The tangled mess that choked Australia
FEW TODAY COULD imagine an Australia where dense, weedy fields of cactus plants choked 240,000sq.km – an area equivalent to the size of the UK – but it was once so, and it was one of the world’s great biological invasions.
The plague would not reach its zenith until the 1920s, but the seeds of this prickly problem arrived with the First Fleet in 1788. Captain Arthur Phillip had been charged with creating a source for the blood-red dye used on the coats of British soldiers.
At the time the hue could only be made from macerated cochineal bugs. These lived on cactuses from the family Opuntia and were produced in Mexico, under Spanish control. In Australia a small cactus and cochineal industry was seeded, but chemical dyes soon replaced the insect-sourced pigment. Rogue cactus nevertheless clung on in backyards and paddocks. Initially, the tangled mess spread slowly, fanning out from Sydney. But removal often proved more expensive than the land was worth, so farmers often abandoned properties where it was a problem.
The situation worsened during a 1901–02 drought, when desperate farmers began to use juicy cactus pads to supplement cattle feed. By 1907 the government was offering the equivalent of $1.3 million to someone who could solve the problem, but nobody claimed it.
The infant federal government sent scientists across the world to find a method of biological control. They went to India, South Africa and the Americas, eventually discovering a promising brown-grey moth called Cactoblastis cactorum in Argentina. These were brought back to Australia and released across inland NSW and Queensland in 1926, resulting in a remarkable biocontrol success story.
By 1932 the fields of prickly pear seemed a bad dream, nibbled into oblivion by millions of moth larvae. Reclaimed land was allocated to the ‘Soldier Settler’ scheme, setting World War I veterans up as small farmers.
This success gave a boost to supporters of biocontrol schemes to manage the many invasives Europeans had brought, but not all schemes were well thought out – in 1935 cane toads were released to control the cane beetles ravaging sugar crops. Biocontrol is still used today, but only when the risks have been studied carefully.
This article originally appeared in the May-June 2016 edition of Australian Geographic (AG#132).