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Cactoblastis
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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 (attached to a pad of prickly pear, 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 extracts 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 - as in this image on the right. 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 CACTORUM

In 1920 the then Commonwealth Prickly Pear Board sent entomologists (including one 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. The whole operation was a massive project, with the outcome a credit to the hard work and perseverance of all those people involved. (Learn more on this project!)

DESCRIPTION OF CACTOBLASTIS

The life cycle of Cactoblastis cactorum goes through four (4) stages: moth, eggs, larvae, cocoon and back to moth. In eastern Australia, there are usually two (2) complete generations per year.

(1) Moth

The moth is plain, grey-brown in colour. The female is much larger than the male. Please click on image, right, for a closer look.

The female cactoblastis moths can cover large areas when it is time for them to lay their eggs. They favour the common prickly-pear plant species, but if desperate their alternate targets may include tiger pear (Opuntia aurantiaca) or velvety tree pear (Opuntia tomentosa).

(2) Egg

The moths lay their eggs on the side of the plant - often attached (carefully, no doubt) to the spines on the plant. The eggsticks average around 25mm long and contain about 50 eggs.

(3) Larvae

All the eggs in the one eggstick hatch at the same time. The tiny larvae move to a nearby point on the plant and then actually work as a team to "chew" a small hole through the tough, outer layer of the plant leaf: they 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 could 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, causing varying degrees of damage to the host plant depending on their numbers and the season. They can devastate a large plant in a matter of weeks! (The image on the right is a good example of a prickly-pear plant completely wiped out by cactoblastis.)

(4) Cocoon

The larvae grow to a length of about 25mm by maturity. When it is their time, they actually leave the plant and find shelter under loose bark, dead leaves or other rubbish at or near the base of the plant. They then enter their cocoon stage.

Eventually, they emerge as moths to start the whole cycle again.

[NB Another, similar-in-appearance, larvae can sometimes be found. Slightly smaller, and a paler version of cactoblastis, Tucamania tapiacola was also introduced into Australia "in the old days" to help control prickly pear. It never had the same impact as cactoblastis, but it's still out there, persisting, and annoying the prickly pear.]

Cactoblastis have their enemies. Ants are the biggest threat, especially when the larvae 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. Birds sometimes cause problems. Extremely hot weather can take its toll too.

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 sp.). 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). In a good season, cactoblastis assists with control of velvety tree pear by attacking seedling growth.

MEANS OF DISTRIBUTION

Because the moths are so mobile, they can fly to prickly pear plants kilometres away to deposit their eggsticks. Depending on plant numbers, this works quite well. 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. Donít 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 piece of paper and pinning the paper to the new plant. 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, common prickly pear. Also please check out Prickly Pear History to learn about the amazing role played by cactoblastis in controlling prickly pear in E. Australia

ACKNOWLEDGMENTS:

"Prickly-pear Pest in NSW" by VH Gray, Prickly-pear Destruction Commissioner 1951-1979.
"The Biological Campaign Against Prickly-pear" Alan P. Dodd (1940)

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 userís independent adviser.
LRT 31/8/2009

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