Weevils help battle Salvinia Weed at Muddy Lake

30 Oct 2023

A biological battle has been underway on the western side of Lake Macquarie as the NSW Government and Lake Macquarie City Council join forces to help stop one of Australia’s worst weeds infesting a high conservation area.

Crown Lands and Lake Macquarie City Council are collaborating on a project to control Salvinia weed that has been growing on the 114-hectare Muddy Lake since late 2020, infesting up to 25 hectares of the waterway.

Salvinia is a Weed of National Significance, and one of Australia’s worst for invasiveness, spread, and environmental impacts. 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.

At Muddy Lake, a tiny weevil (Cyrtobagous salvinia) has been released by Lake Macquarie City Council as a biological control agent in a bid to control Salvinia, which has been achieving strong results by destroying the weed in the south-east of the lake.

Weevils feeding on salvinia — Weevils feeding on Salvinia Weed

The adult weevils are feeding on Salvinia and their larvae are tunnelling into the plant’s stem causing the weed to lose buoyancy and sink. As a result, large areas of the lake are now visible and many water birds have returned.

Clear lake after weevil intervention — Less Salvinia at Muddy Lake after the introduction of weevils

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

Salvinia only survives when salinity levels are low so natural saltwater flushing of Muddy Lake could potentially help control the weed, however the lake faces salinity issues.

Muddy Lake is now 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 2022-2023, have reduced salinity levels and enabled Salvinia to grow.

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 potential options 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 potential impacts to the surrounding environment.

The joint project will help better understand the causes of the weed outbreak and hopefully lead to more ways to better manage it long-term.

Minister for Lands and Property Steve Kamper said Muddy Lake is home to wetlands, water birds and green and golden bell frogs so is an important ecosystem.

“Muddy Lake is on a Crown waterway in Dora Creek and Crown Lands is a member of the Hunter Regional Weeds Committee with a duty to help control priority weeds, so it’s great to see it working with Lake Macquarie City Council on ways to control the spread of Salvinia to protect this habitat.”

Lake Macquarie Mayor Kay Fraser said the council will continue looking at innovative ways to protect and preserve our environment.

“It’s wonderful to see the efforts of this partnership between Council and Crown Lands paying off. 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.”