The Prickly Pear Problem
Prickly pear infestation
All major prickly pear species are
declared as "noxious weeds"
Cactorblastis caterpillars feeding
on a prickly pear plant
Scientists searched through Mexico, United States
and Argentina for insects which would feed on the
cactus and control it's spread. A shipment of 2750
eggs of the moth Cactoblastis cactorum was
received from Argentina in 1925. By 1926, 2.5
million eggs had been produced and 2.25 million
were released into the field. Mass destruction of
prickly pear was evident by 1930, because the
larvae of Cactoblastis (see diagram on left) were
eating the cactus plants.
It is not known how the prickly pear or common pest pear (Opuntia stricta spp first came to Australia from the Americas. It was first recorded as being cultivated for stock fodder in the Parramatta district in the early 1800's. There is also a record of a pot plant being taken to Scone, NSW in 1839 where it was grown in a station garden. The property manager later planted it in various paddocks with the idea that it would be a good stand-by for stock in a drought year. It has also been recorded that a plant of common pear was taken from Sydney to Warwick, Queensland in 1848 for use as a garden plant, with a strong recommendation that it would be a good fruiting and hedge plant!
Early settlers took plants to other parts of New South Wales and Queensland because of its potential use as an alternate food source for stock, especially during dry times. It was also planted at various homesteads as a hedge. The hedges flourished and bore fruit. Excess pieces were dumped in the bush. With all this help, prickly pear quickly established over a large area. From garden plants to hedges and then into the paddock, prickly pear became acclimatised and spread at an alarming rate. Many people were forced off their lands.
In1886 the first (Commonwealth) Prickly-pear Destruction Act was passed that placed obligations upon owners and occupiers of land to destroy pear. Despite this no real way to curtail the advance of the Prickly-pear had been found. By 1925, prickly pear had spread ruining 25 million hectares of farmland.
In 1924, New South Wales brought its own legislation into effect. The (NSW) Prickly-pear Act 1924 provided for the setting up of a Prickly-pear Destruction Commission, with wide powers to deal with the prickly pear problem. Many often draconian measures were taken but with little effect.
The answer to the main prickly pear problem came in the form of biological control. As the amazing spread of prickly pear in eastern Australia was considered to be one of the botanical wonders of the world, its virtual destruction by cactoblastis caterpillars (Cactoblastis cactorum) is still regarded as the world's most spectacular example of successful weed biological control. The first liberations of cactoblastis were made in 1926, after extensive laboratory testing to ensure they would not move into other plant species.
The moths laid their eggs on the prickly pear plants. When the eggs hatched, the moth caterpillars ate the plants. Within six years, most of the original, thick stands of pear were gone. Properties previously abandoned were reclaimed and brought back into production.