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CSIRO researchers are continuing work on a new research initiative aimed at stopping the hop of cane toads (Bufo marinus) across Australia. With the toad creeping west and southward, scientists are fighting back with cutting edge genetic technology.

History

Cane toads were deliberately introduced to Australia from Hawaii in 1935 in an attempt to stop French’s Cane Beetle and the Greyback Cane Beetle from destroying sugar cane crops in North Queensland. The Australian Bureau of Sugar Experimental Stations made the release of 101 cane toads at Gordonvale in Queensland in 1935. They were unsuccessful in controlling the cane beetles.

Since then, the cane toads have spread rapidly, south into New South Wales, with one isolated community in Port Macquarie, and west into the Northern Territory. In March 2001 they reached the wetlands of heritage-listed Kakadu National Park.

Toad Facts

Habitat

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The native habitat of cane toads is in Central and South America. They are found in sand dunes, coastal heath, mangroves and around rainforests. In Australia, most cane toads are found in urban areas, and in areas with grassland or woodland. They are basically a terrestrial animal but require access to water for rehydration and breeding.

Appearance

Cane toads are large, heavily built amphibians with a dry and warty skin. Their colouring ranges between grey and olive brown and their belly is pale with dark, irregular spots. Average-sized adults are 10-15 cm long, but they can grow up to 23 cm or more.

Breeding

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They breed in still or slow-flowing water often tangling the spawn around rocks or water plants. The appearance of cane toad spawn is unique in Australia and consists of long gelatinous strings with double rows of black eggs. Females lay 8,000 to 35,000 eggs at a time and usually breed twice a year. The eggs hatch in 48-72 hours into tadpoles. They develop into toadlets in between 17 days to 6 months. Cane toads need between 6 and 18 months to reach sexual maturity and have a lifespan of about 5 years.

Toxicity

Cane toads have large swellings on each shoulder, the parotoid glands, from where poison is squirted when threatened or handled roughly. They are toxic in all their developmental stages: eggs, tadpoles, toadlets and adult toads. The venom contains 14 different chemicals causing rapid heartbeat, excessive salivation, convulsions and paralysis. No humans have died in Australia from cane toad poison.

Cane toads have no known predator in Australia, with the possible exception of keelback snakes. Freshwater crocodiles, goannas, tiger snakes, dingos and western quolls are known to eat cane toads, but have died from the venom secreted by the toad. Some animals turn the toads on their backs and attack the soft belly, which is only mildly poisonous.

Environmental impacts

Cane toads are not officially recognised as a threatening process in Australia, because not all States consider toads to be a problem. Only animals that are of national significance are officially recognised as pests.

Although no extensive environmental monitoring studies have been undertaken, there is evidence of the environmental impacts of cane toads. A decline in quoll numbers and native frogs in areas where large numbers of cane toads are found has been recorded.

Cane toads eat mainly insects, but will eat any small creature that fits in their mouth. They also eat honey bees and are likely to compete for food with native animals. In addition they may carry diseases that could be transmitted to native frogs and fishes.

Current control activities

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Different control methods for pest animals include conventional control techniques and biological control agents. The former have highlighted concerns in the community to develop more humane control methods of pest animals.

Current control activities are mainly taking place through quarantine checks and public awareness and response. For example, the importation of cane toads into Western Australia is illegal and authorities warn travellers to check carefully that cane toads do not hitch a ride on their vehicles from elsewhere.

There is currently no effective control method that can be applied to the vast area where cane toads have spread. In some areas, bounty systems have been established with community involvement. Such systems carry with them the danger of incorrect identification; at times two-thirds of animals brought in turn out to be harmless native frogs. Although bounties can provide local respite in the short term, they have not proven to be successful in sustainably managing cane toads in the long term or over large areas.

CSIRO scientists are working with gene technology to find a biological control method. Their research is described in the fact sheet CSIRO Cane Toad Research. Scientists of the University of Canberra are studying the environmental impacts of cane toads on native fauna in the Northern Territory. Scientists at the University of Adelaide are trying to find a sex pheromone in cane toads that may be used to disrupt their breeding cycle.

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Sources
M. Tyler (1975) The cane toad (Bufo marinus).

Background

From 1991 to 1997 the Australian Government, through CSIRO, undertook major research on cane toads with a view to discovering methods of control. The more obvious potential control mechanisms, such as a cane toad virus from Venezuela, proved unsuitable in an Australian environment because the virus also killed native frogs.

The work also suggested that although cane toads appear to be an important vertebrate pest, it proved difficult to assess how much impact they have on other species and on Australian ecosystems.

Since 2001 CSIRO has been conducting research into developing a biological control for cane toads. The goal of the research is to interfere with the metamorphosis of the cane toad to prevent it from maturing and reproducing.

The concept is based on research that was done a few years ago in the US on bullfrogs, where inoculation of tadpoles with adult haemoglobin interfered with metamorphosis. It is considered that the mechanism of interference works through the mediation of an antibody.

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Toad genes

The first objective of the research is to identify a gene critical to toad development and the initial focus is on genes critical to metamorphosis. If this gene can be manipulated, it would possibly interfere with metamorphosis, and prevent the transition from tadpole to adult toad.

If this gene can be expressed early in the tadpole stage, the tadpole should see the gene product as a foreign body and initiate an immune response against it. Such a response should interfere with metamorphosis and prevent the toad from maturing and reproducing. The gene must only occur in cane toads and not in other animals.

Virus ‘taxis’

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The second objective is to develop a means of delivering the gene effectively throughout the toad population. To deliver the gene across the wide geographical range of the cane toad, an efficient means of delivery is needed.

One way to do this is to use a virus that can act as a ‘taxi’ in delivering the gene that interferes with metamorphosis. Australian ranaviruses are naturally-occurring viruses that can infect amphibians and fish. Researchers are working on weakening (attenuating) a ranavirus so that, if infected, other non-target amphibians and fish will not suffer from its effects. Toads themselves will be affected by the response to the toad-gene carried by the virus, rather than by the weakened virus itself.

Results to date

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So far researchers have selected several genes that could be used to interfere with the metamorphosis from tadpole to adult cane toad. They are currently looking at how specific these genes are to cane toads.

Researchers are also working on creating recombinant viruses. Because ranaviruses are large, double-stranded DNA viruses, a viral gene (or region) can be deleted and the development-controlling gene inserted in its place. Currently the selected ranavirus is being weakened and its effect tested on at least one amphibian species, as well as its capability to replicate itself.

Risk assessment
There are no plans for field trials at this stage and current research is conducted within a microbiologically secure facility. Before field trials are considered many processes must be undertaken including testing for non-target species to ensure other animals are not affected by the proposed biocontrol, as well as Government and public consultation. In particular the question of non-target testing is critical and intensive consultation is required before this can be attempted.

Even if all the research goes as planned, it could take up to 10 years before a product is available for intended release.

Future steps
To identify potential hazards of the technology a wide program of consultation with community groups needs to be undertaken and must be conducted within the guidelines set out by the Office of the Gene Technology Regulator (OGTR).

Future steps will involve scientists eliminating as many of these identified potential hazards as possible, and those that cannot be completely eliminated will be further subject to a rigorous risk assessment process and minimised as far as possible. This whole process will be targeted towards developing a genetically modified biological control that is both effective and safe and suitable for field trials, subject to permission by the Gene Technology Regulator.

What is the current geographic range of the cane toad in Australia?

Since their introduction in North Queensland, cane toads have spread rapidly, south into New South Wales, with one isolated community in Port Macquarie, and west into the Northern Territory. Their most recent sighting has been at Berry Springs on 3 April 2003 in the Northern Territory.

The map below provides an overview of where cane toads are currently sighted (August 2003).

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This article is reprinted with kind permission from the CSIRO.    The original can be found here
http://www.csiro.au/index.asp?type=faq&id=CaneToadControl 

Enquiries to the CSIRO can be made by mail at:-  

Bag 10, Clayton South, VIC 3169
or by ‘phone 1300 363 400 (National local call)
and fax 61 3 9545 2175
email enquiries@csiro.au