CABI expertise supports national campaign to control invasive floating pennywort on Britain’s rivers and lakes

27 May 2021, United Kingdom: CABI’s expertise on finding a suitable biological control for the highly invasive floating pennywort – which threatens the ecology of Britain’s rivers and lakes – has helped support a new national campaign to control the water weed.

As part of the campaign launch, Djami Djeddour, Senior Scientific Officer at CABI, joined the Angling Trust and the Herts and Middlesex Wildlife Trust on a visit to the Colne Valley where she met the Colne Valley Fisheries Consultative, to help clear a dense mat of floating pennywort.

The event was an opportunity to see first-hand the benefits of working collaboratively to tackle floating pennywort (Hydrocotyle ranunculoides). It is anticipated that later this year, a specialist weevil native to South America – Listronotus elongatus – will be released into the wild to provide a sustainable way to halt the spread of the species.

This follows extensive research undertaken by CABI to determine the suitability of the species for release and presents one of several biological controls that are being used to assist in invasive species management.

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The launch also coincides with Invasive Species Week, an annual event to raise awareness of invasive species and how everyone can help to prevent their spread.

Floating pennywort – first found in the wild in England in the 1990s – can grow at an incredible 20cms a day from tiny fragments to form dense mats that cover the water’s surface. These can block out light, stifle native plants, and make rivers and lakes unsuitable for insects and fish.

The dense rafts can also make waters inaccessible to anglers and present a flood risk when plant mats build up around sluices and drains.

Floating pennywort can last for up to 14 days out of water, allowing for unintentional spread of the plant between water bodies on damp equipment and clothing which has not been thoroughly cleaned and dried. It is also notoriously difficult to control because very small fragments of the plant can regrow. As a result, any effort to control floating pennywort needs to be done regularly, at the right scale and for sufficient duration to be effective.

Djami Djeddour helps clear a water course on the Colne Valley of floating pennywort (Credit: Angling Trust).

Ms Djeddour said, “CABI-UK has already released biocontrol agents against Japanese knotweed, Himalayan balsam and Australian swamp stonecrop (Crassula) in the UK. Weevils have a long history of success in the biocontrol of aquatics. Research into the biocontrol of floating pennywort using the Argentine weevil, Listronotus elongatus, has been funded by Defra since 2011. The weevil has undergone comprehensive screening for safety and efficacy and is now in the last stage of approval for permission to release in the UK.

“CABI are hopeful that this agent will reduce floating pennywort’s dominance on our waterways, providing long-term and sustainable control. A national strategic approach to floating pennywort management provides an excellent opportunity to maximise impact, working closely with partners and stakeholders to integrate this natural solution into conventional management plans.”

Learning from the success of smaller scale strategic partnerships, in September 2020 the Thames basin project was launched, with the Angling Trust, Environment Agency and British Canoeing working together to coordinate management of floating pennywort in the region.

Based on this project and other successful strategic collaborations that have been formed in other parts of the country, the Great Britain Floating Pennywort Strategy is being launched, uniting recreational users, water companies and government to tackle the invasive plant. Through closer, coordinated action, the aim of the strategy is to share best practice and facilitate further partnerships to control the species and prevent further spread into other areas.

Lord Benyon, Defra Minister for biosecurity, said, “I commend this initiative, which will help inform how we manage this highly damaging invasive species. Invasive non-native species not only challenge the survival of some of our rarest species but cost the economy more than £1.7 billion per year.

“Floating pennywort profoundly disrupts aquatic habitats for insects, fish and other wildlife, and disrupts how people interact with Great Britain’s rivers and lakes. Today’s move is a step in the right direction for improving our cherished waterways.”

Trevor Renals, Invasive Species Technical Lead, Environment Agency, said, “Floating pennywort is a highly-damaging non-native weed, which damages the flora and fauna of our rivers and our ability to enjoy recreation such as angling and boating. It also has the potential to increase flood risk and block flood defence assets. No single organisation has a duty to manage floating pennywort, so it is only by working together that we can control this plant. We encourage people to support local partnerships to manage invasive species and use it as an opportunity to enjoy nature and meet people in a socially-distanced environment.”

Listronotus elongatus weevil controlling floating pennywort – currently being researched in the laboratory for potential release (Credit: CABI).

Dr Emily Smith, Environment Manager, Angling Trust, said, “We welcome the launch of this national strategy to tackle floating pennywort and a movement towards a more coordinated approach to invasive species management. Many anglers are already helping to control invasive species, as highlighted by the fantastic ongoing efforts of the Colne Valley Fisheries Consultative. Through this strategy we hope to be able to facilitate further partnerships such as these, and make strides towards tackling this problematic plant to protect the environment and wildlife that we all enjoy.”