Jointed cactus (Opuntia aurantiaca) (Cactaceae)
The weed
Jointed cactus (Opuntia aurantiaca Lindley) is an inconspicuous, low-growing cactus of South American origin, seldom exceeding 500 mm in height, with elongated, almost cylindrical segments or joints. During dry periods, the older segments become tinged with red. Large numbers of long, sharp, barbed spines arise from raised areas or areoles on the surface of the segments, which cause the segments to become attached to passing animals or humans. The distribution of these dislodged segments is the only method of propagation and dispersal for jointed cactus, since the plant does not produce fertile fruits. The densest infestations occur in the Eastern Cape, but infestations have also been reported from all other provinces with the exception of the winter rainfall areas of the Western Cape. It invades a variety of habitats, from sparse, drought-adapted vegetation to dense, moist Valley Bushveld. By injuring livestock and becoming lodged in sheep's wool, jointed cactus causes the devaluation of land and eventually makes stock farming on infested land impossible. It is also a major problem in Australia, where it is commonly referred to as tiger pear.
Control
In terms of the Alien and Invasive Species Regulations (AIS), National Environmental Management: Biodiversity Act (Act No 10 of 2004), jointed cactus has been declared a category 1b species, which necessitates its control, or removal and destruction if possible. No trade or planting is allowed.
Biocontrol of jointed cactus relies mainly on a cochineal insect, Dactylopius austrinus, a native of Central and Western Argentina. It was obtained via Australia, where research into the host-specificity and impact of the insects was carried out. The cochineal exerts satisfactory control on jointed cactus, except in regions where it is too moist and cold for the insects to build up their populations sufficiently. In such areas, registered or recommended herbicides will have to be applied. Cycles of apparent inefficacy of cochineal might be caused by the delay in the recovery of cochineal numbers after having depleted their target weed locally, while the cactus regrows from underground tubers and increases in abundance temporarily. Land owners should, however, allow the cochineal numbers to recover naturally, instead of intervening, because experience has taught us that biocontrol eventually manages to restore the equilibrium. The cactus moth, Cactoblastis cactorum, is of lesser importance in the control of jointed cactus.
More information
- A 1999 review on the biocontrol of jointed cactus in South Africa can be downloaded:
pdf. - Leaflets are available on the following topics:
