We’ve spent some time discussing invasive plants, and their impact on the various environments and ecosystems around the world. We’ve also talked about invasive insects and the problems they can cause. For this blog, we’d like to take a look at a particular case study that brings the two together. Opuntia spp, the prickly pear cactus family, was introduced to Australia in the late 18th century and since then has caused a host of problems across that country. Methods of controlling it led to some issues of their own, and the whole thing functions as a cautionary tale about the risks and dangers posed by invasive species and how we as humans both create and solve those problems. So, join us as we dig in and learn a bit more about the history of prickly pear cactus in Australia.
Gardzen Executive Summary
· This blog is a relatively short summary of some very complex events, so please bear that in mind whilst reading it.
· Opuntia spp are commonly referred to as the prickly pear cactus, albeit of varying species. All species are native to the Americas.
· Opuntia was introduced to Australia in the 18th century and quickly became an invasive.
· Efforts to control the spread of prickly pear in Australia proved futile until the introduction of the Cactoblastis cactorum moth in the 1925.
· The moth was successful in controlling the spread of Opuntia in Australia.
· Similar efforts to use Cactoblastis cactorum to control cactus populations in other parts of the world caused problems.
· There are many lessons to be learned her about introduced species, invasive species, and the ways in which we try to control them.
Opuntia in Australia’s Unique Ecosystem
The first British settlers introduced prickly pear cactus to Australia in the late 18th century—the plant came over with the First Fleet in 1787. The idea at the time was to establish Opuntia as a way to support cochineal insects, which could in turn be used to make red dye for the colony’s nascent textile industry. Colonists from the British Isles cultivated the plant intentionally both as an agricultural crop and as a fast-growing ornamental.
As many of us may know, Australia’s ecosystem is unique. Isolated from the rest of the world for thousands of years, its plants and animals evolved along different lines. There are no native cacti in Australia, and thus no animals that eat cacti plants. Without natural predators, prickly pear spread fast. Though it lacked predators for the plant itself, the fruits were widely consumed and the seeds thus spread by wildlife. Commercial and ornamental cultivation also continued, and the plant continued to spread. While prickly pear many times appears as a low-lying shrub-type cactus, it can grow into a large sprawling plant, a tall tree-like cactus, or both depending on the circumstances.
Rapid, sprawling growth and widespread distribution quickly caused problems, rendering vast swaths of country unusable. Farmland, wilderness, and even entire settled communities were swallowed by prickly pear and entire towns had to be abandoned.
Government Action to Control Prickly Pear
At one point, around 15,000 square miles/40,000 square kilometers of Australian farmland had been rendered unusable and inaccessible due to the invasive cactus. The government took action, beginning in the 1880s with a series of acts and laws designed to control prickly pear and reverse its spread.
These efforts ran into some difficult challenges. Prickly pear is a tough plant. Efforts to remove it via controlled burns were mostly futile. Removal by hand worked, but disposing of the detritus proved problematic; prickly pear doesn’t burn when and when buried it tends to take root and grow into a new plant. The herbicides and other chemical controls available in the early 20th century were dangerous to humans and wildlife, difficult to use, and required repeated applications to be successful. While things have improved since then and better chemical controls are now available, at the Australian government simply wasn’t getting the results they needed. The issues persisted, even after the Australian military got involved, going to battle armed with tanks and flame throwers to little effect.
The problem wasn’t going away, and it was time to think outside the box. So when a possible solution presented itself, Australia jumped at the chance.
How Cactoblastis Moths Saved Australia
The moth Cactoblastis cactorum is native to a small part of Argentina. In its native territory it evolved to eat Opuntia cacti among other things. More specifically, Cactoblastis moths lay their eggs on Opuntia species and the larvae live out the first few steps in their life-cycle on the cactus, eating the plant as they go. Scientists and agronomists in Australia believed that the moth could be used as a natural biological control. While there was some reluctance about introducing another non-native species, Cactoblastis cactorum was introduced to Australia in 1925. Large-scale operation for breeding and releasing the moths sprang up, distributing the insects throughout the affected areas so that they could hopefully reverse the negative impact of prickly pear cactus.
The program proved hugely successful, with the last extensive areas of Opuntia infestation eradicated by the mid 1930s. Farmland could now be reclaimed, families could return to abandoned homes, and a national nightmare had been mitigated. While prickly pear remains invasive across Australia to this day, it is greatly diminished from its 1920s peak.
If you’re in the right part of Queensland, there are two historic sights devoted to these events. There’s a monument to the Cactoblastis cactorum moth in Dalby, Queensland and in the rural settlement of Boonarga you can see the Cactoblastis Memorial Hall, which was built in 1936 to commemorate the moth’s role in controlling prickly pear. The hall is now used as a community venue and meeting place, and it’s the only building we know of dedicated to an insect.
Why Cactoblastis Caused Problems Elsewhere
Cactoblastis cactorum was hugely successful for Australia, which led to international interest in its use as a biological control for Opuntia. This in turn led to Cactoblastis species being introduced in the Caribbean, South Africa, and in parts of North America. The results were quite different: in South African the introduction of Cactoblastis led to a decline in populations of spineless Opuntia that had been useful as cattle fodder. The introduction of the moth into Mexico had a deleterious impact on the agave cactus crop. In the Caribbean, Mexico, and the USA the introduced moth began to prey on native cacti populations which caused environmental problems that are ongoing. In many of these regions control programs are in place to limit the spread of Cactoblastis cactorum, but the insect is expected to keep spreading.
An Ingenious Solution and a Cautionary Tale
The spread of both Opuntia and Cactoblastis cactorum provide a series of lessons. Introduced species, with the best of intentions, may later cause unforeseen problems. The methods used to manage invasive species may work well in one place but cause additional issues in another. It’s very difficult to predict what impact a human-introduced nonnative may have. Australia found an effective solution to a huge problem, saving thousands of square miles of farmland. The same technique used elsewhere caused a cascading series of problems across the Americas and South Africa.
We hope you’ve learned something from this. We certainly did in the process of researching and writing this article. We’re grateful to the libraries and archives across Australia that keep this history alive and the information available online. We also hope you’ll take some time to look at the links and resource we included here and learn a bit more about both the issues caused by Opuntia and about Australia in general!
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