A recently completed project managed by CSIRO has renewed attempts at establishing parasitic flies in parts of South Australia, aiming to suppress Cochlicella acuta conical (or pointed) snail populations. Two additional new projects are under way to enhance the fly’s establishment in SA and introduce it into Western Australia.

Mediterranean snails create significant pre and post-farmgate costs for growers in affected areas, particularly during harvest, as contaminated grain can be downgraded at receival sites. Snails can also create reputational risks for Australian grain in international markets.

The newly completed project was not without its challenges, with researchers aborting attempts to introduce a promising strain of the fly due to its ability to parasitise native snails.

Growers use a range of physical and chemical (baiting) techniques to reduce snail numbers but, to date, the impacts of biological control have been very limited.

Biocontrol is an important component of integrated pest management. However, any new biocontrol species entering Australia must be carefully assessed to ensure it does not disrupt native ecosystems. The cane toad is an obvious example of what can go wrong.

GRDC pest manager Dr Leigh Nelson says Australia has strict biosecurity regulations for safety, including the potential release of biological control agents.

Background to biocontrol

About 20 years ago, a fly from France called Sarcophaga villeneuveana was released at 22 sites on the Yorke Peninsula and south-eastern SA as a biocontrol agent against the conical snail.

The fly lays maggots near an adult snail’s shell opening, which crawl inside and eat the flesh of the snail.

Although the fly from France established itself on the southern Yorke Peninsula, establishment studies found that after 20 years it had not spread widely and only had a small impact.

A recent discovery found that the conical snails in Australia mostly likely came from Morocco, Spain and Portugal, rather than France. Scientists thought that the low level of parasitism was due to a genetic mismatch between the fly and the snail.

In 2019, a GRDC-supported research program investigated whether the same species of fly from these countries would be more effective against the conical snails in Australia than the flies originating from France.

You can’t take any chances … you can’t bring anything bad into the country that poses a risk to native animals.

The project, ‘Snail biological control revisited – phase 2’, was led by CSIRO, in collaboration with the South Australian Research and Development Institute (SARDI) and the University Cadi Ayyad in Morocco.

“We had four populations in the laboratory: flies from Morocco, Spain, France and Australia, and we compared their effectiveness in killing Australian snails,” says project leader Dr Valerie Caron of CSIRO.

“They were all the same species of fly but came from different areas. We sent thousands of conical snails to France – Eskies full – so we could compare how efficient the flies were at killing snails,” Dr Caron says.

The fly strain from Morocco and Moroccan hybrids proved to be the most-efficient conical snail killers, she says. So these were brought to Australia to be tested on native snails in quarantine.

“It turned out that they were too efficient. The Moroccan fly strain was not specific enough because it also parasitised native snails under laboratory conditions, so we had to stop the project.”

While it is likely that the fly from Morocco would not encounter native snails in the locations where releases were planned, its preference to parasitise native species meant it would never pass the test for a release permit to be granted.

Dr Caron says native snail populations are important to ecosystems as they help break down organic matter. Although many of the native snails are forest dwellers, “you can’t take any chances … you can’t bring anything bad into the country that poses a risk to native animals”.

Back to the drawing board

After all the hard work, the flies from Morocco had to be destroyed in quarantine.

Dr Caron says the researchers decided to use the French fly that was already established on the Yorke Peninsula to target other conical snail hotspots.

SARDI was contracted to undertake work for the project, to assist in the spread of the French strain of fly.

SARDI scientists Drs Kym Perry and Kate Muirhead were tasked with breeding the flies and releasing them at target sites.

“We bred 50,000 flies in the laboratory over two years. There were hundreds of containers of flies,” Dr Muirhead says.

The mass-reared flies were released in main target areas – the western Eyre Peninsula and new parts of the Yorke Peninsula – and, more recently, south-eastern SA.

Native vegetation supports fly populations

Dr Muirhead says the fly populations that established on the Yorke Peninsula after the initial release two decades ago tended to be more prolific where there was native vegetation.

“As part of the Phase 2 project, we did detailed work on the Yorke Peninsula to quantify the impact . The fly was not doing very well in areas where there was not a lot of vegetation nearby,” she says.

“But it was doing exceptionally well, with up to 50 per cent parasitisation, in areas with native vegetation with flowers in spring and summer. We hypothesised that flies need the resources from flowers – nectar and pollen – to perform really well. We wanted to investigate that.”

As part of a new, small project, collaborators are establishing monitoring and demonstration sites with plants endemic to the Yorke Peninsula. They will release flies in areas with and without native vegetation. The scientists aim to compare the impact of flowers on flies and parasitisation of two species of conical snails.

The native vegetation project is supported by the South Australian Grain Industry Trust (SAGIT), Viterra and GRDC. The work is a collaboration between SARDI, the University of Adelaide, Trees for Life, Viterra and Grain Producers SA.

“Trees for Life identified a suite of plants endemic to the Yorke Peninsula that flower from spring until the end of summer. They need to produce flowers that are accessible to flies, which have short mouthparts,” Dr Perry says.

“Trees for Life grow tubes of plants, including trees, shrubs, ground covers and daisies. In August, a busload of about 40 volunteers from Viterra and Grains Producers SA helped us plant native vegetation at Port Giles and Ardrossan,” he says.

“It’s good news: our discoveries are promoting revegetation in some areas that are devoid of native vegetation.”

The project began in early 2022, and results should be available by the end of the project in September 2025, but the revegetated areas can serve as demonstration sites for a much longer timeframe.

Project to continue legacy

GRDC is also investing more than $2.8 million in a new four-year project called ‘More effective control of pest snails in Australian grain crops’ led by the University of Adelaide. It aims to develop multifaceted approaches to combat four species of pest snails, including the use of biocontrol.

Collaborators include the University of SA, SARDI, CSIRO and the WA Department of Primary Industries and Regional Development (DPIRD). The value of the whole project is just over $5 million when in-kind contributions from GRDC and collaborators are totalled.

Biocontrol is one small component of the new holistic project. Researchers intend to establish local breeding populations of the fly during this summer and release it in south-western WA for the first time. If successful, it will be another helpful tool in the box in the quest to suppress conical snails.

More information: Valerie Caron, Valerie.caron@csiro.au