Yorkshire Water turns to tiny mite in the fight against invasive weeds

Yorkshire Water is stepping up the fight against one of the UK’s most invasive non-native aquatic weeds by working with CABI scientists to release an army of mites to suppress the pesky weeds.

Approval has been given for the release of a novel biological control agent – the mite, Aculus crassulae – at Gouthwaite reservoir in Nidderdale to assess its ability to suppress the Australian swamp stonecrop (Crassula helmsii) weed. 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 attractive 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 pipes and intakes.

It is hoped that the release of the mites, as part of research commissioned by the Department for the Environment Food and Rural Affairs (Defra), 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.

Rachel Naden, Invasive Non-Native Species and Biosecurity Advisor at Yorkshire Water, said: “We welcome this opportunity to work in partnership with CABI 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.”

Sonal Varia, CABI’s (Centre for Agriculture and Bioscience International) 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.”

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.