A new CABI-led study suggests that a tiny weevil (Stenopelmus rufinasus) has huge benefits in saving Great Britain up to £16.8m in annual management costs of the invasive aquatic fern Azolla filiculoides.
The research, published in the CABI Agriculture and Bioscience journal, estimates that without any biocontrol the expected yearly costs of managing A. filiculoides would range from £8.4m to £16.9m.
The scientists say that the impacts of naturalised S. rufinasus populations on A. filiculoides alone could be expected to reduce management costs to £800,000 to £1.6m a year.
However, they estimate A. filiculoides management costs to be lower still due to additional augmentative releases of the weevil that take place each summer, resulting in annual management costs of £31,500 to £45,800.
Azolla filiculoides, a type of floating water fern, was introduced to Great Britain at the end of the 19th century for ornamental use in ponds and aquaria. But its introduction into the wild has meant it has spread rapidly throughout England and Wales and to a lesser degree, Scotland.
The invasive aquatic fern outcompetes native species by forming a dense covering on the surface of the water. It blocks out light and can also deoxygenate water. A. filiculoides can also block canals, drains and overflows and may lead to an increased risk of flooding. It can affect irrigation systems – both by blocking their water supply and by reducing water quality.
It has been banned from sale in England and Wales since April 2014.
Its specialist natural enemy, S. rufinasus, was first recorded in 1921. It is suspected to have been introduced from America as a stowaway on A. filiculoides. Stenopelmus rufinasus is also reported to be present in numerous additional European countries where A. filiculoides is present.
The study sought to estimate the management cost savings resulting from the presence of S. rufinasus as a biocontrol agent in Great Britain. This includes the value of additional augmentative releases of the weevil made since the mid-2000s, compared with the expected costs of control in the absence of S. rufinasus.
Corin Pratt, lead author and Invasive Species Management Researcher at CABI, said, “The unintentional introduction of the weevil S. rufinasus to Great Britain is estimated to have resulted in millions of pounds of savings annually in management costs for A. filiculoides.
“Additional augmentative releases of the weevil provide further net cost savings, tackling A. filiculoides outbreaks and bolstering naturalised populations.
“The use of herbicides in the aquatic environment is likely greatly reduced due to A. filiculoides biocontrol. Although somewhat climate-limited at present in Great Britain, climate change may result in even more effective biocontrol of A. filiculoides by S. rufinasus.
“This has been observed in warmer regions such as South Africa, where the plant is no longer considered a threat since the introduction of S. rufinasus.”
The scientists conclude by arguing that in the absence of the specialist weevil S. rufinasus, A. filiculoides could be expected to be the dominant aquatic macrophyte in Great Britain. This would require extensive, costly management and likely widespread use of herbicides in the aquatic environment.
They state that the estimated benefit to cost ratio of augmentative S. rufinasus releases to be of 43.7:1 to 88.4:1
Main image: The invasive aquatic fern Azolla filiculoides starts to die back after being treated with the biological control (Credit: CABI).
Full paper reference
Corin Pratt, Suzy Wood, Kate Constantine, ‘A century of Azolla filiculoides biocontrol: The economic value of Stenopelmus rufinasus to Great Britain,’ CABI Agriculture and Bioscience, 14 November 2022, DOI: 10.1186/s43170-022-00136-0
The paper is available to view open access here: https://cabiagbio.biomedcentral.com/articles/10.1186/s43170-022-00136-0
PDF copies of the paper for press use only are available on request prior to the embargo being lifted.
This work was financially supported by the UK Department for Environment Food and Rural Affairs (Defra) Reference 32570. CABI is an international intergovernmental organisation, and we gratefully acknowledge the core financial support from our member countries (and lead agencies) including the United Kingdom (Foreign, Commonwealth & Development Office), China (Chinese Ministry of Agriculture and Rural Affairs), Australia (Australian Centre for International Agricultural Research), Canada (Agriculture and Agri-Food Canada), Netherlands (Directorate-General for International Cooperation), and Switzerland (Swiss Agency for Development and Cooperation). See https://
www.cabi.org/about-cabi/who-we-work-with/key-donors/for full details.
Wayne Coles, Communications Manager, CABI (UK based), email: firstname.lastname@example.org
Corin Pratt, lead author and Invasive Species Researcher, CABI (UK based), email: email@example.com
About CABI Agriculture and Bioscience
CABI Agriculture and Bioscience (CABI A&B) publishes high quality, rigorously peer-reviewed multi-, inter- and transdisciplinary research focused on agriculture, food security, and the environment.
Global agriculture faces many challenges today. How can we produce more safe, nutritious food in the face of climate change? Can we balance greater efficiencies with the need to reduce greenhouse gas emissions and protect biodiversity? Can we meet changing market demands and yet develop more equitable economies? Can agriculture provide a livelihood and opportunities for women and young people?
These problems require increasingly complex and urgent solutions from researchers and policymakers. That is why CABI A&B is committed to encouraging an inclusive culture of scientific discussion and rapid information sharing among researchers worldwide. We publish both large and incremental advances in science in both primary and multidisciplinary fields across the biosciences, agriculture, agronomy, microbiology, social sciences, and the environment.
CABI A&B is an open access journal, with rapid peer review, making findings immediately available to all readers worldwide. We believe it will help engender a clearer understanding of facts and findings, and help challenge assumptions.
CABI A&B is the official journal of CABI – an international, inter-governmental, not-for-profit organization that improves people’s lives worldwide by providing information and applying scientific expertise to solve problems in agriculture and the environment. The organization is governed by 49 Member Countries, many of which are low-income and highly dependent on agriculture to feed their population and generate income. CABI A&B’s supports the organization’s aim to help achieve 8 of the Sustainable Development Goals: (1) No Poverty; (2) Zero Hunger; (3) Quality Education; (5) Gender Equality; (12) Responsible Consumption and Production; (13) Climate Action; (15) Life on Land; and (17) Partnerships for the Goals. CABI A&B is integral to delivering on the organization’s mission, and any surplus derived from the journal is reinvested in its international development activities.
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