Tuesday, 3 April 2007

#14 Prickly pests

Australia has no native cacti, but cacti and introduced succulents are a common sight on roadsides and in rural and bushland areas of New South Wales and Queensland, and possibly beyond. Gardeners and early settlers have introduced these plants to the Australian countryside, and although some species can be attractive, they are all foreign to the Australian landscape.

Common Prickly Pear in rural Hunter Valley

But although they are foreign plants, they are well suited to the hot dry conditions of many parts of Australia, and therefore have adapted well.

Australian history of the Prickly Pear

Although the Smooth Tree Pear from Brazil, Opuntia vulgaris, was introduced to Australia by the First Fleet to establish a cochineal dye industry, the introduction of the highly invasive Common Prickly Pear, Opuntia stricta, has not been documented. The Common Prickly Pear overran NSW and Queensland between 1900 and 1930.

The Hunter Valley infestation was in part due to a pot plant of Opuntia stricta being taken to Scone where it was grown in a station garden. The property manager later planted it in various paddocks with the view that it would make a good standby for stock feed during times of drought. It was also grown for its fruit, and as a fence substitute.

Due to appropriate climate and lack of natural enemies, the Prickly Pear spread throughout NSW and Queensland at an alarming rate, forcing people from their land.

By 1925 it was spreading at a rate of 1/2 million hectares a year. In 1926 biological control by Cactoblastis caterpillars, Cactoblastis cactorum, was implemented with astounding success. Within 6 years most of the thick stands of cacti were gone, and abandoned properties were reclaimed. There is more information on the interesting history of Opuntia stricta here.

Flower buds of the Common Prickly Pear

As the introduced Cactoblastis caterpillar has a specialised diet, it has not had a detrimental effect upon the environment.

Prickly Pear characteristics

Prickly Pear species are shallow-rooted succulent shrubs with segmented swollen fleshy stems, but no leaves. The Common Prickly Pear has flat plate-like stems and clusters of bristles located at raised 'eyes' (or more correctly, areoles). There may, or may not, be one spine protruding up to 60mm from the 'eyes'. Here is a distribution map of the Common Prickly Pear.

Attractive flowers of the Common Prickly Pear

The Tiger Pear, Opuntia aurantiaca, is considered the most troublesome of all cactus species in NSW and Queensland. Here is a distribution map of the Tiger Pear.


All Opuntia species will reproduce vegetatively. Segments will take root from the 'eyes' if left in contact with the ground, and are capable of taking root several months after being detached from the parent plant. Movement in floods can lead to infestations along river banks.

The Tiger Pear in the images below had 15 young plants growing within a metre of the parent plant, with many plants within a few metres, indicating the ease at which it reproduces vegetatively.

Fruit of the Common Prickly Pear

Spines of the Tiger Pear have a slight hook at the tip which will attach to animals, vehicles and humans, resulting in transportation to other areas.

Tiger Pear growing amongst fallen timber

Opuntia species flower in spring and fruit in summer and autumn. Birds and animals spread seed from the Common Prickly Pear, but the Tiger Pear does not produce viable seed in Australia.

A noxious weed

Prickly Pear is at home on rocky slopes and riverbanks. It can dominate vegetation of rocky outcrops displacing native flora, some of which may be restricted to such outcrops, and consequently be relatively rare.

All Opuntia species except Opuntia ficus-indica (Indian Fig), are 'Class 4' Noxious Weeds in NSW and Queensland, meaning they must not be sold, propagated or purchased.

A flowering Tiger Pear

As segments of Prickly and Tiger Pear will take root if separated from the parent plant, this pest can not be left on the ground to rot after being dug up. Deep burial is one method of disposal. They can also be poisoned, but copious amounts of strong poison will need to be applied more than once, and this will be detrimental to the soil and surrounding vegetation.

I am going to dig several Tiger Pear plants from my roadside verge, place them in buckets in a sheltered position for several months, with the intention of disposing of them in the local landfill when they are completely dehydrated. I will update this blog with the outcome.

A local Opuntia species that I have not identified.

Weeds will always be a part of the ecosystem. Many species strangle and displace native flora, and ultimately cause the displacement of native animals and invertebrates that depend upon indigenous plants. It can be useful and rewarding to be able to recognise weeds in your locality, and to play a role in their control. Here is an easy to navigate weed identification site for those wishing to know more about common weeds.


Esperance Blog said...

A very good example of biological control Gaye, also educational and well presented. Well done.


Gaye from the Hunter said...

hi Jack, it is indeed a good example of biological control, and I have learned a lot researching the topic. I found it interesting.

Prickly Pear do not appear to be much of a problem in the Hunter Valley today, but Tiger Pear spreads virtually unnoticed hugging the ground amongst grasses. I imagine it also gets spread by council roadside slashing.


VanGoghFer said...

Although a pest in Australia, Prickly Pear Cacti are rare in America because the moth used to control them in Australia and South Africa was accidentally introduced here!

The "textbook example" of biological control has become a mess here in the states.