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
DESCRIPTION OF CACTOBLASTIS
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).
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.)
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
"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
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.
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
"Prickly-pear Pest in
NSW" by VH Gray, Prickly-pear Destruction Commissioner 1951-1979.
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
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