A Kruger National Park visitor and Kruger Park Times reader, Lorraine Hofreiter, sent us an image taken on March 13, 2008 at the Eileen Orpen Dam in the Kruger National Park (KNP). Dr Llewellyn Foxcroft, SANParks' scientist, invasion ecology and editor-in-chief of the Sanparks journal, Koedoe- African Protected Area Conservation and Science, identified the plants in the photo as water lettuce. He shed some more light on theproblem in Kruger.
The plants seen on Orpen Dam are water lettuce (Pistia stratiotes). Unfortunately this plant has long been in the KNP (more than 20 years), and has also been a problem on the Sabie River and Sunset dam, as well as in other areas. It is alien to Africa and a declared weed in South Africa. The plant, originally from South America, is now found in tropical and subtropical areas throughout the world. Its spread was probably due to the popularity of this weed as a plant in water gardens and aquaria. It now occurs at scattered localities throughout South Africa and unfortunately also occurs in Kruger.
Chemical control of water lettuce plants in the KNP was initially exercised through the application of herbicides on the Sabie River from Skukuza to the high level bridge about 15km downstream of Skukuza. The weed was initially considered "under control" for three years, but later had to be sprayed again and thereafter annually. These efforts were extremely costly and labour intensive, with the added danger of crocodiles and hippopotami in the dams and rivers.
Over time, the cost of herbicidal and labour intensive operations increased dramatically and financial limitations started affecting the Park's ability to control the weed. It also became evident that it was not possible to eradicate water lettuce completely and that the Park would always have the plant in the eco-system. Long-term, viable cost-effective solutions were therefore sought, placing more emphasis on the role of biological control.
In 1986 it was proved on Nhlangaluwe and Dakamila pans, in the Pafuri region of the KNP, that a host-specific snout weevil, Neohydronomus affinis, was able to control water lettuce. The weevil, which is specific to water lettuce, lays its eggs on the edge of the water lettuce leaves, and on hatching, the larvae bore into and feed on the internal spongy tissue of the leaves. The larvae pupate on the plant and after emergence the adults feed on the leaves causing small round holes. During the summer months the total life cycle takes 25-30 days to complete.
However, during the colder winter months, the life cycle takes longer to complete because insects are cold blooded and thus affected by the lower temperatures. Fortunately, the growth of water lettuce also slows down in winter due to cooler temperatures. In spring, the weevils become active and multiply quickly as temperatures increase and similarly, the water lettuce populations respond to the warmer weather and begin multiplying, also providing more food for the hungry multiplying weevils. In this way the insect and plant populations track one another, with the weevil population increases and decreases mirroring, but slightly lagging behind the plant population's trends, showing synchrony between insect and plant populations.
In time, both populations will decrease. With biological control there will never be total eradication of the weed, but long-term control of the plants at a level which can be tolerated and accepted ecologically.
This weevil was released on water lettuce on the Sabie River at Lower Sabie in 1987 and by 1992 this infestation was considered to be adequately under biological control. The management response was to stop all chemical control efforts on the Sabie River in 1993, as it was evident that the biocontrol agents were capable of maintaining the plant populations at acceptable minimum levels.
By spring, September 1997, Sunset Dam was completely clear of water lettuce - an incredible crash in the alien plant population was experienced due to the effect of the host-specific weevil. With this dramatic crash of its food resource, the insect population followed suit and weevil numbers dwindled, with only a few remaining on the remnant plant population.
Throughout the summer season, the water lettuce population slowly built up and increased again and by late autumn and winter in 1998 Sunset Dam was again completely covered in water lettuce. Nevertheless, the cycle repeated itself and by November 1998 the Dam was again clear of water lettuce. This cycle of increasing and dwindling water lettuce cover on Sunset Dam, tracked by increases and decreases in weevil populations, was repeated annually until 2003.
This pattern of alternating 'clear and covered' states of Sunset Dam continued until 2003. During these periods intense pressure was put on management to again begin chemical control of the water lettuce problem on the dam, a favoured sundowner spot for tourists. Even internally there was not always agreement that biological control options would solve the problem and pressure to begin spraying herbicides reappeared at the end of each summer.
Nevertheless, researchers accepted that the fluctuations in plant and weevil populations would lessen and stabilise at some stage, and that less dramatic differences between "covered" and "clear" periods would emerge. On the Sabie River it took six years to reach this stage, while on Sunset Dam it took seven years (1996-2003), with the dam being completely clear during 2004/05.
Thus these cyclical fluctuations of 'open and closed' water bodies are normal and can be expected over time. It is now important to survey the plants to determine whether the biocontrol insects are present. This will be done by Ezekiel Khoza of the KNP alien biota section in early April, who will reintroduce the insects if not found present.
Although there were many times when KNP managers and tourists were not convinced of the snout weevils' ability to control the water lettuce, especially on Sunset dam, this has been achieved through perseverance against chemical control. Although the populations of insects and plants took seven years to stabilise, the effects will be long-term, sustainable and natural.