Chain-fruit cholla occurs naturally in northwestern Mexico and southwestern USA. It was introduced into South Africa as an ornamental around the 1940s, but was initially misidentified as rosea cactus (Cylindropuntia pallida/Opuntia rosea) in this country.
It grows as a shrub or small tree, 1-3 m tall, with a distinct trunk and spreading side branches. The branches are arranged in whorls and consist of easily detachable segments. They are densely covered by long (< 30 mm), barbed, white to yellow spines with baggy sheaths. The common name of the plant refers to the fact that new flowers grow from the tip of the previous season's fruit to create long, hanging chains of fruit. The flowers are pink to magenta, with the "petals" curving backwards and open during the late afternoon, while the fruits are spineless and grey-green in colour, or sometimes tinged with red. In South Africa, the fruit contains no seeds.
Chain-fruit cholla has invaded the dry savanna in the Northern Cape and Limpopo Provinces, with the latter infestation extending into Zimbabwe. In the absence of seeds, it reproduces purely vegetatively, whereby any part of the plant that breaks off (including the fruit) can take root. Detached segments adhere to the feet or coat of wild or domestic animals and are thus dispersed over long distances. The long, dense, barbed spines cause severe injuries and even the death of game, livestock, small mammals, reptiles and birds and, as a result, the value and productivity of infested areas is significantly reduced.
In terms of the Alien and Invasive Species Regulations (AIS), National Environmental Management: Biodiversity Act (Act No 10 of 2004), chain-fruit cholla has been declared a category 1b, which necessitates its control, or removal and destruction if possible. No trade or planting is allowed. Although herbicides have been registered against the cactus, the chemicals are expensive and it is a never-ending process because small plants are easily overlooked when spraying. Manual removal of the plants is also ineffective, because every piece that remains can regrow. In contrast, biological control is free of charge to land managers, requires little labour and, once established in an area, spreads naturally to find even the smallest cactus plants.
A cochineal species, Dactylopius tomentosus (Homoptera: Dactylopiidae), had been introduced into South Africa and released during 1970 to control a different but related cactus species, imbricate cactus (Cylindropuntia imbricata). Literature records led South African researchers to suspect that the same cochineal species might control chain-fruit cholla as well, resulting in its transferal to infestations near Douglas (Northern Cape) during the 1970s. The cochineal did persist on chain-fruit cholla, but failed to control it effectively.
Joint research by Drs Helmuth Zimmermann (then ARC-PPRI) and Catherine Mathenge (then based at PPRI, Pretoria, now residing in Australia) revealed the existence of a biotype of this cochineal species on a closely related cactus, Cylindropuntia cholla, in Mexico, which was highly damaging to chain-fruit cholla. This 'cholla' biotype of D. tomentosus was released near Douglas (Northern Cape) and near Musina (Limpopo Province) during October 2008, from where it has also colonized the cactus infestations on the Zimbabwean side of the Limpopo River. Biological control of the cactus plants near Musina was spectacularly successful, but the age and size of the plants growing near Douglas, as well as climatic differences, resulted in incomplete biological control in that area. Research indicated that large, woody cactus plants in the Northern Cape need to be felled about two years after having been infested with cochineal to prevent them from regrowing.
A review of this biocontrol project was published in 2011. pdf
>>>Specific IAP Species and their control according to botanical names