Alors! Our snails appeal to ze French flies by Denys Slee

Megan Leyson

NO, MEGAN Leyson isn't into pottery. In fact, the container she is holding is made of plastic and contains water to keep 200 flies refreshed — French flies, which are a natural parasite of conical snails.

Ms Leyson, research officer with the entomology unit at the South Australian Research and Development Institute (SARDI), was pictured on Yorke Peninsula late last year in a small tent where the flies were released.

The release was part of the establishment of three fly nursery sites on the Peninsula where the plan is to build up fly numbers so that Peninsula farmers can ultimately collect snails containing fly larvae and release them on their own properties.

However, it could be 5-10 years before it is known whether the flies are having an impact on conical snail numbers. Meanwhile the SARDI team will continue with the release program in the hope that this biological control measure will become part of an integrated approach to snail control.

The fly, Sarcophaga penicillata, was imported from France and, after quarantine specifications were met, bred at the insectary at SARDI. This species is quite snail-specific and therefore should not add to our fly burden or impinge on the Aussie barbecue.

Over the past two years, Ms Leyson and fellow SARDI officers have released flies at 17 sites on the Peninsula and established that they do persist and do kill conical snails which damage crops and pastures and contaminate grain at harvest.

"The flies parasitise the snails over summer and then go into a dormant stage during the cooler months.

"We will be monitoring the flies' activity at the nursery sites over 2002-03 and will then decide when it is appropriate to invite farmers to collect infected snails for release on their own properties. Depending on the success of the Yorke Peninsula program, it is possible something similar could be set up in other areas infested with conical snails. "

Ms Leyson said research into integrated control for conical and white snails, supported by the GRDC and the SA Grain Industry Trust, had established a number of key guidelines.

  • Snail cover in stubbles needs to be reduced. Cabling, where a heavy steel cable is strung between two vehicles enabling large areas to be covered in a short time, has proved effective. Cabling dislodges snails from straw and forces them to move over hot ground, causing dehydration. Cabling also exposes snails hiding under rocks, which is of particular benefit in areas with high densities of conical snails. Taking into account any erosion risk, burning of stubbles is also an effective control measure.
  • Trials and farmer experiences have shown that baiting in April-May, after rain and before snails begin breeding, can effectively reduce snail numbers. Snails larger than 7 mm have responded particularly well to early baiting. All evidence suggests that if no control measures are implemented by the end of autumn, it can be difficult to get good control in winter.
  • Remove weeds and trash around fencelines and roadsides to reduce the habitat for snails and limit their survival over summer and autumn.

"In 2002 many farmers tried baiting at 10 kg/ha along fencelines while baiting the rest of the paddock at 5 kg/ha, " Ms Leyson said. "The results have been promising with this investment in fenceline control aiding in snail control over the whole paddock. "

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Contact: Ms Megan Leyson 08 8303 9670