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The problem

Crassula

Australian swamp stonecrop (Crassula helmsii) is an invasive aquatic plant that dominates still and slow-flowing waterbodies. It was initially introduced from Australia in the early 1900s as a garden pond plant but is now spreading across waterbodies in the UK and parts of Western Europe.

It is particularly problematic in sensitive aquatic habitats where it has the potential to outcompete native flora and reduce oxygen levels by forming dense, impenetrable mats. This weed can also have negative impacts on recreation and can block filters necessary for water treatment. Australian swamp stonecrop tolerates extreme environmental conditions and, as such, management can be very challenging and often unsuccessful, especially for infestations in areas of high conservation value.

The project

In 2010, CABI was commissioned by the UK government to investigate the possibility of controlling the weed using biocontrol. Together with Australian collaborators, CABI’s scientific team carried out surveys throughout the plant’s native range in Australia in order to identify natural enemies that could be considered as biocontrol agents in the UK and Northwest Europe.

Many of the natural enemies (both fungal and insect species) collected and identified during the surveys were rejected as potential biocontrol agents because they were able to attack other plants closely related to Australian swamp stonecrop. Further testing, however, revealed the mite, Aculus crassulae (Acari: Eriophyidae), to be the most suitable agent.

Mites from this family are renowned for their host specificity and ability to reduce the plants’ reproductive fitness or success. Mite feeding causes the growing shoots of the terrestrial and emergent growth forms of C. helmsii to develop galls or swellings, significantly reducing growth.

Research to date

Between 2012 and 2018, safety testing was carried out under quarantine conditions, demonstrating that the mite is specific to its host, C. helmsii, and will only feed and complete development on this species. Subsequent studies in the laboratory also showed that the mites had the potential to survive and establish under the environmental conditions in the UK.

CABI scientists compiled this research in a pest risk analysis which was reviewed by Defra, an expert scientific panel, the devolved governments and by stakeholder groups before ministerial approval was granted in June 2018.

The mites have since been released at sites across England and Wales and continue to be released at new sites and monitored. This initial restriction at the early stages of the release programme allows for more detailed assessments to take place to increase the understanding of field results before upscaling its release at a later stage.

Studies have also been taking place under field conditions and results have shown that the mites can survive and develop populations under UK conditions. The aim is now to ensure that the mites can establish robust and sustainable populations at the release sites.

Project-specific articles, reports and papers

Varia, S., Wood, S., Pratt, C., Adair, R., & Murphy, S. (2019). A mitey solution to Australian swamp stonecrop. In H. L. Hinz, M. C. Bon, G. Bourdôt, M. Cristofaro, G. Desurmont, D. Kurose, H. Müller-Schärer, M. Rafter, U. Schaffner, M. Seier, R.F.H. Sforza, L. Smith, S. Stutz, S. Thomas, P. Weyl & R. Winston (Eds.), Proceedings of the XV International Symposium on Biological Control of Weeds, Engelberg, Switzerland, 26-31 August 2018 (pp. 242–243). Engelberg, Switzerland. Retrieved from https://www.ibiocontrol.org/proceedings

Knihinicki DK, Petanović R, Cvrković T, Varia S. A new species of Aculus mite (Acari: Eriophyidae), a potential biocontrol agent for Australian swamp stonecrop, Crassula helmsii (Crassulaceae). Zootaxa. 2018 Oct 11;4497(4):573-85.