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Opuntia stricta (Haw.) Haw. 1812
pronounced: op-UN-tee-uh STRICK-tuh
(Cactaceae — the cactus family)
common name: Prickly Pear
Prickly pear is a general term used to describe some plants of the Cactaceae family. The term includes Opuntia, Nopalea and Acanthocereus. All of these plants originate in the Americas. The term ‘prickly pear’ relates to the fruit that is often spiny and pear-shaped.
There is no information on the introduction of Opuntia stricta into Australia. It was recorded as being cultivated for stock fodder in the Parramatta district in the early 1800s. There is also a record of a plant in a pot being taken to Scone, NSW, in 1839, when it was planted in a station garden. The manager of the property later planted it in some of his paddocks with the idea that it would be a good standby fodder in a drought year. Another plant is recorded as being taken to Warwick, Queensland, in 1848 for use as a garden plant, with the thought that it would make a good hedge. The hedges grown from this plant, and from others introduced in other places for similar reasons, flourished and bore fruit, which spread into the paddocks. The plant soon became acclimatized and spread at an alarming rate.
There are several clumps of the pear on Magnetic Island. The one pictured is near the old Picnic Bay Surf Lifesaving hut. There is a small clump on the bush track behind Butler Street in Picnic Bay, and there is quite a large clump near the beginning of the Balding Bay track past of the last houses on the eastern end of Horseshoe Bay.
Opuntia stricta is found in semi-arid, subtropical, tropical and warmer temperate regions, in open woodlands, grasslands, pastures, on the banks of watercourses, on roadsides and by railway lines, in disturbed sites and waste areas. It will also invade rocky slopes. It is an erect or spreading succulent shrub usually growing 50-100 cm tall, but occasionally reaching 2 m in height.
The stems are much-branched, and consist of a series of of flattened succulent cladodes that are usually 10-35 cm long, 7-20 cm wide, and 1-2 cm thick. They are obovate in shape, and green, or sometimes bluish green in colour. They are glabrous, and covered in small areoles that bear tiny spiny bristles. The areoles may have one or more sharp spines 2-4 cm long. Each areole contains a growing point that can produce roots or shoots. The leaves are reduced to tiny terete or conical structures up to about 6 mm long, and are quickly shed from the developing cladodes.
The flowers, up to 7 cm long and 6-8 cm across, are bright yellow, but sometimes have pinkish or reddish coloured markings on the outer petaloids. They are borne singly on fleshy bases. There are numerous petaloids and stamens.
At first the fruits, berries, are green in colour, and turn reddish purple as they mature. They can grow up to about 8 cm in length and 4 cm in width; they are obovoid in shape, usually with slightly depressed tips. Each fruit has several tufts of small barbed bristles on its surface, and should be handled with great care. Inside the fruit is a reddish or purplish pulp containing many seeds. These seeds, 5 mm or so in size, are generally yellow or pale brown in colour, and subglobose.
The introduction and spread of prickly pears into Queensland and New South Wales is one of the greatest environmental invasions of modern times. By 1900 there were over 4 million hectares infested, and by 1925 the pest had taken over more than 24 million hectares. Many graziers, farmers and smallholders, including my grandfather, were forced off the land. Prickly pears are drought-resistant because of their succulent nature, their lack of leaves, and their thick, tough skins. These features result in plants that use the majority of their internal tissues for water storage, and their outer tough and thorny parts to reduce water loss and damage by grazing animals. They remain vigorous in hot dry conditions that kill most other plants – they are, of course, desert plants in the Americas. Some species develop underground bulbs that enable the plant to resist both fire and mechanical means of eradication. They can reproduce both sexually and by cloning. Birds and other animals eat the many-seeded fruits, and deposit the seeds in their droppings. Cloning occurs when pads or fruits located on the ground take root and produce shoots. Both animals and flood waters can move broken pads long distances, and these pads can survive long periods of drought before weather conditions become suitable for them to root. Up until the first world war, the only means of control was mechanical. Many shires, in trying to control the spread of the pear, had regulations requiring property-holders to keep the areas adjoining their boundary fences clear of the pest. The regulations, no doubt, varied from shire to shire: but in my grandfather’s case, he was required to keep one chain (the length of a cricket pitch) of land clear of the pear, right along his boundary. My father and his brothers spent their boyhood years, after school and on the weekends, hoeing out the pear from this buffer zone. It was relentless work, and of little avail; but failure to obey the regulations meant that the council was empowered to have the work carried out by contractors at the farmer’s expense.
Research for biological control agents began just before the first world war, and in 1914 cochineal insects were released to control one of the minor species. This was rather ironic, as it was because of its value as host to the cochineal insects that the very first prickly pears were brought to Australia by Governor Phillip in the first fleet. At the instigation of Sir Joseph Banks, Captain Phillip collected a number of cochineal-infested plants from Brazil on his way out to New South Wales in order to establish a cochineal-dye industry. Opuntia vulgaris, the ‘smooth tree pear’, is the host of this insect, which at the time was the world’s main source of red dye, obtained by crushing countless numbers of these tiny insects. Apart from its importance to the textile industry, the cochineal dye was used to colour the British soldiers’ red coats – a very important factor indeed! At this time, Spain and Portugal had the monopoly on this product, through their South American colonies, and the British government was keen to establish a source of supply in one of its own colonies. This variety of the pear never did become a major pest, and is still to be found in some coastal areas of New South Wales.
The major break-through in the control of the Opuntia stricta pest was the introduction of Cactoblastis cactorum (the cactoblastis moth) from Argentina and its release into the infested areas in the early 1920s. The moth lays its eggs in the pads of the cactus, and the larvae eat the contents of the pads, leaving empty skins and piles of mushy droppings. Within a few years millions of hectares of the pest had been destroyed. Colonies of the moth are still often to be found in residual prickly pear patches, and keep the pear under control.
This was probably the most successful biological control agent of a pest ever discovered and put into effect anywhere in the world, and, had it not been introduced, most of eastern Australia would today be covered with prickly pear. BUT – there is a down-side. Ever since then, Australian agricultural scientists have sought biological control agents for every pest that has become rampant in the country, and the ‘cure’ has often been worse than the disease. The major example of this was the introduction of the cane toad as a control agent for a beetle infesting sugar-cane areas of Queensland; and we all know what has become of that odious creature! Another that comes to mind is the introduction of the Indian Myna bird to eat ticks from the bodies of cattle. This bird has not only become a lice-ridden suburban pest in many areas, but has displaced many of our (much more attractive) native bird species.
Photographs taken at Picnic Bay 2017, 2018
Page last updated 21st March 2018