Biological control of Crassula helmsii
Crassula helmsii is an aquatic plant species which can grow in a number of forms (submerged, emergent or semi-terrestrial) depending on environmental conditions. The species is native to Australia and has been introduced into the USA and the EPPO region as an ornamental water plant. Since being introduced to the United Kingdom in 1911 as an oxygenating plant for garden ponds, it has been gradually increasing its range in the United Kingdom and other parts of Europe by escaping from gardens and through incorrect disposal by aquarium and pond owners. It has now spread to at least 2000 sites in the UK, particularly threatening conservation sites that are home to rare and endangered organisms. A biological control programme has been conducted against the species in the United Kingdom which has seen surveys conducted in the plants’ native range to identify natural enemies which could potentially be utilised as biocontrol agents. Most of the natural enemies collected during these surveys were rejected as potential agents due to their wide host range. However, one species an eriophyid mite species (Aculus crassulae) has been shown to be host specific to C. helmsii and approval has been given for its release in the UK. The mite will be released on reservoirs and in wetlands in England and it will be monitored to evaluate if it can reduce populations of this invasive plant species.
CABI website: https://www.cabi.org/news-and-media/2018/