Biological control of Crassula helmsii
Crassula helmsii (Crassulaceae) is a semi-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 oxygenating plant for aquaria and ponds. The species has become a major problem in aquatic habitats in the United Kingdom and North-West Europe where it can have negative impacts on ecosystem services. A biological control programme was conducted against the species and a mite (Aculus crassulae), native to Australia, was identified as a promising biological control agent. Host range studies revealed that A. crassulae has a very narrow host range and it is unlikely to pose a risk to the flora of the United Kingdom and Europe. Additionally, impact studies of A. crassulae on C. helmsii showed that the mite has a negative impact on the plant compared to control plants without the mite. Plants infested with the mite had lower height, and a reduction in the number of new leaf pairs, new nodes and secondary shoots. The biological control agent shows great potential to negatively affect C. helmsii growth under natural field conditions.
Sources
Varia S, Wood SV, Allen RMS, Murphy ST (2022) Assessment of the host-range and impact of the mite, Aculus crassulae, a potential biological control agent for Australian swamp stonecrop, Crassula helmsii. Biological Control 167, 104854 . https://doi.org/10.1016/j.biocontrol.2022.104854