Two businessmen of George in the Southern Cape have tackled the thorny issue of invasive, alien cactus species that are adored as garden plants, but which are seriously threatening to destroy pasture and cultivated land.
Three years ago, Ken Coetzee, Wallie Stroebel and Bruce Taplin of Conservation Management Services (CMS) extended their soil conservation activities to also combat the rampant spreading of invasive alien cactus species.
These plants were imported and cultivated for use as livestock feed, but also as ornamental garden plants.
They tackle the invasive plants by means of biological control methods, which takes time. Where dense stands of impenetrable cacti are resistant to biological methods, chemicals with the least possible negative effect on the environment (soil, water, and air) are used. In extreme cases, mechanical methods are used to eradicate dense stands by chopping up the offensive aliens.
These cactus species, which include prickly pear species and others, were mostly imported to South Africa from arid areas of Southern America, Mexico, and parts of the USA. They spread rapidly wherever the soil has been degraded to such an extent by drought, overgrazing and erosion that indigenous plant species cannot survive.
These cactus species are rapidly gaining ground in the Western, Eastern, and Northern Cape, but also in other parts of the country.
Ken identified the top ten species that are already invasive and have the potential to develop into pest plants. According to the Conservation of Agricultural Resources Act, as well as the National Environmental Management: Biodiversity Act, these plants are classified as pest plants and farmers are under an obligation to control it and preferably remove it.
Regeneration and distribution
All these plants, except the prickly pear and thornless prickly pear, were imported as ornamental plants for gardens. But many of them escaped from gardens and invaded large areas of land.
Top ten intruders
Opuntia ficus-indica – Prickly pear (gewone turksvy)
The prickly pear is a native plant of Southern America. The fruit is eaten, and the leaves are used as animal feed. Because of the thorny leaves, these plants form a formidable, impenetrable hedge. In some countries they are planted to stop desertification.
These species occur in dry, warm areas and are widely distributed throughout South Africa and Namibia. Although the fruit were once exported, these plants are at present mainly used as animal feed. It is highly digestible and tasty, and it is easy and cheap to establish.
Opuntia monacantha – thornless prickly pear (doringlose turksvy)
The thornless prickly pear is also a native plant of Southern America. It easily encroaches and must be controlled. The leaves are also used as animal feed.
Ornamental cactus species
The rest of these cactus species were imported as ornamental plants.
Opuntia aurantiaca – jointed cactus (litjieskaktus)
The stem of the low-growing jointed cactus consists of segments, which easily break off and spread, as the thorns attach in them to humans and animals, especially wool-bearing sheep. The thorns reduce the value of the wool, and where these plants grow profusely, it eventually becomes impossible to farm sheep.
Dense stands occur in the Eastern Cape, but also in other provinces, excluding the winter rainfall areas of the Western Cape.
Opuntia stricta – Australian pest pear (suurturksvy)
The Australian pest pear is causing a massive problem in Australia where it has invaded large areas. It is gaining ground in the Karoo and on grasslands all over the country. The seed is distributed by baboons and elephants that eat the fruit. Leaves that break off propogate the plant vegetatively and quickly sprout roots.
This cactus must by law be eradicated, but even though a chemical herbicide has been registered for it, the only real solution is eradication by means of biological control with cochineal (Dactylopius austrinus) or the cactus moth (Cactoblastis cactorum).
Opuntia microdasys – bunny-ears (haasoortjies)
The bunny-ear cactus with its short, yellow clusters of thorns, is a pest plant in the Western Cape, Eastern Cape, Northern Cape, and Gauteng.
It is spread by water, humans and vehicles, and poses a serious threat to indigenous plant species. Dense stands under trees rob livestock of shade in summer. Extensive stands push out indigenous species, and decrease the value of the land.
Animals and birds eat the fruits, which may be shades of red to purple, but basically the plant has no other value besides being pretty.
Opuntia humifusa – creeping prickly pear, devil’s tongue (kruipkaktus)
The low growing, spreading creeping prickly pear has small, pointed leaves and no thorns. It bears bright yellow, red, or purple fruit in summer. It is spread by means of seed and is becoming a problem throughout South Africa.
Cylindropuntia imbricata – imbricate cactus (imbrikaat-kaktus)
The imbricate cactus is difficult to control once it is established, as every shred of leaf that breaks off from the mother plant, forms a new plant. No chemical herbicide is effective, and it can only be controlled by cochineal.
The stems must be cut down at ground level, stacked and left to the insects to devour the leaves from the inside.
Cylindropuntia fulgida var. mammilata – boxing glove cactus (bokshandskoen-kaktus)
The boxing glove cactus occurs widely spread in the dry Northern and Eastern Cape, Limpopo province and Namibia. It grows up to a metre or a metre and a half. The tips break off and fall on the ground near the mother plant, where it takes root. It is also distributed by water or animals. The sharp thorns injure man and beast, and its presence dramatically lowers the value of the land.
There is no registered chemical herbicide that can effectively control it. It is also practically impossible to control it mechanically, as every inch of leaf that breaks off, immediately takes root. Biological control with cochineal or cactus moth are the only options for eradicating it.
Trichocereus spachianus – torch cactus (orrelkaktus)
The torch cactus with its thorny, cylindrical, tall, and upright stem is regarded as the most serious invasive plant in the Eastern Cape and Karoo.
Tephrocactus articulatus – pinecone cactus (dennebol-kaktus)
The pinecone cactus grows up to 30 cm tall and bears white flowers. The corklike, winged seeds are distributed by wind and water. The plant occurs along dry and seasonal waterways, drainage ditches and wherever the land has been disturbed by human action.
It is regarded as a pest plant, and it is rapidly spreading in the Western and Northern Cape.
Cochineal (Dactylopius austrinus) spread from Central and Western Argentina, via Australia, to South Africa. It was introduced to South Africa in the 1970s and has since been used to control prickly pears and other cacti in hot, dry areas.
In areas such as the Western Cape, where the winters are normally cold and wet, chemical herbicide is the only means to these species. The prickly pear moth (Cactoblastis cactorum), which is a native of South America, preys specifically on cactus species.
To eradicate dense stands of prickly pear, CMS construct a greenhouse or a hutlike structure with plastic in the veld where these plants grow. Infected and healthy prickly pear leaves are placed in buckets inside the greenhouse, where the cochineal and cactus moths are allowed to spread from the infected leaves to the healthy ones. Once these leaves have been infected, they are placed near or on stands of prickly pear to allow the insects to invade the plants. By using this method, it takes about two years for a dense stand to be eradicated.
CMS makes use of a mobile diesel chipper to mechanically fight invasive plants. The leaves of prickly pear plants are finely chopped up and used as a mulch to cover bare soil in the fight against soil erosion. The mulch protects and enriches degraded soil, allowing indigenous plants and pasture to reclaim the land.
The roots are treated with herbicide once the leaves have been removed from the prickly pear plant, and the fruits are treated separately to prevent further distribution by means of seed.
For more information contact Ken Coetzee at (+27)76-227-5056 or email@example.com.