MIL-OSI United Kingdom: Weevil used to control floating pennywort in the Cam Washes

Led by CABI (www.cabi.org), a not-for-profit inter-governmental organisation specialising in natural solutions for invasive non-native plants, the project is funded by the Environment Agency, working in partnership with CABI scientists.

Floating pennywort (Hydrocotyle ranunculoides), an invasive aquatic weed plant native to South America, has become a major threat to the UK’s waterbodies. It is known for its rapid growth and ability to spread up to 20cm a day, forming dense mats that disrupt ecosystems, causing deoxygenation, blocking flood defences, impacting recreational activities, and contributing to flood risk issues.

The cost of controlling floating pennywort, together with the impact on tourism and recreation is estimated to exceed £25 million annually in the UK. To address this growing problem and reduce the spread of the plant, a South American weevil Listronotus elongatus, was approved for release in the UK in 2021 after a decade of safety testing; this is the fourth weed biological control solution approved for release in the UK.

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.

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.

Djami Djeddour, Senior CABI project lead 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”.  

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.”

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