The prickly pear (Opuntia) represents one of the most significant biological invasion stories in Australian history. Its introduction, initially viewed as beneficial, spiraled into an overwhelming environmental disaster across the eastern states. By the early 20th century, the infestation transformed millions of hectares of productive land into an impenetrable, spiky wilderness. The eventual defeat of the cactus through a targeted scientific response stands as a globally recognized case study in pest management.
The Introduction and Rapid Spread
The first recorded introduction of prickly pear occurred with the First Fleet in 1788, when Opuntia plants were brought from Brazil to Port Jackson. The initial purpose was to establish a cochineal dye industry, as the insects that produce the scarlet dye feed on the cactus. While that early species did not spread extensively, numerous other varieties were subsequently introduced by settlers in the mid-1800s. These later introductions, particularly Opuntia inermis and Opuntia stricta, were used for hedging, as ornamental plants, and as emergency fodder.
The cactus thrived in the semi-arid climate west of the Great Dividing Range, where Australian conditions lacked the natural enemies that kept the plant in check in its native Americas. Its reproductive success relied on both seed dispersal and vegetative propagation. The tough, coated seeds were consumed by native fauna, such as emus and crows, which spread the seeds across vast distances. Furthermore, the fleshy pads (cladodes) easily detached and took root wherever they landed, quickly establishing new colonies. This combination allowed the infestation to grow exponentially, covering 10 million acres by 1900. By the early 1920s, the prickly pear had overrun approximately 60 million acres of land across Queensland and New South Wales.
The Environmental and Economic Crisis
The rampant spread of the prickly pear quickly escalated into a national catastrophe, earning the infested areas the local moniker of the “green hell.” The economic destruction was profound, as the cactus rendered vast tracts of prime grazing and agricultural land unproductive. Land values plummeted, and the dense, spiky growth made farming or moving livestock impossible, forcing countless farmers and graziers off their properties. The infestation was advancing at an estimated rate of up to 2.4 million acres each year, overwhelming human efforts to contain it.
Early attempts at control, including mechanical methods like digging up and crushing the plants, proved too slow and labor-intensive against the sheer volume of the cactus. Chemical control, often involving hazardous arsenic pentoxide, was also ineffective and costly, sometimes reaching £20 per acre. Beyond the agricultural losses, the ecological impact was severe, as the prickly pear formed an impenetrable canopy that could reach heights of up to six meters. This dense growth outcompeted and smothered native grasses and other flora, leading to a significant loss of biodiversity.
The Triumph of Biological Control
With mechanical and chemical methods failing, the focus shifted to a coordinated scientific strategy centered on biological control. The Commonwealth Prickly-Pear Board (CPPB) was established in 1920 by the federal and state governments of New South Wales and Queensland to spearhead the research. The Board sent entomologists to North and South America to search for the cactus’s natural enemies, ultimately testing approximately 50 insect species. The search focused on finding an organism that fed exclusively on Opuntia to prevent it from attacking native Australian plants.
The most promising candidate identified was the Cactoblastis cactorum moth, a species native to South America. After rigorous quarantine and testing at research stations in Queensland, the first major release of the moth’s eggs occurred in 1926. The moth’s efficacy stemmed from its life cycle: the female lays her eggs in a characteristic “stick” formation on the cactus spine. Once the larvae hatch, they bore directly into the cactus pad (cladode), devouring the internal tissue. This feeding behavior hollows out the pad, causing it to rot and collapse within a few weeks.
The moth reproduced rapidly, and its larvae became a highly efficient control agent against the dense prickly pear growth. Within a decade of its introduction, the Cactoblastis moth caused a dramatic reversal of the infestation. By 1933, the cactus had been effectively destroyed over much of the previously infested area, with up to 80 percent of the land in Queensland cleared. This success is globally recognized as a watershed moment in biological pest management.
Current Status and Utilization
Despite the success of the Cactoblastis cactorum moth, the prickly pear was not completely eliminated and requires ongoing management. Patches of the cactus persist in the landscape, often in smaller formations, and new varieties less susceptible to the moth’s larvae have become established. The moth maintains a natural check on the dominant species, preventing a return to the catastrophic levels of the 1920s. Residual populations sometimes necessitate the use of chemical herbicides or mechanical removal by government agencies.
The plant’s status has shifted from an agricultural pest to a resource with limited utilization. The fruit of some Opuntia species is edible and consumed globally. However, due to its history as a severe invasive species, the prickly pear is classified as a declared pest in Australia, and its commercial cultivation is heavily restricted. While small-scale operations exist under strict regulatory control, its widespread use as a commercial crop or fodder remains constrained by the need to prevent another major outbreak.