CABI has supported the first-time releases of two specialist biocontrol agents, a weevil and a mite, targeting their respective invasive weed hosts in the Netherlands, namely floating pennywort (Hydrocotyle ranunucloides) and Australian swamp stonecrop (Crassula helmsii).
These exotic aquatic plants form dense monospecific stands, can out-compete native plants and animals, fundamentally changing natural ecosystems and threatening biodiversity. Conventional control is proving challenging and costly and as such, a more sustainable and natural approach is required.
CABI had previously shared expertise in invasive species management in the Netherlands with the pioneering collaboration targeting Japanese knotweed using the psyllid Aphalara itadori, released in the Netherlands in 2020 following approval from the Dutch authorities.
This latest initiative to use natural solutions to target two invasive aquatic species is supported by STOWA (Dutch Foundation for Applied Water Research) and brings together a consortium of stakeholders and partners including the Water Authorities of Aa en Maas, De Dommel, Hoogheemraadschap De Stichtse Rijnlanden, Wetterskip Fryslân, the knowledge institute Probos, the University of Leiden and CABI.
The background
To achieve the Water Framework Directive objectives, in 2010 the UK Department for the Environment, Food and Rural Affairs (Defra) commissioned CABI to investigate the potential of controlling several non-native aquatic and riparian weeds, including C. helmsii and H. ranunculoides using classical biological control.
The biology and safety of prioritised agents, the mite Aculus crassulae for C. helmsii and the weevil Listronotus elongatus for H. ranunculoides were investigated and following review of the risk assessments and consultation, ministerial approval was subsequently granted for release of the mite in England and Wales in 2018 and the weevil in England in 2021.
Years of monitoring field trials in the UK have confirmed specificity and safety for both agents and highlighted their potential for long term management of their host.
Collaborations in the Netherlands facilitated additional research by CABI to confirm safety to key Dutch plant species and applications for release of the two biocontrol agents submitted to the Enterprise Agency (Rijksdienst voor Ondernemend Nederland, RVO). Both species were approved in early 2025 and suitable sites subsequently sought for releases and monitoring in the autumn.
The releases – Listronotus weevils
Weevils feeding on floating pennywort leaves and ready for release.
Sites near Vught and Beers in the Netherlands saw the release of a total 500 weevils on floating pennywort mats. The adult weevils, originally sourced in Argentina, were reared at CABI in the United Kingdom and transported to the Netherlands. These herbivorous beetles feed and develop in the floating pennywort and their impact and spread will be assessed in subsequent visits. In the UK, results have been very encouraging with spread over a kilometre and sustained, heavy impact on the plant, stunting growth and allowing native plants to recolonise.
CABI team at the floating pennywort site in Vught near Den Bosch, working area of the Dommel water board.
The releases – Aculus mite
Thousands of tiny mites, Aculus crassulae, were released on grounds managed by Natuurmonumenten in the working area of the Vallei en Veluwe water board, near Apeldoorn and another site managed by the Hoogheemraadschap Stichtse Rijnlanden near Utrecht. CABI-led research, published in the journal Biological Control, revealed that A. crassulae can help reduce the overall vegetative growth of Crassula – particularly in primary stem and secondary shoot growth. In the field, the mites have started to develop robust populations in the UK, and have been recorded spreading many metres from the original release plots.
CABI team and partners assessing area for mite release at Natuurmonumenten in the working area of the Vallei en Veluwe water board, near Apeldoorn.
The lead partner from Aa en Maas, Dr Janneke van Der Loop, was interviewed by the National News NOS to highlight the impacts of the plant and the potential of the mite to reduce its spread.
Lead scientists for floating pennywort, Djamila Djeddour, and for Crassula, Dr Sonal Varia, said, “Both the mite and the weevil offer viable biological solutions to these highly invasive plants in the Netherlands; both agents have very narrow host ranges and results from the UK confirm that they are unlikely to pose a risk to flora in the Netherlands. We’re excited to see how the field trials progress and are hugely encouraged to see biological control being more widely adopted in Europe.”
Aculus crassulae infested galls released in the Netherlands field trials.
Additional information
CABI Biocontrol website
Find out more about CABI’s biocontrol work on five species in the UK, including Australian swamp stonecrop and Floating pennywort here.
STOWA website: Biocops | STOWA
Links to Dutch news articles
https://www.aaenmaas.nl/actueel/nieuws/2025/innovatie/primeur-nederland-500-extra-collega/
Relevant news stories
‘Tiny mite could prove a ‘mighty’ weapon in the fight against one of the UK’s most invasive weeds.’
‘Weevil used to control floating pennywort in the Cam Washes.’
‘Wonder Weevil’ released in fight against invasive floating pennywort
Acknowledgements
We are grateful to the Dutch financiers, STOWA (Dutch Foundation for Applied Water Research) and the water boards who have supported this research. In addition we would like to thank the UK Department for Environment Food and Rural Affairs (Defra) commitment to the Water Framework Directive, 2000/60/EC, for supporting and funding the original and continuing research on a classical biological control approach to Crassula helmsii and Hydrocotyle ranunculoides. We would like to extend our thanks to the Argentinian collaborators (FuEDEI) and Provincial authorities for the export of Listronotus elongatus and to the Australian partners (Australis Biological) for assisting in the export of Aculus crassulae to facilitate the original research in the UK.
