Australian
Screen

Teacher’s notes

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This clip shows an excerpt from a silent, black-and-white newsreel item about the prickly pear infestation in Australia in 1926, and the steps being taken by scientists to control it. It opens with a map of Queensland and northern New South Wales, indicating the extent of the problem, and includes footage of affected countryside. Scientists are shown experimenting with biological control of the plant, and the clip includes shots of cochineal insects overrunning prickly pear plants. A Cactoblastis cactorum moth emerges from a chrysalis. Packing and distribution of Cactoblastis moth eggs is shown, and the clip concludes with shots of a sugar cane plantation and cattle grazing in a field, with both plantation and field cleared of prickly pear.

Educational value points

• The spread of prickly pear in Australia is recognised as one of the greatest biological invasions of modern times. Prickly pear may have come to Australia with the First Fleet; however, from about the 1840s many farmers planted it as a fence for stock and because it could feed animals in times of drought. By the 1900s large tracts of agricultural and pastoral land were infested by the plants, which were spreading at a rate of about 1 million acres (404,686 hectares) a year. By 1925 about 25 million hectares in Queensland and New South Wales were infested.

• The prickly pear originated in the Americas and is a member of the cactus family. Prickly pear shrubs are leafless succulents that have large flowers in spring, and their stems are divided into segments that are flat and covered with clusters of spines. The term 'prickly pear’ actually refers to the edible fruit, which has very fine spines and is pear-shaped. Different species were introduced to Australia, but the worst is the common prickly pear (Opuntia stricta), a bushy spreading plant that grows up to 1.5 m in height.

• The effects of the prickly pear highlight the dangers of introducing plant species into Australia. In the absence of natural predators, many plants introduced into Australia over the past 200 years have spread unchecked and are now environmental weeds. Plants such as the prickly pear, Paterson’s curse, blackberry and lantana threaten native plant species and reduce the plant diversity needed to support indigenous Australian insects, birds and animals. Their spread can cause environmental damage such as soil erosion and lead to a greater risk of bushfire. The control of weeds is estimated to cost Australian agriculture up to $4 billion every year.

• From about 1912 Australian scientists were engaged in research to find biological control agents (a living organism used to control another). Early attempts to control the plants through slashing and poisoning had been ineffective because prickly pear pads can easily re-establish. After testing 52 species, 18 insect species and 1 mite species were released in Queensland. The most successful of these were the moth Cactoblastis cactorum and five varieties of cochineal mealybugs. By the 1930s the plants were controlled but not completely eradicated.

• Female cochineal mealybugs, seen in the clip on prickly pear segments, spend their adult lives permanently attached to the host plant, in this case the prickly pear, sucking plant juices and eventually killing the plant. The insects are covered with a fine waxy secretion that, when crushed, produces a carmine (red) colouring that was used as a dye into the 19th century. Different species of the insect were effective against different types of prickly pear and, as the clip demonstrates, the insects were often distributed onto plants manually.

• The Cactoblastis cactorum moth, shown emerging from a chrysalis, was imported from Argentina in 1925 as larvae and introduced as a control measure the following year. The larvae eat the prickly pears from the inside, leaving the empty pad skins and piles of droppings, as seen in this clip. They have largely controlled the spread of both the common and spiny prickly pear.

• The control of prickly pear by the Cactoblastis moth is regarded as one of the most successful examples of pest plant repression by biological means. Other attempts at biological control in Australia have, however, proved disastrous. For example, the cane toad was introduced to control the cane beetle (another introduced species) that was attacking sugar cane in Queensland, and is itself now a major environmental problem. Today, biological control agents must be rigorously tested before release to establish what effect they will have on species other than the targeted pest species.

• The scientists featured in the clip may be at the Sherwood quarantine station in Queensland, where Cactoblastis moths were initially reared in cages, and their eggs collected for distribution. About 12 million eggs were distributed between 1926 and 1931. Eggs were wrapped in paper quills and then packed in boxes, with pins to attach the quills to the pear plants. Each box contained 100,000 eggs and printed instructions for landholders on the method of dispersal. A fleet of seven trucks distributed the boxes across Queensland. Once the moth was established, eggs for further distribution were collected in the field.