Alien Plant Meets a Mighty Foe at Llanelli Wetland Centre

In a first for Wales, a microscopic sap-sucking mite has been introduced in a bid to help control an invasive Australian wetland plant that chokes up bodies of water and vastly out-competes our native plants.

After years of staff and volunteers battling valiantly to prevent the spread and growth of this prolific alien, WWT Llanelli Wetland Centre has been supported by Dŵr Cymru Welsh Water and other generous funders to be the first Welsh site for the Centre for Agriculture and Bioscience International (CABI) trial of a brand new biological control agent that feeds exclusively on Australian Swamp Stonecrop (Crassula helmsii).

Brought to the UK through the ornamental pond industry in the early 1900s, Swamp Stonecrop soon found its way into wild waters and is now a serious problem across the UK. It quickly forms carpets of growth and has spread through a number of waterbodies in the Llanelli Wetland Centre reserve. It doesn’t die back in winter, smothers native plants, and can reduce the amount of oxygen in water, which is bad news for all kinds of wetland creatures.

Llanelli Wetland Centre is an important haven for many vulnerable species, including water vole, lapwing and otter, and it’s vital that a delicate balance is maintained to ensure that the ecosystem remains biodiverse and healthy.

After initial investigations, CABI found that the mite most effective at slowing the growth of Swamp Stonecrop was Aculus crassulae, a natural biological control of the plant in Australia.

CABI’s Project Scientist, Sonal Varia, said: “Feeding by these mites causes Crassula plants to develop galls in the growing shoots, known as big bud galls, leading to a significant reduction in overall plant fitness by reducing plant growth. It is hoped that over time, the introduction of these mites will contribute to the long-term management of this weed by reducing its competitive ability, allowing less competitive native plants to grow where Crassula currently dominates.”

WWT Llanelli Wetland Centre Reserve Manager, Dr Brian Briggs, said: “We are excited to be part of this important trial. Australian Swamp Stonecrop is an extremely invasive plant that takes up a huge amount of our time and resources and we have been hoping for an effective way to control it for years. We will be eagerly monitoring the release areas for signs of the mite getting to work.”

The mite was released at Llanelli on 20 June and there are already early indications that the mites are having an effect. Only time and careful monitoring by CABI and WWT staff will tell if this tiny sap-sucker is the answer.

If the project is a success the mite is likely to be introduced to countless other sites around the UK which have been affected and reduce the vigour of the invasive plant and prevent it from choking ditches and ponds in future.