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Weevils lead biological battle at Muddy Lake

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The NSW Government and Lake Macquarie City Council are joining forces to help stop one of Australia’s worst weeds invading a high conservation area.

The two organisations will collaborate on a project to control the unwanted Salvinia, which has been growing on the 114-hectare Muddy Lake since late 2020.

As a result of its rapid evolution, it’s infested up to 25 hectares of the waterway.

Salvinia weed at Muddy Lake.

Muddy Lake is largely disconnected from Lake Eraring and the broader estuary where higher salinity levels naturally occur.

Mangroves and sediment have created a barrier to all but the highest tides which, together with higher-than-average wet weather in 2022/23, have reduced salinity levels and enabled Salvinia – a Weed of National Significance – to grow.

It can double in size every three days in ideal conditions and forms a dense mat on the water, blocking light and oxygen and threatening ecological health.

Tiny weevils feeding on invasive salvinia weed at Muddy Lake.

But, to combat the hazard, a tiny weevil (Cyrtobagous salvinia) has been released by council as a biological control agent.

Already, they’ve achieved strong outcomes by destroying the weed in the south-east of the lake.

The adult weevils feed on Salvinia and their larvae are tunnelling into the plant’s stem causing it to lose buoyancy and sink.

Large areas of the lake are now visible and many water birds have returned.

“It’s wonderful to see the efforts of this partnership between council and Crown Lands paying off,” Lake Macquarie City mayor Kay Fraser said.

“Muddy Lake looked more like a paddock than a waterway for a long time due to the Salvinia infestation.

“So, it’s very pleasing to see it returning to a much healthier state. 

“We’ll continue looking at innovative ways to protect and preserve our environment.”

Lake Macquarie City Council biosecurity officer Brad Shultz and Crown Lands senior natural resource management officer Hannah Curtis inspect the salvinia infestation at Muddy Lake.

It’s a sentiment shared by Lake Macquarie MP Greg Piper.

“Muddy Lake is home to wetlands, water birds and green and golden bell frogs, so it’s an important ecosystem that we want to keep healthy,” he said.

“This joint project will help us better understand the causes of the weed outbreak and, hopefully, lead to more ways to better manage it long-term.”

While the weevils are working well, environmentally-friendly and low cost, they are not enough to eradicate Salvinia, which could again spread as the warmer weather arrives leading into summer.

The weed only survives when salinity levels dip, so natural saltwater flushing of Muddy Lake could potentially help control the weed.

However, the lake then faces salinity issues.

Crown Lands and council are working with the University of NSW’s Water Research Laboratory to investigate potential options to reinstate tidal flows at Muddy Lake.

Other possibilities to control Salvinia include manual removal of the weed, which is expensive and ongoing, or the use of herbicide, which is not supported due to environmental impacts.

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