Sonal Varia, Suzy Wood, Corin Pratt and Sean Murphy
CABI Europe – UK, Bakeham Lane, Egham, Surrey, TW20 9TY, United Kingdom
Crassula helmsii or Australian swamp stonecrop is an invasive aquatic weed native to Australia and New Zealand which has been invading Europe since its introduction in the early 1900s. It was first introduced from its native range as an ornamental pond plant and is now spreading across Western Europe, especially in the United Kingdom where it is most widespread. With few available control options, due to its ability to tolerate extreme environmental conditions and regrow from small fragments, biological control could be an effective and sustainable method of control for this weed. Surveys in the native range had previously identified the Australian gall-forming mite, Aculus crassulae (Acari: Eriophyidae) as a species of interest due to the host specificity of species in the Eriophyidae family and its distinctive impact in field populations of C. helmsii. Aculus crassulae was found to feed and reproduce exclusively on C. helmsii in laboratory testing and therefore was not deemed a risk to European species. In 2018, after six years of safety testing under quarantine conditions and after undergoing an extensive pest risk analysis (PRA), permission was granted to release Aculus crassulae from quarantine as a biocontrol agent of C. helmsii. Since its release from quarantine, the mite has demonstrated its ability to overwinter and survive under UK conditions using field-based, mesocosm studies combined with laboratory-based studies and 2019 will see the first full year of releases into C. helmsii populations in England and Wales. The release of A. crassulae represents only the fourth application of a weed biological control agent in the EU and the third in the UK.