Insect set to tackle Lake Benalla weed problem

In a first for Victoria, a weevil with a taste for cabomba, a highly-invasive introduced weed impacting the Benalla lake, has been released in a trial to tackle the problem.

Goulburn Broken Catchment Management Authority Environmental Water and Wetlands Coordinator, Jo Geddes, said the insect was smaller than a grain of rice and about 65 of them were released into the lake this week.

“We’re working with CSIRO, Benalla Rural City Council and Goulburn-Murray Water to trial this weevil as a method to reduce the cabomba weed in Lake Benalla,” Ms Geddes said.

“In February the weed was detected in the lake again, following an outbreak about five years ago. Managing it has been a long-term challenge.”

Cabomba is a fast-growing weed that can quickly dominate waterways, out-compete native plants, reduce water quality and impede recreational use. It’s an introduced plant originally used in aquariums.

As well as smothering native aquatic vegetation that provides valuable habitat for native animals, fish and waterbugs, it can clog pumps and filters.

“Management of the weed over the years has relied on lake drawdowns in winter and summer, along with targeted excavation where conditions allowed. While these methods have provided temporary reductions, cabomba is persistent and can quickly recolonise,” Ms Geddes said.

The CSIRO-led Cabomba Biocontrol & Drought Resilience Project is a nation-wide project that aims to suppress the cabomba weed through the release of an approved biocontrol agent, the cabomba weevil Hydrotimetes natans.

Dr Kumaran Nagalingam from CSIRO, Australia’s national science agency, said the trial release of the cabomba weevil was a promising new approach to controlling the invasive weed.

The tiny aquatic insect feeds exclusively on cabomba, laying eggs within the plant tissue and boring into stems, weakening the weed and reducing its ability to grow and spread,

“The weevil has undergone extensive testing by CSIRO, including rigorous assessments to ensure it does not pose risks to native or desirable aquatic plants.”

This is the first release of the cabomba weevil in Victoria, following successful trials in Queensland and New South Wales.

Ms Geddes said the introduction of the cabomba weevil provided another tool to help keep the weed under control in Lake Benalla, in the long term.

“While we don’t expect the weevil to eradicate cabomba completely, it may help weaken infestations and make them much easier to manage using our existing methods,” she said.

Monitoring will assess establishment of the weevil population, its impacts on cabomba - which is expected to take about a year - and any broader ecological changes. If successful, the trial may pave the way for broader use of the cabomba weevil across Victoria in the future.

This project is part of a 'Nation-wide weed biocontrol mass-rearing and release network for enhanced drought resilience in Australia’s agricultural landscapes' initiative, funded by the Australian Government’s Future Drought Fund and led by the Centre for Invasive Species Solutions (CISS).