediate, but temporary, relief. However, the use of harvesting as a primary control tool is constrained by the cost of control, limited capacity to address large-scale infestations, and more importantly, to control the rapid regrowth of this weed. Julie says that manual control might be an option in SA where hydrilla is present at low densities; however, it will be nearly impossible to remove the reproductive turions and tubers from the soil, and any hydrilla fragment left behind will be capable of regenerating the infestation.
The most potentially effective, yet long-term, hydrilla control method would be through biological control. This method offers economical and sustainable control of the weed, yet as things stand, availability of a suitable biocontrol agent for hydrilla in SA is still at least two years away.
International research into biocontrol of hydrilla has revealed several potential agents – two leaf-mining fly species, Hydrellia pakistanae and H balciunasi, and two weevil species, Bagous affinis and B hydrillae. The leaf-mining flies have shown positive results for hydrilla control in the US, while the populations of released weevils there have seemingly failed to establish themselves.
“Research into the suitability of the already established biocontrol agents from the US as potential agents in South Africa will be conducted as soon as we get an imported culture of the flies,” Julie says. “We are in a very fortunate position in terms of implementing a hydrilla biocontrol programme in South Africa because nearly three decades of research has been conducted on all aspects of the Hydrellia flies in the US, from their biology through to what makes them successful biocontrol agents.”
Meanwhile, the WfW programme, members of provincial conservation organisation Ezemvelo KZN Wildlife, and private landowners around Pongolapoort Dam are doing their best to ensure that hydrilla does not spread to any other freshwater sources in SA. However, with waterbirds able to travel long distances between water bodies, there is a chance that a strand of hydrilla could attach itself to a bird’s leg in Pongolapoort, only to fall into the water at the bird’s next stopover. Pongolapoort’s stakeholders have set up high-pressure watercraft washing facilities at all exits from the dam to try to minimise the chances of a piece of hydrilla being spread by watercraft.
“Hydrilla is really tough,” Debbie points out. “Take a strand of it and leave it to dry out, and then put it back into freshwater and it will start growing again within hours. People must be aware of the threat posed by this weed, and how easily they could unknowingly spread it from Pongolapoort. Fortunately, the dam users are all on our side because they realise that they could lose valuable fishing and recreational spots if they don’t take care.”