North West Weeds 
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Origin│ Description │ When to use │How to distribute│ More information │ Disclaimer

Biological control of prickly pear has been one of Australia's success stories when it comes to weed control.  And, biological control continues to play a very significant role in controlling prickly pear species in New South Wales, particularly in the drier, warmer areas.

The two main biological control agents are  CACTOBLASTIS (left) and COCHINEAL (right).  The two are entirely different:  Cactoblastis (Cactoblastis cactorum) is a black and yellow striped "grub" that tunnels into and devours the inside of the plant.  Cochineal (Dactylopius spp.) on the other hand, attaches to the outside of the plant and (like a mosquito) inserts its mouthpiece and literally sucks all of the moisture out of the plant.

The Cactoblastis success story
By the year 1920, prickly pear (mainly Opuntia stricta spp.) infested 58 million acres (23 million hectares) of land in New South Wales and Queensland.  It was spreading at an alarming rate of 1 million acres a year when the Cactoblastis insect was first released.  Within six (6) years most of the original pear had been destroyed. It was an amazing result. (See Prickly Pear History)

The success of cactoblastis is still regarded as the world's most spectacular example of effective control of a weed by biological means.

The origin of Cactoblastis
In 1920 a Commonwealth Prickly Pear Board sent entomologists (including one Dr Alan P. Dodd) to America (that's where the pear came from originally) to seek suitable biological control agents.  Out of 150 different species, 12 were brought to Australia to undergo strict breeding and feeding evaluation to ensure they would not impact on other plant material.  By 1926, Cactoblastis was approved for release.

DESCRIPTION OF CACTOBLASTIS

The life cycle of Cactoblastis cactorum goes through the stages of moth, eggs, larvae, cocoon and back to moth.  There are usually two (2) generations per year.

Moth The moth is plain, grey-brown in colour.  The female is much larger than the male.  Click on photo, right, for a closer look.

Eggsticks The moths fly from plant t plant, laying their eggsticks usually on the undersides or protected parts of the plant leaves/pads.  Each eggstick is about 25mm long and contains about 50 eggs.

Larvae When the tiny larvae hatch, they move to a nearby point on the plant and then work as a team to "chew" a small hole through the tough, outer layer of the plant leaf/pad.  The tiny larvae "form a circle" and take turns to nibble at the selected site until eventually they work their way through the outer skin and into the soft, edible interior of the plant. As one would imagine, the tiny larvae work quickly to enter the plant as quickly as possible - while out in the open they are very vulnerable to ants, birds and even bad weather!

The larvae feast their way through the plants for some weeks before retiring.  By this time they are 25mm long.  They actually leave the plant and shelter under dead leaves or other rubbish at the base of the plant.  They then spin their cocoons, later emerging as a moth to start the whole process again.

Cactoblastis have their enemies.  Ants are the biggest threat, especially when they attempt to move from one plant to another.  For this reason, the winter generation of Cactoblastis seems to have the best results in northern New South Wales because of the reduced ant presence.

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WHERE AND WHEN TO USE CACTOBLASTIS?

"Where" relates to how much common pest pear there is.  If you only have a small number of plants and you want to keep your property really clean, don't rely on insects.  On the other hand, if you have pear over a large and/or inaccessible area, biological control is an excellent, cost-effective and long-term option (but not if your property is located in a cool climate).

"When" to use Cactoblastis?  There are two cycles per year, so there is no real time barrier.  The main thing is to keep a number of prickly pear stacks around the property to encourage the moths to call and deliver their eggs.

Cactoblastis is mainly used for control of common pest pear (Opuntia stricta spp.).  In some seasons they will also significantly reduce the bulk of individual tiger plants (very rare for them to destroy the whole plant, and unless cochineal is also present, the cacto often creates a bigger mess because there are more loose tiger pear segments on the ground).  Cactoblastis assists with control of velvety tree pear by decimating seedling growth.

MEANS OF DISTRIBUTION

Because the moths are so mobile, cactoblastis eggsticks can be distributed to prickly pear plants kilometres away from known insect populations.  However, you can assist the process by making a series of stacks (each several plants together, on rocks or logs but even two or three thrown on top of a large plant) to 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.

Physically transferring pads of prickly pear containing larvae to un-infected plants works to some extent but ants can be a problem.  Ants often attack the vulnerable larvae through the broken, open ends of the pads.

Transferring eggsticks to new plants was the method originally used to distribute cactoblastis to new areas.  The job involved pasting the eggsticks to small pieces of paper and attaching them with a pin to new plants. We find that, generally, this is unnecessary because the moths are periodically moving through the pear areas and will lay their eggs on new plants if they find them.

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MORE INFORMATION

Cochineal is the other valuable prickly pear biological control agent.  Also please return to Weeds List to access information on biological control for individual plant species like tiger pear, velvety tree pear, prickly pear.  Also please check out Prickly Pear History.

ACKNOWLEDGMENTS:

"Prickly-pear Pest in NSW" by VH Gray, Prickly-pear Commissioner 1951-1980.

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 11/11.2003

Send mail to lestanner@aapt.net.au with questions or comments about this web site.
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