The prickly pear (genus Opuntia) is a succulent plant with flat, jointed stems and vibrant flowers and fruits. Native to the Americas, it became a devastating environmental and economic threat in Australia after its introduction. Its story highlights the profound impact an invasive species can have on an ecosystem.
Arrival and Rapid Spread
Prickly pear was first introduced to Australia with the First Fleet in 1788, primarily Opuntia monacantha, to establish a cochineal dye industry; the cochineal insect feeds on certain cactus species. Early settlers also cultivated various prickly pear species for hedges, garden ornamentals, and as emergency fodder for livestock during droughts.
Australia’s semi-arid and sub-humid regions, particularly in Queensland and New South Wales, proved highly accommodating. Without natural predators or diseases, prickly pear spread rapidly. Its ability to reproduce both by seeds, dispersed by animals and water, and vegetatively from detached stem segments (pads) contributed to its quick colonization. By 1920, infestations covered an estimated 24 million hectares, advancing at approximately 1 million hectares per year.
The Ecological and Economic Crisis
The proliferation of prickly pear led to an ecological and economic crisis across eastern Australia. Dense infestations formed impenetrable thickets, some reaching 6 meters high, rendering vast tracts of land unproductive for grazing and farming. This reduced pasture availability and livestock access to water, causing financial hardship and forcing many farmers to abandon their properties.
Beyond agriculture, the invasive cactus altered native ecosystems. It outcompeted indigenous flora, forming monocultures that reduced species abundance and threatened biodiversity. The sharp spines and glochids caused injury to livestock, native animals, and humans, limiting movement through infested areas. The plant’s ability to harbor pest animals like rabbits also exacerbated environmental issues.
A Biological Control Triumph
Faced with an escalating crisis, Australia embarked on a successful biological control program. After initial mechanical and chemical methods proved ineffective or too costly, the Commonwealth Prickly Pear Board was established in 1920. Scientists explored over 150 insect species, eventually introducing approximately 50 to Australia for host testing.
The most successful agents were the Cactoblastis cactorum moth, introduced from Argentina in 1925, and several Dactylopius cochineal insects, introduced between 1913 and 1922. Cactoblastis moth larvae burrow into and consume cactus pads. This moth, along with cochineal insects like Dactylopius opuntiae which feed on the plant’s sap, decimated prickly pear populations. By 1932, Cactoblastis cactorum had controlled common prickly pear on nearly 7 million hectares, reclaiming land for agricultural use.
Prickly Pear Today in Australia
Following the biological control program’s success, prickly pear populations in Australia are largely under control. The Cactoblastis cactorum moth and Dactylopius cochineal insects continue to suppress most prickly pear species, maintaining manageable levels. However, the plant has not been entirely eradicated.
Isolated patches of prickly pear may still exist, particularly in areas where the biological control agents are less effective, such as cooler, wetter, or very dry regions. Ongoing management efforts involve monitoring these residual populations and, when necessary, using chemical treatments or targeted manual removal for specific outbreaks. While the historical threat has largely subsided, the plant’s legacy as a devastating invasive species remains a significant part of Australia’s environmental history.