Over 200 species of cactus have been introduced into South Africa, mainly for their ornamental value. The first to be introduced was a spineless form of the sweet prickly pear, Opuntia ficus-indicus, in the 17th century, for its fruit, and fodder value. By the beginning of the 20th century, infestations of this plant, spiny variants of which began to dominate, had become so severe countrywide that steps were taken to bring it under control, mainly through the use of the cochineal insect.
In the Waterberg, the following species of cactus are of importance as declared Category 1 Declared Weeds, meaning that they serve no economic purpose, but possess characteristics that are harmful to humans, animals and/or the environment. They are prohibited and need to be eliminated:
|Harrisia martinii||moon cactus||toukaktus|
|Opuntia aurantiaca||jointed cactus||litjieskaktus|
|Cylindropuntia imbricata||imbricate prickly pear||kabelturksvy|
|Opuntia ficus-indica||sweet prickly pear||boereturksvy (spined variant only)|
|Opuntia humifusa||large-flowered or creeping||prickly pear|
|Opuntia stricta||Australian pest-pear||suurturksvy|
|Echinopsis schikendantzii||torch cactus||orrelkaktus|
Description: The great majority of cacti are leafless, spiny, terrestrial, stem-succulents. Growth habit ranges from low-growing dwarf shrubs to erect or sprawling shrubs, climbers and trees. Stems may be segmented or unsegmented, cylindrical, columnar, spherical, tuberculed (with knobby protuberances), ribbed, winged or flattened. Spines vary in number, size, shape and colour. Flowers are showy, usually solitary and radially symmetric, with numerous colourful segments, nocturnal or diurnal, and pollinated by insects, birds and bats. The ovary, which develops into the fruit, is enveloped in a pericarp (fleshy receptacle), sometimes referred to as a floral tube. Fruits are berries, usually fleshy, and range from smooth to scaly, hairy, bristly or spiny (SAPIA News, July 2012).
Origin: All cacti are alien to South Africa, originating in the Americas.
Occurrence: almost anywhere, but often spreading away from human habitation or stock kraals. Some cacti spread very effectively from detached segments e.g. jointed cactus and imbricate cactus, but most dispersion is due to transport of seeds by birds, baboons and monkeys (and people) from eating the fruit.
Why they are a problem: except for the fruits, spiny cacti are largely unpalatable to stock and game. Left un-controlled, they develop into dense thickets, in which the spines are both a deterrent to animals and a source of severe skin irritation. Spiny variants of sweet prickly pear, at one time under control, appear to be regaining ground; and bio-control agents need to be re-introduced into areas where this is happening.
The principal herbicide registered for use against the plant, with great success, is MSMA 720 SL (L7279). Dilute 1 litre of MSMA in 2 litres of water and inject the solution into pre-made holes in the stem of the plant at ~2.5m intervals, with only 2 ml per hole. Repeated treatment of up to 8 injections may be necessary
NB: SANBI has advised that MSMA has recently been withdrawn from use by state departments because in isolated cases, it has been found to be highly toxic to grazing stock and wildlife, as it readily contaminates the grass surrounding the target plants. However, MSMA can be used safely if it is injected into the target plant, not sprayed onto it. It is strongly advised that the area to be treated is enclosed by a simple temporary fence to keep livestock and game out; and that the treated plants, once they have died off, are collected, burned in a pit and covered over with soil, before allowing animals into the area again.
The active herbicide ingredient glyphosate has also been registered, for use only against Opuntia ficus-indica (sweet prickly pear), always applied as a stem injection. However, it does not deliver results as good as MSMA. Numerous herbicides containing this chemical have been registered, for example:
• Roundup Max (L6790) (glyphosate(ammonium) 680g a.e./kg: water dispersible granules
• Buggie 360 (L6806); Glyphosate 360 (L4732, L4756); Profit (L4774); Roundup (L407); Strip (L6752); Sunup (L4687) (glyphosate(isopropylamine) 360g a.e./L: solutions.
• Roundup Turbo (L7166) (glyphosate (isopropylamine) 450g a.e./L): a solution.
• Touchdown Forte Hi Tech (L7305) glyphosate (potassium) 500g a.e./L): a solution.
Others include: Duiker 180, Roundup Max, Nexus, Cobra, Springbok.
All herbicides should be used when freshly mixed (do not leave the solution overnight).
NB: Follow carefully the instructions provided on herbicide label. Many herbicides can be toxic to other plants and or game and livestock if used inappropriately. (Mis-use of herbicides is also a criminal offence in terms of Act No. 36 of 1947).
ARC-LNR Weeds & Invasive Plants website: www.agis.agric.za/wip
ARC-LNR SAPIA News 25 (July 2012); Focus on Cacti in South Africa. ARC – Plant Protection Research Institute, Pretoria. www.arc.agric.za
Bromilow, Clive (2010): Problem Plants and Alien Weeds of South Africa. Briza. Pretoria.
Henderson, Lesley (2001): Alien Weeds & Invasive Plants. Agricultural Research Council (ARC), Pretoria.
Schmidt, Ernst, Mervyn Lotter & Warren McCleland (2007): Trees and Shrubs of Mpumalanga and Kruger National Park (2nd Edition). Jacana, Johannesburg.
Van Zyl, Kathy (compiler) (2005): Control of Unwanted Plants. Xact Information, Pretoria.
Special thanks to Dr Gerhard Verdoorn of Griffon Poison Information Centre, to Mr Ferdie Jordaan of Arysta Lifescience and to Ms Lesley Henderson of ARC for their invaluable advice and guidance. Their enthusiastic support for this voluntary project is greatly appreciated.