Weevils released to tackle invasive floating pennywort in the Cam Washes
The weevil Listronotus elongatus was approved for release in the UK in 2021 following a decade of safety testing
A project has been launched to control floating pennywort in the Cam Washes using a South American weevil. The initiative, led by scientists from CABI and funded by the Environment Agency, aims to reduce the spread of the invasive aquatic plant, which threatens water ecosystems in the UK.
Floating pennywort (Hydrocotyle ranunculoides) is a fast-growing invasive species that can spread up to 20cm per day. It forms dense mats on the water surface, blocking flood defences, depleting oxygen levels, and increasing flood risks. Managing the plant costs an estimated £25 million annually, including its impact on tourism and recreation.
To address this, the weevil Listronotus elongatus was approved for release in the UK in 2021 following a decade of safety testing. The weevil, which only feeds on floating pennywort, is expected to provide a long-term, self-sustaining control method, unlike manual removal or herbicide use, which can be costly and ineffective.
The trial involved releasing weevils in the Cam Washes, a Site of Special Scientific Interest (SSSI), where traditional control methods have struggled to manage the plant. Drone footage recorded the extent of the floating pennywort before the release, and further monitoring will assess the weevil's impact.
Djami Djeddour, a senior scientist at CABI, 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 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.”
Pippa Keynes, the Environment Agency’s 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.”
The project will continue to monitor the effectiveness of the weevil in reducing floating pennywort infestations, with hopes that it will provide a cost-effective solution for managing the species in the UK’s waterways.