Harrisia cactus, a native of Argentina, is a sprawling or clambering succulent shrub with spiny, cylindrical branch segments. The ribbed stems, which have the appearance of plaited rope, frequently arch downward and root where they touch the ground, thus creating dense, impenetrable thickets. The central spine on each areole is up to 4 cm long, with several shorter spines surrounding it. The cactus lacks glochids. The flowers are large and funnel-shaped, up to 20 cm wide, and nocturnal. The inner floral parts are white to pale pink, while the outer ones are greenish white and curve backwards. They give rise to spherical, red fruit, about 3 cm diameter, which are armed with long, white spines. The fruit splits open along one side to reveal edible, white pulp dotted with small black seeds. Frugivorous birds feed on the fruit and spread the seeds in their droppings.
cactus invades the savanna biome of South Africa, especially in the Gauteng,
North West, Limpopo and KwaZulu-Natal Provinces. It forms dense patches
underneath trees, thus making them inaccessible to wild and domestic animals
that seek shelter from the sun, in addition to injuries caused by the long,
sharp spines. This reduces the production potential and therefore the value of
In terms of the Alien
and Invasive Species Regulations (AIS), National Environmental Management:
Biodiversity Act (Act No 10 of 2004) , harrisia cactus was declared a category 1b species, and land owners are compelled to control it, or to remove and destroy it if possible. No trade or planting is allowed.Herbicides are registered to control the plant chemically, but the underground tubers prevent the translocation of the active ingredients.
Biological control is the preferred control option, and two biocontrol agents are available in South Africa. They are a mealybug, Hypogeococcus festerianus (according to some taxonomists it should be called H. pungens), that causes stunted and distorted growth before eventually killing the plant, and a long-horn stemborer, Nealcidion cereicola, which tunnels in the stems and also has the potential to kill the plant. Both these agents were introduced via Australia, where they formed part of a very successful biological control programme against harrisia cactus. The mealybug is readily available for redistribution, but the stemborer is only established in a few sites and is difficult to mass-rear. In addition to harrisia cactus, both agents are also effective in controlling another iinvasive cactus, queen of the night (Cereus jamacaru) in South Africa.
A review on this biocontrol project was published during 1999. pdf
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