A wildlife preservation group has recruited a new ally in its battle to control an invasive river plant – the weevil.

Floating pennywort is a highly damaging plant originally from the Americas that has been suffocating parts of the River Medway between Tonbridge and Maidstone.

First introduced to Britain in the 1980s through the ornamental aquatic nursery trade, by the 1990s floating pennywort – Latin name hydrocotyle ranunculoides – had escaped into wild waterbodies.

It is capable of growing up to 20cm a day and has rapidly spread across the country, forming dense floating mats, which can hinder navigation and cut oxygen levels and light to the aquatic environment, negatively affecting biodiversity.

The Medway Valley Countryside Partnership (MVCP) has been battling floating pennywort on the River Medway for more than a decade by ripping it out manually.

But now, thanks to a collaboration with the Environment Agency and the Centre for Agriculture and Bioscience International (CABI), it has been able to release a species-specific weevil, which feeds on nothing other than floating pennywort.

The South American weevil had to undergo rigorous safety and efficacy tests before being approved for release in an invaded inlet of the River Medway in July.

There is already some evidence of damage to the pennywort mat from the first generation of 400 weevils, and the MVCP is now hoping that the weevil will survive the winter and continue to reduce the size of the mat over subsequent seasons.

Meanwhile, MVCP volunteers will continue to manually remove mats of floating pennywort in other areas, hoping that in time the weevils will do their work for them.

Andrea Griffiths of MVCP said: ‘Removing floating pennywort by hand is physically demanding, difficult and sometimes dangerous work.

“So naturally we’re very excited about the possibilities of this biocontrol trial.

“It would be lovely in the future if the weevils provide a more sustainable, long-term solution to controlling floating pennywort.”

Suzy Wood of CABI said: “The weevil’s release site will be closely monitored to assess their survival, impact and spread.”

“Biological control of aquatic weeds using specialist insects (weevils in particular) has proved highly effective globally.”