Invasive cactus species in Parys & surrounds – a pretty, thorny issue! (Part 2)

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:

 Imbricate cactus / Kabelturksvy (Cylindropuntia imbricata)

Mature Imbricate cactus with flowers

Young Imbricate cactus

In this part of the country, this weed is not nearly as common as O. humifusa or O. stricta, but author has observed isolated patches around Parys and in the Koepel area in the North West province.  While there is a registered chemical remedy for its control, it comes with a warning – a 60 day withholding period where livestock, including game, must not be allowed to graze treated paddocks.  Newer, safer herbicides are being developed, but until such products are registered, one should rely on biological control, like Conchineal (Dactylopius tomentosus, the imbricata biotype) or mechanical control which involves physical removal of the whole plant (and possibly burning in a safe area).

• Torch cactus / Orrelkaktus (Trichocereus spachianus)

Also less common, but like Imbricate cactus, it can be seen near the homesteads on some farms where it was probably introduced as ornamentals in days past.  It has, however, been classified as an alien invasive plant that needs to be controlled wherever it is growing (Category 1b, NEM:BA). No biological agent has proven to be highly effective, but there is a reliable chemical product that was registered a few years ago.

• Queen of the night / Nagblom (Cereus jamacaru)

Queen of the night is also classified as a Category 1b weed in NEM:BA and has to be controlled wherever it happens to grow.  Its was imported from South America, probably as an ornamental plant.  Seeds are spread by birds and other wildlife, but easily grows from segments of the plant that comes into direct contact with the soil.

There are several biological control agents available, but it sometimes takes years for it to make a dent in established populations of Queen of the night.  There is a chemical solution, but with an extended 60-day withholding period for livestock, including game.

Mature C. jamacaru plants

Queen of the night is one heavily armed invader!

Giraffe damage to a tall Queen of the night.  During the dry season in Limpopo, the spikes do not deter hungry kudu or giraffe.

Huge (9m) Queen of the night that was killed by means of an injection treatment directly in the main stem.  It took nearly 3 years to achieve this result.  Patience is indeed a virtue.

Examples of biological control agents available in South Africa

Cochineal infestation on a cactus plant

Cactoblastis cactorum larva (above) and moth (below)

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.

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)