Australia’s Cactus Story
Australia is known for its unique flora and fauna, making the presence of cacti seem unusual. While cacti are found across the Australian landscape, almost all species are introduced, not native. Many have become highly invasive, posing significant environmental and agricultural challenges. Most established cacti originate from the Americas, spreading primarily due to human activity.
Australia’s Unwelcome Cactus Guests
No cactus species are indigenous to Australia, with the exception of Rhipsalis baccifera, sometimes found in Queensland’s rainforests. The Opuntia genus, commonly known as prickly pear, represents the most significant and problematic introduction. Early European settlers brought various prickly pear species, arriving with the First Fleet in 1788.
These plants were initially introduced for diverse purposes, including a cochineal dye industry, ornamental use, living fences, or livestock fodder during droughts. However, the cacti quickly escaped cultivation. Lacking natural predators and diseases from their native Americas, species like Opuntia stricta and Opuntia inermis spread rapidly across vast agricultural and pastoral lands in Queensland and New South Wales. By the early 20th century, prickly pear had infested an estimated 24 to 60 million hectares, rendering much of this land unusable.
The Fight Against Invasive Cacti
The widespread invasion of prickly pear caused severe ecological and agricultural damage. Dense thickets prevented livestock access, reduced land productivity, and outcompeted native vegetation. The sharp spines also posed a significant injury risk to humans and animals. Early control attempts, including mechanical removal and chemical treatments, proved largely ineffective and very costly.
A turning point arrived with an innovative biological control program in the early 20th century. Following research, the Cactoblastis cactorum moth, native to South America, was introduced to Australia in 1926. Its larvae specifically feed on prickly pear, consuming the plant’s internal tissues. This biological agent proved remarkably successful, decimating large infestations. By 1933, an estimated 80% of infested land in Queensland and 50-60% in New South Wales had been cleared, establishing this as one of the most successful examples of biological pest control globally.
Cacti in Modern Australia
While major prickly pear infestations were largely controlled by the Cactoblastis cactorum moth, many Opuntia species and other introduced cacti persist. These plants often exist as isolated populations or escapees from gardens, continuing to pose a threat. Various cactus species are now classified as noxious weeds or restricted plants under state and territory legislation.
This legal framework prohibits or heavily regulates the sale, cultivation, and movement of many cactus species to prevent further spread and environmental harm. Most Opuntia species are declared Weeds of National Significance, with a few exceptions like the spineless prickly pear (Opuntia ficus-indica) permitted under controlled conditions due to its less invasive nature. Ongoing management efforts focus on containing existing populations and preventing new introductions.