Elizabeth Pendlington, Junior Scientist – UK Invasive Species Management at CABI, presents an exciting update on the biological control of Crassula helmsii at the Royal Society for the Protection of Birds (RSPB) Nature Reserve in Conwy, North Wales.
Firstly, what is Crassula and why is it a problem?
Crassula helmsii, also known as Australian swamp stonecrop, New Zealand pygmyweed or just Crassula, is an aquatic plant from Australia and New Zealand. It was introduced to the UK in the early 1900s and has since become invasive across the country and parts of Northwest Europe.
Crassula has several adaptations which allow it to grow quickly and take over habitats. This is a problem ecologically as native plants can be outcompeted for resources such as space and light. Crassula can be particularly dominant in habitats with bare ground and a low abundance of other plant species, as often found at nature reserves for wading birds (see photo below).
Crassula infested lagoon at RSPB Conwy, North Wales, UK, inset; zoomed in section of Crassula (Credit: CABI).
How can Crassula be managed?
Manual control – tools or diggers can be used to scrape the Crassula. This can be quite expensive and is only a short-term solution, as the plant can quickly grow back from remaining fragments and even be spread further by disturbance.
Chemical control – spraying the Crassula with herbicide can cause plant mortality. But herbicide use can have other negative ecological and environmental effects, particularly as the plant is found in sensitive aquatic systems.
Other control methods – hot foam and saltwater have also been trialled to control Crassula with mixed results.
Biological control – living organisms have the potential to reduce Crassula infestations to an appropriate ecological or economic threshold. This is the ideal management strategy as it is more environmentally friendly, cost-effective and sustainable.
At CABI, we have been researching the biological control of Crassula since 2010 and have one agent approved for release by the Department for Environmental, Food & Rural Affairs (Defra). You can read more about the research behind the Crassula biological control programme here.
Introducing, the mite…
In 2018, approval to release the mite Aculus crassulae as a biological controlagent against Crassula was granted*.* This highly host-specific mite causes abnormal growth, swelling and the development of a gall at the growing shoot tip within which the mites feed and live (see image below). This reduces plant growth and overall plant fitness which can prevent the Crassula spreading further.
Since the mite was approved for release, the Invasive Species Management (ISM) team at CABI have been releasing it as a biological control agent at various trial sites around the UK. A crucial stage in any biocontrol programme, once the agent is released from the lab into the field, is to achieve successful establishment. Here, establishment is defined as a population that survives for more than two successive years without any further releases.
Conwy Crassula
Thanks to funding from the Welsh Government and in collaboration with the RSPB, CABI released the mite at a trial site located in the RSPB nature reserve at Conwy in 2023.
The mite was released within two 10m2 plots of Crassula during the summer of 2023 (see photo below). Scientists from the ISM team then conducted regular monitoring visits to record the number of galls found on the Crassula in the release areas as an indicator of mite population.
The first monitoring trip in autumn 2023 found that the mite population had increased moderately over the summer – from around 400 to 1000 galls per m2 – and when the team returned in early summer 2024, they found that the mites had successfully survived the winter.
Release areas in 2023 at RSPB Conwy, green flags show mite release patches (Credit: CABI).
The “mitey” update
Excitingly, when the team returned again in August 2024, the mite population had expanded significantly, to a greater density and spread than had been seen at other sites. The average density of the galls was over 3000 per m2 in the release area and some mites had spread nearly 100 metres away.
High density of mite-infested Crassula (left side of image), compared with healthier Crassula with a lower density of mite infestation (right side of image) at RSPB Conwy (Credit: CABI).
Two months later, the average density of galls had more than doubled again, to over 6500 per m2. One quadrat even found a density of 10,000 per m2!
Although survival of the mite across the different UK trial sites has varied, such promising progress at RSPB Conwy has provided a great example of the mite’s potential as a biological control.
Graph of average gall density per m2 in release area (Credit: CABI).
What’s next?
The team at CABI will continue monitoring the release areas at RSPB Conwy in collaboration with RSPB staff, with a particular focus on how this high density of mites survive the winter and whether the success continues into 2025. If the mite does survive long-term, it will be important to understand the factors behind the success of the mite at the Conwy site and monitor any improvements for native plant species over time.
Additional information
Main image: Close up of high density of mite-infested Crassula in the first release area at RSPB Conwy(Credit: CABI).
Relevant blog
‘Crassula – the potent invasive plant being challenged by a microscopic mite.’
Project page
Learn more about the research into Crassula from the project page here.
RSPB Conwy
To read more about RSPB Conwy visit: https://www.rspb.org.uk/days-out/reserves/conwy