Weevil scheme: Why scientists have put bugs in SEQ’s water supply

A quiet south-east Queensland lake is the site of a world-first test to see if introducing a small insect will have a big impact on an incredibly invasive weed.

CSIRO experts have been working for nearly two decades to find something to control the fast-growing cabomba plant.

Ay, cabomba: The weevils are smaller than a grain of rice, but it’s hoped they’ll have a big impact on the invasive weed.Credit: CSIRO

Cabomba was first introduced to Australia in the 1960s as a decorative plant for aquariums, but it got into the local ecosystem and took off, with Seqwater alone spending $170,000 a year to control it in just three of the lakes it manages.

The only effective control method until now was to send divers in the physically rip the weed out, but scientists now say they have a solution.

In its native South America, cabomba is preyed on by the cabomba weevil. Importantly, it is the only thing the cabomba weevil eats.

A diver covered in cabomba at Lake Kurwongbah.

Much of the research conducted by CSIRO over the last 19 years has been to confirm that the cabomba weevil will not suddenly get a taste for native Australian plants.

Hearing about introducing a species to combat another species will raise the spectre of the cane toad for many people, but CSIRO scientist Kumaran Nagalingam said this was a very different situation.

“The cane toad was not introduced by scientists, and it would not meet the standards of a biological control agent if it were being considered today,” he said.

“We have done testing where we placed the weevil in proximity to both cabomba and other similar Australian plants, and the weevil only ate the cabomba.”

Having been satisfied that the weevil will not become a pest itself, scientists have now released them into Lake Kurwongbah, a small man-made lake in the Moreton Bay region which is used for recreating and additional drinking water supply.

Researchers preparing to release the weevils.

Seqwater senior research scientist David Roberts said the weed had caused significant issues over the years, and they were glad to finally have something to tackle it directly.

“Cabomba is so thick it makes recreational activities such as swimming, fishing and canoeing both difficult and dangerous,” he said.

“The weed also reduces the water holding capacity of dams and significantly adds to the cost of treating drinking water.”

It’s not expected that the weevil will completely eradicate cabomba, but it is expected to significantly control its spread, eliminating many of the issues associated with it.

Removing cabomba is also expected to help native wildlife, with platypuses and water rats recording lower populations in areas where cabomba had taken over.

Seqwater has also established a weevil “nursery” at Lake Kurwongbah to continue to provide weevils for future releases.

Nagalingam said releases were planned for Lake Macdonald, near Noosa, and they could potentially expand to releases in central Queensland, northern NSW and some sites in the Northern Territory.

“It’s been fantastic to collaborate with Seqwater, so we can develop natural solutions for problems like invasive weeds,” he said.