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Improving lives by solving problems in agriculture and the environment

Finding a biocontrol agent for Crassula

Crassula helmsii is an invasive water weed that dominates still or slow flowing water bodies. It’s spreading throughout the UK and has the potential to out-compete native flora and reduce oxygen levels by forming dense mats. Chemicals are not an option so CABI were commissioned by the UK government to investigate the possibility of controlling it using biological control. This includes lots of testing by our scientists to ensure that any potential agent is safe for release.

Project Overview

So, what's the problem

Crassula helmsii or Australian swamp stonecrop is an invasive, aquatic weed from Australia and New Zealand. It was introduced to the United Kingdom in the early 1900s as a garden pond plant and has since spread throughout the country. This weed can dominate still and slow moving water bodies, smothering native species and potentially depleting the water of oxygen. 

The introduction of the European Water Framework Directive requires European waterways to reach a ‘good ecological status’ but control of this species is a problem owing to restrictions on the use of chemicals near waterbodies. Manual control also often generates new plants by spreading viable fragments.

The UK Government has therefore commissioned CABI to investigate the potential of controlling C. helmsii in a sustainable and biological way. This involves assessing the suitability of the natural enemies that help to keep it under control in its native range in Australia and New Zealand.

What is this project doing?

Together with the University of Tasmania, the Department of Primary Industries, Victoria and Australis Biological, a scientific team from CABI conducted surveys throughout the plant’s native range in order to identify natural enemies that could be considered as biocontrol agents in the introduced range (the UK). 

Those with the most potential will then be brought back to the UK for further testing in our quarantine facilities. If deemed safe, the research will be presented to the Government in a Pest Risk Assessment to ensure that there will be no damage to the UK’s natural environment. Assessments will be made on whether the agent is safe enough to be released.  


Many of the natural enemies (both fungal and insect species) collected and identified during surveys, were rejected as potential control agents because they were able to attack other plants closely related to Australian swamp stonecrop. 

Further testing, however, revealed the gall-forming eriophyid mite (Aculus sp.) as the most suitable agent. This family of mites are renowned for their host specificity and ability to reduce the plants reproductive success (fitness). To-date, CABI scientists have compiled the research in a Pest Risk Analysis (PRA) which was reviewed by Defra, an expert scientific panel, who gave an external review and technical consultation before ministerial approval was granted in June 2018. The mite was released at a small number of sites in 2018. This initial restriction of release is to allow for more detailed assessments to take place to increase the understanding of field results before upscaling its release at a later stage.

The team

Project Manager

Staff image of Sonal Varia

Sonal Varia  Project Scientist

Bakeham Lane
United Kingdom
T +44(0)1491 829040

Project team

Staff image of Richard Shaw

Richard Shaw

Country Director CABI UK/ Regional Coordinator Invasives

Staff image of Marion Seier

Marion Seier

Senior Plant Pathologist; Team Leader - Invasive Species, UK

Staff image of Nikolai Thom

Nikolai Thom

Plant Production Manager


  • Australia


  • Start: 01/04/2011
  • End: