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Biological Control in Australia

Australia has a long history of attempted biological control of pests.  The results of these vary from amazing successes to catastrophic failures.

Cats Released to Stop Rabbits

In the 1890’s 300 Cats were released in the west of South Australia in an attempt to stop the Rabbits invading Western Australia.

It definitely did not stop the Rabbits.  The people who released them must have been optimists if they expected 300 Cats to stop the advancing plague of Rabbits.

Some of the Cats would have starved.  Some would have gone wild, contributing to the population of feral Cats.  No doubt the Cats did kill a few Rabbits, but they would also have had much more effect in killing the young Goannas and other potential Rabbit predators, so the Cats would have contributed to the survival of the Rabbits.


European Carp were released by the government into the River Murray to control the plants growing there.  This attempt was successful.  The introduced Carp did eat a lot of Native water plants.  Unfortunately they also did enormous damage to the river’s ecosystem.  Now Carp are a major problem in the river, and governments are looking at ways of controlling them.

Carp have seriously disrupted the Ecosystem of our biggest river system.  In planning control methods we need to consider the effect on the whole ecosystem.

Mosquito Fish

Mosquito Fish, Gambusia affinis, and its relatives were introduced by the government to control Mosquitoes.  This was a dismal failure.  The Mosquito Fish is a poor eater of Mosquito Larvae.  Instead it has severely reduced the numbers of native fish which were good predators of mosquitoes.  It is now illegal to possess Mosquito fish in many areas.


A moth, Cactoblastis cactorum, was introduced in 1926 to control the introduced Prickly Pear.  This is described in more detail in the article about Prickly Pear.

This was perhaps the greatest success of all biological control attempts anywhere in the world. 

The moth had been extensively tested to make sure it would not harm any Australian native plants.

Cane Toad

Sugar Cane growing in Australia was being attacked by two native beetles, the Greyback Cane Beetle, Dermolepida albohirtum, and Frenchi’s Cane Beetle, Lepidiota frenchi.

The Cane Toad is native to the central portions of the two American continents.  It had been introduced to Hawaii to control scarab beetles in Sugar Cane.  Apparently it was successful at this.

Despite protests in Australia, the Cane Toad was released in the 1930’s after only about six months of controlled breeding and perhaps testing.

The Cane Toad is now a major pest in Australia.  No one appears to have even studied how successful it is in controlling the native pests it was introduced to eat.

Personally, I wonder if the amazing success of Cactoblastis was one of the reasons for the shortness of the period of testing the Cane Toad.  Another thing I wonder about is what sort of pressure the scientists testing the Cane Toad were under from government and business.

Biological Control of the Cane Toad

Now, having introduced the Cane Toad for biological control, scientists are looking at controlling the Cane Toad.

It was believed that all the parasites of this Toad had been left behind, but careful analysis was done of a lung worm attacking the Cane Toad.  This parasitic nematode worm turned out to be Rhabdias pseudosphaerocephala.  This is a South American species and is different from the similar looking worms attacking native frogs.

This worm kills about 30 percent of the Cane Toads, and the ones left are much smaller than unaffected Toads.

The Cane Toad is spreading rapidly across tropical Australia.  The bigger toads move faster.  This means that the parasite is being left behind.  One option is to take the parasite to infect young toad tadpoles at the front of the spreading toad plague.  If this parasite had not been accidentally introduced it would have had to be extensively tested before release.  If it could have infected native frogs, it could have caused serious problems of its own.

Native Meat Ants are a major predator of Cane Toads.  Simple ways have been found of increasing their effectiveness.

One feeble attempt to control Rabbits with Cats is mentioned at the start of this article.  The introduction of Foxes to Australia was not an attempt at the Biological control of Rabbits.  It was done about 20 years before there was a Rabbit problem on the mainland.  However, some of the secondary introductions might have been done in a misinformed attempt to control Rabbits.


In the late 1940’s, or possibly in 1950, Myxomatosis was introduced to Australia to control Rabbits.  This disease kills Rabbits slowly and with considerable suffering.  In 1963, the New Zealand government decided not to introduce this disease to their country. 

The head of the Victorian Royal Society for the Prevention of Cruelty to Animals in 1985, Peter Barber, stated that the public outcry against introducing this disease in 1985 would have prevented its introduction.  Attitudes had changed in the previous 40 or so years.  There were protests even in the 1940’s against the introduction.

Myxomatosis was successful in getting a temporary major reduction in the numbers of Rabbits, but the numbers have built up again.  There is very strong selective pressure on the Rabbits to evolve resistance to this disease in Australia.

The longer term effect of Myxomatosis may have been less favourable.  Foxes and Cats no longer had so many Rabbits to eat.  They ate more native animals, including the young ones of good Rabbit predators.  To set against that effect, there was chance for some regeneration of vegetation which would have been good for native animals.

However; we still have an ecosystem in much of Australia dominated by Rabbits, Foxes, Cats and introduced grazing animals.  In fact the domination of the introduced predators appears to have increased since the introduction of Myxomatosis.  In that sense it was a total failure.  In fact people have argued that it actually was responsible for the increased domination of these three species.
The use of native predators for Rabbit control is usually not considered.  However, encouraging native Rabbit predators like Goannas is much less likely to cause permanent harm than introducing a species from another continent.  The Goanna population has been seriously damaged by Foxes and Feral Cats eating the young Goannas.  The only place I know of where they killed all the Rabbits is on Kangaroo Island where there are no Foxes.
Probably better than just one predator being deliberately built up would be the encouragement of a range of them.   Wedge-tailed Eagles are a major Rabbit predator, but they were killed in the millions in the mistaken belief that they kill a lot of lambs. Quolls, especially the larger ones, are also good Rabbit predators. These have been greatly reduced in numbers by Foxes.


Related to this disease was the introduction in 1969 of the Rabbit Flea to help spread the disease.

Rabbit Calicivirus

Rabbit Haemorrhagic Disease appeared in China in 1984.  Together with many other agents this Rabbit disease was considered for importation to Australia.

In 1991 it was brought in.  It was tested under strict quarantine in Geelong.  This program was jointly funded by the Australian and the New Zealand Governments.

In March 1995 testing in a fenced enclosure at Wardang Island was commenced.  Wardang Island is 4 Kilometres off the coast of Yorke Peninsular in South Australia’s Spencer Gulf.  This island already had wild Rabbits, and had been used for testing Myxomatosis.

The testing in Geelong had suggested that the fence would be sufficient to prevent the disease getting into the wild population of the island.  However, on October the 1st 1995, it was realised that the wild population of the island was infected.  In accordance with the procedure originally worked out, the decision was made to exterminate all the wild Rabbits on the island.

But on October the 17th 1995 a Rabbit victim of the disease was found at Point Pearce on the mainland just opposite Wardang Island.

This is an extraordinary rate of spread.

How it Escaped

Of course, we do not really know how it reached the mainland so quickly.  The official belief is that it was carried by an insect like the bush fly.  Bush flies do not generally fly out to sea, but there are a lot of them.  It is likely that some are carried by the wind the 4 Kilometres to the mainland.


The official story is widely disbelieved in Australia.  Many people think that it was spread by a Human with malice aforethought.  That is, malice towards the wild Rabbits.

Before the release there had been public speculation that the government would decide not to release the virus. 

As I said, we do not know how is got out.  Either the scientists who set up the conditions of the experiment got it wrong, or a Human was involved.


Rabbit Haemorrhagic Disease was renamed Rabbit Calicivirus.  It was stated that death from this disease involved much less suffering than death from Myxomatosis. On average death is faster but people disagree about how much the Rabbits suffer.

In Australia the decision was never made by the government to release the virus.  Any public consultation was rendered irrelevant by the premature release.  Potentially this release was very dangerous because testing had not been completed.


As far as is known, Rabbits are the only animal that can get sick from Rabbit Calicivirus.  Even Hares appear to be immune.  Caliciviruses of other types infect many other species, including Humans.  There is no evidence that it is likely that the Rabbit Calicivirus will cross the species barrier.  It seems that we were lucky in this case.


The Rabbit Calicivirus is killing large numbers of wild Rabbits.  However, it is very much less effective than it would have been if testing had been completed and a proper plan worked out for its release.

In Australia

Like Myxomatosis Rabbit Calicivirus is reducing the numbers of Rabbits but proper management of the whole ecosystem needs to be done.  If Rabbits cannot be exterminated they need to be managed on a permanent basis.

New Zealand

In New Zealand careful consideration was given to importing the Rabbit Calicivirus.  It was decided not to bring it in.  In 1997 it appeared in the country.  It is almost certain that this was done deliberately.  The antisocial criminal responsible has not yet been caught.

It was deliberately spread by people who did not really know what they were doing and was very much less effective than it could have been.

Control of

Foxes are too close to Dogs for it to be safe to introduce any biological controls in the normal sense.  I hope no one has deliberately introduced Fox Mange to any area to control Foxes.  I have no evidence that this has happened, but some people do stupid things.

Fox Mange can spread to Dogs.  If your Dog has it, get advice from your Vet; it is treatable.  Unfortunately, Fox Mange is also killing Wombats and is putting even more pressure on these threatened animals.

Biological control of Foxes would have to be something specific to this species that could not spread to related species.  With diseases this is almost impossible to guarantee.

However, Foxes do not produce fertile babies if crossed with a dog.  So something regarding Foxes breeding is a possibility.  One thing that has been seriously suggested is to make a gene that allows the male Fox to breed, but means that he only has sons. 

If this daughterless gene could be introduced onto a wild population, at first there would be no change in the numbers, but after a while there would be a shortage of vixens.  The dog Foxes would move further afield looking for mates and the gene would spread through the population.  Eventually there would be a steady drop in Fox numbers without killing any Foxes, and with a minimum of cruelty.

The biggest cost of this would be the research to produce the daughterless gene.  Once it had been invented, the costs would be much lower.  Some of the research would improve our understanding of genetics in general.

It is interesting to speculate whether the Foxes would evolve an answer to this gene.  Personally I would predict that an answer would be evolved before Australian Foxes disapeared completely.

Another way that would does not require any research is simply to release Male Foxes that have been made infertile without removing their sex drive.  This is also a very long term solution, but if persisted in could exterminate all the Foxes on the mainland without killing any.  It would be an expensive solution, but Foxes cost the country hundreds of millions of dollars.  Eventually this cost would be gone.


Although only a few examples of biological control have been given here, it would appear that introducing any organism from overseas needs to be done with extreme caution.  Using native organisms is less likely to permanently disrupt the ecosystem.


European Carp
Cyprinus carpio
Mosquito Fish
Gambusia affinis
Cactoblastis catorum
Cane Toad
By Sam Fraser-Smith from Brisbane, Australia [CC-BY-2.0 (], via Wikimedia Commons
This file is licensed under the Creative Commons Attribution 2.0 Generic license.

Rabbit with myxomatosis
Shropshire, Eng;land
By ChrisJB at en.wikipedia (Transferred from en.wikipedia) [CC-BY-2.5 (], from Wikimedia Commons.
This file is licensed under the Creative Commons Attribution 2.5 Generic license.
Red Fox
Wedge-Tailed Eagle at Raptor World on Kangaroo Island.
Photo by Steve Challis
Rabbits on Wardang Island in 1938
In 1938, Wardang Island was used for a Myxomatosis trial.

From 1910-1968 Wardang Island was used as a source of lime sand
This picture was taken in 1925
Not even the closely related European Hare seems to get the Rabbit Calicivirus.
By Hans-Jörg Hellwig (Original photo: [1]) [CC-BY-SA-3.0 (], via Wikimedia Commons.
This file is licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license.
Dog Suffering from Mange
Dogs and Foxes are closely related and can generally suffer from the same diseases.
Photo taken in Indonesia byJack Merridew.