Invasive cactus species in Parys & surrounds – a pretty, thorny issue!
Several exotic (alien, imported) cactus species that have the ability to adversely affect bio-diversity, are present in the Parys and surrounding areas.
In some cases, biological control agents are available to limit populations of these weeds, but it may take years for the agent populations to establish in large enough numbers to have a severe impact on problematic cactus species.
There may also be cases where a herbicide had been registered for their control, but registrations are specific to certain species only and does not apply to cactus species in general.
It is also worth pointing out that one of the active ingredients contained in registered products, are toxic to livestock, including game, and carries a 60-day withholding period, where livestock should be prevented from grazing an area where monosodium methanearsonate (MSMA, an organic arsenical) has been applied. There are however, safer alternatives – in some cases.
Ideally one should consider integrated control methods which incorporate chemical (if available), biological (if available and effective) and mechanical measures.
All the weeds mentioned here are classified as a Category 1b weed in NEM:BA (NationalEnvironmental Management: Biodiversity Act) and has to be controlled wherever it occurs. Landowners, take note.
The following cactus species are generally viewed as an environmental threat:
Opuntia humifusa (Large flowerd prickly pear / Creeping prickly pear)
This invasive species can really spread rapidly and have a severe impact on grazing.
There was a farm near Koffiefontein, where around 200 hectares were heavily infested when chemical trials were conducted. In some areas one could literally walk from one patch to the next without ever touching bare ground! Needless to say, on this sheep farm, where cactus infestations had reached such a magnitude, grazing capacity was virtually nil. As with perhaps all cactus species, propagation can be via seeds, often spread by animals, birds (crows love the ripe fruit) and humans, as well as segments of plants that are broken off and come into direct direct contact with the soil.
Opunia humifusa (Large flowered prickly pear)
O. humifusa – early stage of root development on broken-off cladode
Level of infestation of O. humifusa on the farm Oude Werf, Koffiefontein, 13 December 2016. .
Opuntia stricta (Australian pest pear)
While this weed may be more common than is generally known, the author was only aware of one site where this pest had reached populations that was already damaging the environment. The farm was actually in the North West province, in the Koepel area, just a few kilometers from the Vaal River. Two biological control agents were already present (having been introduced to help manage the problem), namely Cochineal and Cactus moth (Cactoblastis cactorum). It took several years for these bio-control agents to achieve significant results and while chemical control would have offered a much quicker solution, it
would also have been more costly due to the level of the infestation
- Flower of Opuntia stricta
Australian pest pear with ripe fruit
Large numbers of O. stricta seedlings infesting a new area.
O stricta – impressive roots of a young seedling.
Control of Cochineal
Producers of prickly pear fruit, may from time-to-time experience Cochineal infestations to their Sweet prickly pear (Boereturksvy) orchards. On a small scale, one can use a high-pressure sprayer to rinse Cochineal off the targeted plants. On a commercial scale, the more realistic option would be to consider using a registered systemic insecticide. Products containing the active ingredient, carbaryl (carbamate insecticide) can be considered. As with all pesticides, care should be taken to avoid exposure to the compound during the application process. Study and adhere to all label requirements at all times to avoid unintended consequences.
Examples of biological control agents available in South Africa
Cochineal infestation on a cactus plant
Cactoblastis cactorum larva (above) and moth (below)
Next week we will discuss
- Imbricate cactus / Kabelturksvy (Cylindropuntia imbricata)
- Torch cactus / Orrelkaktus (Trichocereus spachianus)
- Queen of the night / Nagblom (Cereus jamacaru)
ENQUIRIES:
Products and prices: Chris Richter (082 458 4558)
Author of this article: Ferdie Jordaan (083 636 7368)
References:
Klein, H., Zimmerman, H.G., ‘Invasive cacti in South Africa. Their identification and control’ Plant Protection Research Handbook Series No. 20, Agricultural Research Council, 2020
Jordaan, F., Unpublished observations (2003 – 2022)
