Invasives Blog

CABI scientists have been supporting the UK Environment Agency with the release of a South American weevil to serve as a sustainable biological control solution to control floating pennywort at the Cam Washes Site of Special Scientific Interest (SSSI) west of Wicken, Cambridgeshire, UK.

Floating pennywort (Hydrocotyle ranunculoides) is a highly damaging non-native plant that was first introduced to Britain as an ornamental pond plant in the 1990’s. In the wild, this species can grow up to 20cm a day and rapidly forms dense floating mats of weed across waterbodies.

These crowd out native species, block light impacting on the aquatic ecosystem, interfere with activities such as angling, paddling and sailing, and potentially increase the flood risk.

Cost of controlling floating pennywort

The cost of controlling floating pennywort, together with the impact on tourism and recreation is estimated to exceed £25 million annually in the UK.

However, it is hoped that the Floating Pennywort Weevil (Listronotus elongatus), which was approved as a biocontrol for release into the wild in England in autumn 2021 after a decade of safety and efficacy testing by CABI, can help halt the spread of floating pennywort at the Cam Washes SSSI.

Unlike traditional control methods such as manual or mechanical removal and herbicide applications, which are costly, labour-intensive, and often ineffective, the biocontrol with the weevil presents a long-term, self-sustaining solution. The weevil has evolved to only feed and develop on floating pennywort, reducing biosecurity issues that can arise from disturbance.

The trial involved the release of weevils on the Cam Washes SSSI. This site was chosen for the weevil release due to the difficulties of controlling floating pennywort using traditional methods and the need to manage the reinfestation of the River Cam.

Sustainable control of floating pennywort

Djami Djeddour, Senior CABI project lead and Weed Biocontrol Scientist, said: “We are thrilled to be able to work in partnership with the Environment Agency to help tackle floating pennywort in the Cam Washes. Biological control of non-native, invasive aquatic weeds using specialist insects has proved highly effective globally, and we are hopeful that these released weevils (beetles) will be the plant’s Achilles’ heel.

“Once the weevil populations are successfully established, they will play an important role in the sustainable control of floating pennywort, reducing its impact on native habitats and enhancing their biological diversity and function.”  

Prior to the release, the extent of floating pennywort spread was captured using drone footage. It is hoped that continued drone monitoring will help to show how effective these weevils could be as a cost-effective solution to managing this invasive species.

Pippa Keynes, Environment Agency project lead, said “We are very happy to partner with CABI in this important project. Following on from extensive Floating Pennywort control work that has been carried out on the Bedford Ouse, we are keen to explore different ways of controlling this invasive plant which can cause havoc on our waterways.”

Additional information

Main image: The Floating Pennywort Weevil Listronotus elongatus (Credit C. Pratt, CABI).

Environment Agency press release

You can read the Environment Agency press release on this story here.

Relevant stories

CABI supports floating pennywort weevil release in Barnsley, UK.

‘Wonder Weevil’ released in fight against invasive floating pennywort.

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

Invasive non-native species cost UK economy an estimated £4bn a year, new CABI-led study reveals.