North West Weeds 
          Your local guide to local noxious weed control (NSW, Australia)

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  PRINT VERSION    Identification │ The problem │ More information │ Control methods │Using pesticides │Disclaimer 

"Common pest pear" or “prickly pear” (Opuntia stricta and sub species) was the variety of pear that caused such widespread devastation in eastern Australia during the late 1800s and early 1900s.

Originally imported from the Americas during the early 1800s, prickly pear was widely promoted and grown as a source of stock fodder during periods of drought.  It didn’t take long for prickly pear to become established over large areas of New South Wales and Queensland.  The perfect climate and lack of natural enemies accounted for its amazing spread - still considered by many experts to be one of the botanical wonders of the world.

Common pest pear is still present throughout New South Wales.  Biological control agents cactoblastis and cochineal generally keep this pest under control – especially with a little bit of help from the land holder.  Photo (right) shows an area of common pear between Lightning Ridge and Collarenebri - this area has since been cleaned up by cochineal insects.  They usually work very well in this type of country because it's warm, dry and there are no cracks in the ground for them to fall into…

IDENTIFICATION

Common pest pear is a flat-leaved cactus plant, usually around 60 cm high but up to 1 metre.  The leaves, or pads, grow to about “the size of a man’s hand”.  Flowers are yellow, the prolific fruit purple when ripe.  Birds, foxes and pigs eat the fruit and spread seed over large areas.  New plants grow from seed or from pieces of the plant.  The two main growing periods for the plant are in February and October each year.

 THE PROBLEM

Common pest pear is an invasive plant.  It spreads quickly if not controlled and can completely take over a paddock.  The plant is “armed” with two sets of thorns:  the 25mm ones are very sharp, but they aren’t really significant.  The clusters of small, golden thorns are the ones that cause the problem.  The pattern on the leaf (Photo of leaf, courtesy NSW DPI) marks the clusters of thorns. 

Touch one of these clusters and you need tweezers to remove the thorns.  These tiny thorns penetrate tongues and mouths of animals tempted to eat the plant, as cattle and sheep often do when feed is scarce.  The thorns cause irritation and inflammatory sores.  Sheep brushing amongst the pear accumulate numerous thorns in their wool and hides, causing downgrading of the wool and discomfort to the sheep.  Of course, the shearers are never very happy about shearing these sheep either because the thorns transfer from the wool to the shearers!

CONTROL METHODS

PLOUGHING and cultivation takes care of prickly pear, but plants need to broken up and disturbed a number of times before they give up.  One run with a plough will multiply each plant by the number of plant parts left lying on top of the ground! The plant pieces need to be buried or badly damaged before they rot away completely.  [NB.  State laws apply in respect of land clearing.]

BURNING. It takes a lot of heat to kill a plant, especially one with a large butt still under the ground.  Plants pulled out of the ground are easier to kill.  Burning is really only appropriate for small infestations.  [NB.  Subject to compliance with local bushfire restrictions].

CHEMICAL TREATMENT of common pest pear is effective, because the plants are relatively easy to find.  However, there are plenty of seeds in the ground, and new plants will keep coming (it has been reported that the seeds are viable for up to 30 years).  According to NSW DPI’s excellent booklet “Noxious and Environmental Weed Control Handbook 2001/2002”, and other sources, herbicide treatments include:

 IMPORTANT:  USE OF PESTICIDES – ALWAYS READ THE LABEL

Pesticides must only be used for the purpose for which they are registered and must not be used in any other situation or in any manner contrary to the directions on the label.  Never use a herbicide in any way contrary to the label recommendations.

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BIOLOGICAL CONTROL is a good control option for large areas of pear, especially in the dry and warmer areas of the State.  There are two biological control agents for common pest pear – cactoblastis and cochineal:

 1.      CACTOBLASTIS (Cactoblastis cactorum) has always been the main biological control agent for common pest pear and they're still out there doing a great job in many areas.  Cactoblastis moths fly from plant to plant, laying their eggs.  These eggs hatch into larvae (black & orange striped “grubs” up to about 25mm long) which eat their way through the plant, hollowing out the stems and leaves (see photo, right).  Cactoblastis are capable of knocking over a large plant in a few weeks.  Sometimes they will destroy 100% of the plant.

Assisting the further distribution of cactoblastis?  Stacking several plants together will provide a better target for the moths to find, and a better food source for the hatching larvae.  Don’t make the stacks too big, because the lower plant material can begin to rot under the weight.  Shifting plants laden with cacto grubs from one site to another is usually not an option because the process exposes the grubs to attack by ants.

Remember, cactoblastis is not a totally reliable means of control.  Some years are better than others.  Sometimes ants and birds take their toll on the young larvae.  The main thing is to maintain a number of stacks in the core areas to encourage cactoblastis moths to pay a visit.

2.     COCHINEAL (Dactylopious opuntiae) insects are not as spectacular as cactoblastis but they often prove to be more reliable and effective over the long term.  Once established on individual plants, adult cochineal provide a continuous supply of new insects to attack the fruit, new growth and surrounding plants.  They are very effective on seedling growth (which is good because prickly pear seeds can remain viable in the ground for thirty or more years!).  So, even though the original parent plant may never give in to the cochineal, it is being controlled, and the insects are attacking surrounding plants.

Many write off cochineal, saying it's too slow.  The reality is that we have to give the tiny insects a hand.  Too often, the insects will work really well on one plant, and die of starvation before they find their way to a new plant (which may sometimes only be a matter of metres away.

Assisting the further distribution of cochineal?  Cochineal start their lives as tiny, crawling insects, venturing off to locate a new food source.  Their dispersal method includes being blown by the wind, which is great if the wind takes them to a new patch of common pest pear!  Their main means of travel is by crawling along the ground until they find a plant.

Cochineal insects can only travel along the ground for about 4 or 5 metres (or to the first large crack in the ground, eg as in black soil situations, into which they might disappear forever).  They need our assistance to get them into the pear areas.  A bucket , a pair of tongs and a bush knife (photo, left) is all the equipment needed to transfer infected segments into new plants. And, it's environmentally friendly!

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Other tips to help the cochineal? 

a)      Always remember that cochineal insects are very delicate.  They don’t like cold and wet weather.  When you put the insects into a new plant (especially in the approach to winter) give them some protection from the elements.  Place the infected leaves on their flat, with the main body of insects on the “underneath side”, to reduce their exposure to the elements.  It will also help if you cover that part of the plant with branches, bark, cow pads etc.

b)      Covering newly-infected plants with cardboard, plywood or even a bit of old corrugated iron will offer the cochineal insects maximum protection from the weather.  You’ll be surprised how well the insects build up in this situation (NB be aware that snakes may also decide to nest in these sheltered areas). 

c)      Another trick is to breed the insects indoors, in readiness for release in early summer, the optimum time for release.  Store infected plant material in cardboard boxes in a dry, warm area.  Over a period of 6 to 8 weeks, you’ll have good supplies to put out when the weather warms up.

USE THE RIGHT COCHINEAL…  Tiger pear, common pear and rope pear cochineal all look the same.  But, they’re each specific to their host plants.  Only common pear cochineal – Dactylopious opuntiae - works on common pear!

MORE INFORMATION

Cochineal.htm on this website.  Information on individual prickly pear species is also included in this website.  Return to Weeds List for rope pear, harrisia, velvety tree pear, prickly pear history etc. 

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DISCLAIMER:   The information contained in this web site is based on knowledge and understanding at the time of writing. However, because of advances in knowledge, users are reminded of the need to ensure that information upon which they rely is up to date and to check currency of the information with the appropriate officer of North West Weeds or the user’s independent adviser.  (LRT 28/06/06)
 

Send mail to lestanner@aapt.net.au with questions or comments about this web site.
Copyright © 2007-2008 Last modified: 22/06/08 - L R Tanner