Australian pest pear (Opuntia stricta) [pdf] is a native of the south-eastern USA, Mexico and Cuba. It is a low, spreading shrub, approximately 1 m tall, with elliptic, blue-green, flattened stem segments (cladodes or leaf pads) that become narrower towards the base. Leaf pads are either spineless or have only a few spines, usually single, stout, straight, yellow spines restricted to the margin of the leaf pad. The glochids (short, fine bristles that occur in tufts on the areoles) are inconspicuous. Yellow flowers, approximately 5-6 cm across, give rise to dark-red or purple, pear-shaped fruits with a slender "neck". The fruits are spineless and smooth, except for a few areoles with a tuft of glochids. The fruit pulp is deep purple and has a very sour taste.
A variety of this cactus, O. stricta var. dillenii, which occurs only in a few parts in KwaZulu-Natal, has 4-7 spines per areole. The spines also differ in being hard, curved, flattened and yellow to brown. The fruits are often curved.
Australian pest pear has probably spread from gardens and rockeries, and is now invading the savannah and grassland regions of all South African provinces. Wild animals (mainly baboons and elephants) that eat the ripe fruit, spread the seeds, but detached leaf pads will also readily root to form new plants.
This species is also problematic in several southern African countries, Ethiopia, Yemen and Saudi Arabia. It used to be regarded as Australia's worst ever weed, where it covered almost 24 million ha of Queensland and New South Wales, but has been brought under effective biological control.
In terms of the Alien and Invasive Species Regulations (AIS), National Environmental Management: Biodiversity Act (Act No 10 of 2004), Australian pest pear has been declared a category 1b species. Land users therefore have to control it, or to remove and destroy it if possible. No trade or planting is allowed. Certain herbicides have been registered for use on this weed, but biological control is exceptionally effective and is the preferred control method. The biocontrol agents are the 'stricta' biotype of the cochineal insect Dactylopius opuntiae [pdf] and the cactus moth Cactoblastis cactorum [pdf]. Chemical control is only recommended for outlying, isolated cactus plants.
The biological control of Australian pest pear in South Africa was recently discussed in a review published during 2011. [pdf]
The following leaflets can be downloaded:
A detailed account of cochineal insects (Dactylopius species). [pdf]
A control strategy for Australian pest pear. [pdf]
A detailed description of Australian pest pear and its weed status in South Africa. [pdf]