The Common Prickly Pear

Widely acknowledged as one of the greatest biological invasions of modern times, the introduction and subsequent spread of prickly pear into Queensland and New South Wales infested millions of hectares of rural land by the 1920s, rendering it useless for agriculture. Prickly pear proved so difficult and costly to control by chemical and mechanical means, that enormous areas were completely abandoned by their owners. The eventual control of this pest mainly by one insect, in the space of a few short years, lead to further successful developments in the field of biological pest control.

“The speed of prickly pear is like the invasion of a dangerous enemy, advancing slowly but steadily, and gradually taking possession of the continent. At the outset the foe was met with knives and spades; at the present time it is usually fought by the employment of poison sprays and gas, whose utilisation is due to scientific research. We are, however, not holding our own in the contest: the prickly pear still advances.”

Opuntia stricta is a shrubby, low-spreading to erect, evergreen cactus, forming part of the Cactaceae family (figure 1). Usually growing 50-100 cm tall, the stems are multi-branched and consist of a series of flattened, fleshy cladodes. Opuntia stricta has bright yellow flowers and pear-shaped fruit. The immature fruit is green, however they turn a reddish-purple colour as they mature.

The first recorded introduction of prickly pear to Australia was traced to Governor Phillip at Port Jackson in approximately 1788. It is thought that the shipment consisted of the drooping tree pear (Opuntia monacantha) and possibly one or two other species from Brazil. A potted plant was then brought to Scone, NSW, in 1839 and it spread rapidly north into QLD.

The first species of Opuntia was introduced to establish a cochineal dye industry. At that time, Spain had a world-wide monopoly on the important cochineal dye industry and the British Government was keen to set up its own supply source. The cochineal insects fed off the pads of the prickly pear and were harvested by being squashed for the production of a scarlet dye. In the eighteenth century, the red dye derived from cochineal insects (figure 2) was very valuable for the world clothing and garment trade, with the dye being used to colour the British soldiers distinctive red coats. The plants were also introduced to be used as a natural agricultural fence (figure 3) with attractive flowers and tasty fruit high in vitamin C. The pads also made a good supplementary drought fodder for cattle because of their high water content. By the mid 1800’s, synthetic red dyes replaced cochineal, however prickly pear continued to thrive in parts of Australia.

The Australian habitat suitability, especially along the eastern seaboard, made it a perfect breeding ground for this invasive species (Figure 4). By 1925, Opuntia stricita had spread across 25 million acres of Queensland and NSW agricultural land, “spreading at a rate of 1 million ha per year”.

As noted in Figure 5, the area of infestation reported in 1925 was significant but we can see that pockets of Prickly Pear had spread towards the QLD coast and further inland within 40-50 years. While the areas of infestation was significant, Figure 6 indicates the potential pattern of distribution across the continent if not brought under control.

Opuntia stricta is found in every state (Figure 7) but is largely widespread throughout the eastern seaboard of Australia with scattered areas in other parts of the country. It is most abundant in central and southern QLD and northern NSW and relatively common in other parts of NSW. It is common in northern and western Victoria and from the Flinders Ranges south to the Murray Region in SA. It also occurs in the western and south-western regions of WA and in scattered locations in northern QLD and in the southern part of the Northern Territory. This invasive species is classified as a Weed of National Significance (WoNS) and must be controlled, eliminated or minimised under the Biosecurity Act 2015 due to the threat they impose to the natural environment. 

O. stricta is considered an environmental weed in many parts of the world. It is recognised as being among 100 of the "World's Worst" invaders by the IUCN Invasive Species Specialist Group and it has been listed as a noxious weed in most Australian states. The plants invaded a huge area very quickly due to their hardiness and ability to spread (Figure 8). Plant segments are easily detached from the parent plant by animals, wind or flood waters and establish where they remain. It is believed that the flood of 1893 spread seed and plant parts to many new areas, further spreading the plant.

These invasive cacti impact Australia’s environmental, agricultural and aesthetic values. The spines are capable of causing serious injury to humans, stock and native animals. Farmers have also encountered problems when stock roamed on pear infested land. Horses used for mustering cattle have had pear thorns pulled out of their legs which become septic and contaminate hides. Infestations can reduce or prevent grazing activities, also reducing productivity. Large cacti stands of cacti can harbour feral animals and vertebrate pests such as foxes, mice and rabbits, limiting access for stock mustering and recreational activities (Figure 9). Opuntioid cacti also attracts fruit flies impacting horticulture. O. stricta has the ability to out-compete all other vegetation. The invasion process is worsened by selective grazing of stock on the few remaining native plants which eventually results in monocultures of O. stricta, causing a dramatic loss of biodiversity. They damage environmentally sensitive areas, especially along watercourses. Opuntioid cacti can restrict access or completely impair landscapes and reshape the natural structure of the land. 

However, there are also multiple benefits to Prickly-pear cactus. One being its relationship with natural areas to protect soils from eroding. The eradication of the cactus will alter the nutrients available in soil profiles and affect microbial communities. This will then affect the composition and structure of other plant communities. Vegetation structure Opuntia have been identified as nurse plants, facilitating the establishment of other plant species by providing a more moderate (cooler, moister, shaded) and protected growing environment. Reduction of nurse plants can lead to lower forage availability, increased bare ground, and erosion in arid landscapes. Many species of bird (Figure 10), mammal, reptile, and insect species eat, nest and rely on Opuntia species. Direct and indirect losses of Opuntia food and nesting resources impact native species interactions, habitat structure (shade, shelter, nesting areas) and sustainability of desert ecosystems.

Questions and answers:

What allowed the Prickly Pear to spread so rapidly in Australia?

The accommodating climate of Australia and the general lack of natural enemies accounted for the Prickly Pear’s rapid spread – still considered by many to be one of the botanical wonders of the world. There are two main methods by which the pear spreads; the growth of new plants from joints or unripe fruits which have become detached from the parent plant, and by the germination of seed. Slabs and fruits are readily detached from the plants by animals and wind and may be carried considerable distances down streams in time of flood or by cattle travelling along the stock routes (Figure 12). Plant segments easily take root where they lodge. It is believed that the flood of 1893 spread seed and plant parts to many new areas. The heavy crop of fruit produced by pear plants is particularly palatable to birds such as crows, emus and magpies and are also eaten by cattle. The seeds pass through the intestines without injury and subsequently germinate in their droppings. 

How did Prickly Pear affect the biodiversity of the Australian desert ecosystem?

Invasive species are the second largest threat to biodiversity after habitat loss. Invasive species such as the Prickly Pear have had a major impact on Australia’s environment, threatening biodiversity, reducing overall species abundance and affecting the health of endemic species. This non-native species is an aggressive and vigorous grower and can overwhelm and out-compete local native species. This upsets the natural balance of the ecosystem and results in the loss of the native species and sometimes whole ecological communities, thereby lowering the overall biodiversity of the located area. Prickly Pear lowered the biodiversity so greatly that it created a monotypic community (Figure 13). By completely invading the land, the whole equilibrium of the ecosystem changed. This led to habitat modification as well as the depletion and replacement of food sources for animals who relied on other producer plants. Some species such as the Yellow footed rock wallaby were not able to exist in this altered landscape as available land was occupied, further affecting the biodiversity of the ecosystem as only few species were able to thrive in this environment.

Opuntia cacti has the considerable potential to markedly increase distribution density and significantly impact native species as it;

• It directly competes with native vegetation by limiting the growth of small shrubs and smothering the natural landscape. This results in monoculture by displacing naturally occurring species.
• Affects water availability and damages the quality of soil nutrients. Once the plant has invaded a habitat, it changes the conditions of that environment which can make the ecosystem unsuitable for other plants and animals.
• Threatens extinction to native plant and animal species.

Mostly, if Opuntia cacti continues to invade natural grass and shrub lands, coastal dunes, forests and riverbanks, it essentially removes that species from the environment and causes an unbalanced ecosystem (Figure 14). Rare and protected native animal species and native flora and fauna would become endangered to the point of elimination all together.

Early control methods, such as digging up and burning and crushing with rollers drawn by horses and bullocks, all proved inadequate to eliminate the problem. Early trials of chemical control of prickly pear confirmed that best results were achieved by using arsenic pentoxide (Figure 15). This chemical, although effective, was highly toxic, expensive and hazardous to operators and livestock. A shortage of supply of the chemical during World War I further limited its usage. Demands for the chemical led to mining of the essential component, arsenic, in Queensland at Jibbenbar (near Stanthorpe) and the development of a new industry. In 1901, the Crown offered a £5000 reward for the discovery of an effective control method. Regardless of an increase in reward to £10 000 in 1907, it was never invented. Other avenues for commercial use of prickly pear for paper, alcohol, dye, soap and oil were all investigated but without success.

In 1914, the Prickly Pear Travelling Commission imported specimens of several promising insect species. These included two species of the cochineal insect (Dactylopius ceylonicus and D. confuses) and the cactoblastis moth (cactoblastis sp). Encouraging investigative work with the cochineal insects led to a determination to pursue the concept of biological control. The first field releases of the cochineal insect were made in May 1914. Within three years of release, most stands of drooping prickly pear, found primarily in the Charters Towers, Townsville and Bowen regions were destroyed. The cactoblastis larvae failed to reach maturity, however, the moth was investigated again in 1924. Specimens arrived at Sherwood in May 1925 and rearing of the moths proved successful. 10 million eggs were distributed in 61 localities throughout affected areas during 1926 and 1927 and a further 2.2 billion eggs were released between 1927 and 1931. 

The eggs are placed onto the leaves of plants and the larvae fed on the pads and seed pods of the Prickly Pear (Figure 18), eating it from the insides out and leaving mere shells, destroying an entire plant in a matter of weeks (Figure 17). Once the larvae reach maturity, they fall from the plant and enter the cocoon stage. When the moth hatches, the whole cycle starts again. This is why it has been so successful, as it only takes a short period of time to eradicate large areas of infestation.

This insect proved to be spectacularly successful in destroying the weed. By 1932, the stem-boring cactoblastis larvae had caused the general collapse and destruction of most of the original, thick stands of Prickly Pear (Figure 19). By 1932, almost 7 million ha of previously infested land was made available to 1165 settlers. Townships that had been stagnant in the 1920s were revitalised; public buildings, offices, shops and residences were built. Cactoblastis achieved an impressive reduction of prickly pear species when conventional methods of control could not successfully manage the pest.

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