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Tiny mite could prove a ‘mighty’ weapon in the fight against one of the UK’s most invasive weeds

12th September 2018

Scientists are stepping up the fight against one of the UK’s most invasive non-native aquatic weeds and an old garden favourite.


Approval has been given for the release of a novel biological control agent – the mite, Aculus crassulae – to assess its ability in the real-world environment to suppress Australian swamp stonecrop (Crassula helmsii), also known as New Zealand pigmyweed.This follows carefully controlled laboratory testing to ensure the safe, controlled release of the mite in the UK.

The weed was introduced to the UK in the early 1900s as a decorative garden pond plant, but is no longer considered appropriate due to its impact on native biodiversity. The plant can form dense mats in still and slow moving water bodies, dominating sensitive aquatic environments and threatening native plants and invertebrates. It can also exacerbate flood risk as well as clogging the pipes and intakes of the water companies.

Crassula helmsii is also present across Western Europe, though is most widespread and problematic in the UK. It is present throughout Cornwall and Devon, but is very under-recorded.

It is hoped that the release of Aculus crassulae, as part of research commissioned by the Department for the Environment Food and Rural Affairs (Defra) and in partnership with the University of Tasmania, the Department of Primary Industries, Victoria and Australis Biological, will help to reduce the spread of Crassula helmsii in water bodies and areas of high conservation status.

The research aims to serve the European Water Framework Directive requirements of ensuring that European waterways reach a ‘good ecological status’ through the management of invasive species.

Sonal Varia, CABI’s lead scientist on the project, said, “Many of the natural enemies – both fungal plant pathogens and arthropod species – collected and identified during surveys were rejected early in the project as potential control agents because they were able to attack other plants closely related to Australian swamp stonecrop.

“However, testing of Aculus crassulae in the laboratory revealed it to be a suitable agent. Mites in this family are renowned for their host specificity and ability to reduce a plant’s fitness. The safety testing demonstrated that this species only feeds and reproduces successfully on its host Crassula helmsii and can significantly reduce plant growth. The next step is to now see how it works under natural field conditions.”

Dr Niall Moore, Chief Non-native Species Officer, Defra, said: “Aquatic invasive non-native species like Crassula helmsii can cause significant harm to our waterbodies and the native plants and animals. After rigorous testing in quarantine, biocontrol approaches such as the release of this mite can provide an effective and sustainable tool to manage invasive non-native weeds.

“Good biosecurity is key to reducing the spread of invasive non-native species; we urge all users of waterbodies to Check-Clean-Dry* their clothing and equipment and gardeners and pond owners to Be Plant Wise**, choosing and disposing of their plants with care.”

This new, release phase of the project, which is also part-funded by Natural England, South West Water and Yorkshire Water, will see the mite released at reservoirs and wetlands in England to determine if the gall-forming eriophyid mite can reduce the weed’s dominance.

Kate Hills, Invasive Non-Native Species Ecologist at South West Water, the first water company to create a dedicated INNS post and to support CABI on these innovative trials, said, “Aquatic invasives are particularly problematic and cause a range of problems for water companies at considerable cost. To date there have been no known effective measures to control Crassula. We are concerned about the range of impacts of Crassula, not least the health and safety issues it can create for recreational water users.”

Rachel Naden, Invasive Non-Native Species and Biosecurity Advisor at Yorkshire Water, said, “We welcome this opportunity to work in partnership with CABI and South West Water to further the development of this novel control measure and hopefully reduce the impact of Crassula helmsii on our assets and Yorkshire’s ecosystems.

“Invasive species are detrimental to our infrastructure, native biodiversity and public enjoyment of our sites and we are committed to playing our part in their control. The use of biocontrol potentially delivers a sustainable and efficient solution to deliver a greater customer outcome at a much lower cost than alternative measures such as the use of herbicides.”

CABI’s research on the mite was presented to Defra as part of a Pest Risk Analysis (PRA) to assess the safety of the mite. The PRA gained ministerial approval after being accepted by an expert scientific panel and by various stakeholders in a public consultation, meaning it is now allowed to be released from quarantine. A summary of stakeholder responses can be found on Defra’s Plant Health Portal.

*Check, Clean, Dry

Invasive plants and animals can carry diseases that kill fish, block up waterways, harm the environment, can damage boats, and cost the UK economy over £1.8 per annum. 

Water users may unknowingly be helping to spread invasive plants and animals as they can be small and hard to spot so are easily spread on damp equipment and clothing.

Following three simple steps after leaving the water: Check, Clean, Dry – can prevent this from happening:

  • Check your equipment, clothing and footwear when you leave the water for mud or plant material. Remove anything you find and leave it at the water body, don’t put anything down the drain.
  • Clean everything thoroughly as soon as you can. Use hot water if possible.
  • Dry everything for as long as you can before using it again as some invasive plants and animals can live for over two weeks in damp conditions.

**Be Plant Wise

Pond owners and gardeners can unknowingly assist the spread of aquatic plants into our countryside, where they can smother our native plants, clog our waterways, exacerbate flooding and remove oxygen from the water, which can harm fish. 

Be Plant Wise by following three simple steps:

  • Know what you grow
  • Stop the spread
  • Compost with care

 Find out more at www.nonnativespecies.org/beplantwise

 What is an invasive non-native species?

An invasive non-native species is any non-native animal or plant that has the ability to spread, causing damage to the environment, the economy, our health and the way we live. 

Find out more at www.nonnativespecies.org/checkcleandry.

More about Australian swamp stonecrop (Crassula helmsii)

Australian swamp stonecrop is a semi-aquatic, succulent, perennial herb that can grow in three growth forms; terrestrial, emergent and submerged. Submerged growth forms are generally more elongated than the former two growth forms and can be found in water up to 3m deep. Roots form at the nodes and in the terrestrial habitats of damp margins of water bodies, the growth form is creeping or erect. Flowers are found on emergent and terrestrial plants, are borne singly on axils and are pale pink to white in colour. Reproduction is mainly vegetative, however there is some evidence that Crassula helmsii can also reproduce by seed in the introduced range. Dispersal is mainly assisted by human activity but the species can also be spread by wildfowl. 

More on the biological control of Australian swamp stonecrop using the mite Aculus crassulae

A scientific research project initiated in 2009 was coordinated by a Steering Committee for the project which included Defra, the Environment Agency, Natural England and scientific advisers from the University of Reading and the Natural History Museum. The purpose of the project is to determine whether biological control is a feasible method for the long- term, sustainable management of Australian swamp stonecrop in Great Britain. The research is being carried out by CABI. 

A number of surveys were carried out in the native range of Australian swamp stonecrop in Australia and New Zealand between 2009 and 2013. The mite, Aculus crassulae was observed causing damage to the plant and was subsequently imported into CABI’s quarantine facility to undergo safety testing. The mite has been tested on 40 non-target species. These included species that are closely related to Australian swamp stonecrop, ornamental plants and plant species which grow in a similar habitat to the target weed. 

Find out more from the project page ‘Finding a biocontrol agent for Crassula’ on CABI.org https://www.cabi.org/projects/project/33138  

About CABI

CABI is an international not-for-profit organization that improves people’s lives by providing information and applying scientific expertise to solve problems in agriculture and the environment. 

Through knowledge sharing and science, CABI helps address issues of global concern such as improving global food security and safeguarding the environment. We do this by helping farmers grow more and lose less of what they produce, combating threats to agriculture and the environment from pests and diseases, protecting biodiversity from invasive species, and improving access to agricultural and environmental scientific knowledge. Our 49 member countries guide and influence our core areas of work, which include development and research projects, scientific publishing and microbial services.


About the Department for Environment Food & Rural Affairs (Defra)

Defra is the UK government department responsible for safeguarding the natural environment, supporting the UK’s world-leading food and farming industry, and sustaining a thriving rural economy. This broad remit means Defra plays a major role in people’s day-to-day life, from the food we all eat, and the air we all breathe, to the water we all drink.


For further information please contact:

South West Water