The spread of prickly pear is like the invasion of a dangerous enemy, advancing slowly but steadily and gradually taking possession of our continent.

The Sydney Mail, 1923

Rabbits, pigs, blackberries … Australia has a troubled history of introduced species wreaking havoc in its unique environment.

The prickly pear was one such species. Introduced in the early days of the colony, by the 1900s it had taken over large tracts of land in Australia's eastern states.

Attempts to stop the thorny pest were nothing less than full-scale war – as records in the National Archives reveal.

A thorny issue – who's to blame?

Many years ago people used to claim the honour of having introduced species of prickly pear into Australia as useful plants, but … their descendants are now silent on this point.

WB Alexander, biologist to the Prickly Pear Board

Arthur Phillip is said to have picked up some prickly pear plants in Brazil on his way out to the fledgling colony as a food source for the cochineal beetle. At the time, the beetle was crushed and used for red dye. 'It appears probable,' noted WB Alexander, 'that Sir Joseph Banks … suggested the possibility of introducing the cochineal industry in the new settlement at Botany Bay.'

It is likely, however, that these original specimens died and that the plants that were to cause so much trouble were introduced later. Prickly pear was planted as hedging, used as an ornamental in gardens, and even cultivated as a potential source of stock feed.

Whoever the culprit was, once the species started its deadly march across the eastern states, no one was game to own up.

The enemy advances

By around 1870, the prickly pear was out of control. Eaten by birds, cattle and horses, and spread far and wide in their droppings, it soon took over large tracts of farming and grazing land.

By 1919, the 'Pest Pear' (Opuntia inermis) had occupied over 22 million acres (89,000 square kilometres) of land in Australia – an area larger than the size of Scotland. It was estimated to be spreading at the rate of a million acres a year.

Tanks and flamethrowers

What an ideal job to give the returned soldiers! Let them attack the prickly vegetable with the same vigour they did the spiky helmets, and, having rid the land of the pest, occupy it themselves.

'Prickly pear: a suggestion'

In 1919, the Commonwealth Prickly Pear Board was established to try to find a solution, and a travelling commission searched the world looking for answers.

Inventors put forward their ideas for uprooting tools and mincing machines, and the general populace weighed in, too.

Mr MS Baldwin, from Delungra, New South Wales, proposed combatting the prickly pear with 'liquid fire'.He had read of the use of flemmenwerfers (flamethrowers) by the Germans during the war and thought that the weapons could be of ‘considerable value in fighting these pests'.

In a similarly warlike vein, one newspaper correspondent suggested adapting the tanks used in Flanders. He declared that, 'Some adaption of the "caterpillar" tractor should give us a machine that would roll down, chop up, and plough under the succulent stuff in one act'.

Blast that cactus!

Ultimately, the Prickly Pear Board found that 'mechanical or chemical treatment [poisoning] is of little real value in dealing with the pest'. Both would be costly. Neither were likely to stop the pest from growing back.

The Board turned its attention instead to methods of biological control. Research stations were established in Queensland and New South Wales and over 30 species of insects tested in carefully controlled environments. The trick was to find a species that would destroy the prickly pear without also taking out Australia's 'economic' species – that is, crops.

The Cactoblastis cactorum moth from Argentina was found to fit the bill. Female moths lay eggs on the plant and their larvae eat and destroy it in a few weeks.

The moth was released in 1926. Within 10 years, prickly pear had been largely eradicated in New South Wales and Queensland.

Memoralising the moth

The Cactoblastis Memorial Hall in Boonarga, Queensland, was built in 1936, while a cairn to the moth was erected by the Queensland Women's Historical Association in 1965.

The cairn's plaque records: 'the indebtedness of the people of Queensland, and Dalby in particular, to the Cactoblastis cactorum, and their gratitude for deliverance from that scourge'.