Assessment of the host-range and impact of the mite, Aculus crassulae, a potential biological control agent for Australian swamp stonecrop, Crassula helmsii

•Eriophyoid mite screened as a biological control agent for Crassula helmsii.•No significant non-target effects observed.•A. crassulae demonstrated significant impact on C. helmsii. The non-native aquatic weed Crassula helmsii, or Australian swamp stonecrop, has become widespread and a major problem in aquatic habitats in the UK and North-West Europe, with negative impacts reported from ecologically important habitats. As very few effective traditional management tools are available to land managers, a biological control project was initiated in the UK. An Australian phytophagous mite, Aculus crassulae (Acari: Eriophyidae) was evaluated as a potential agent for C. helmsii under laboratory conditions in no-choice and choice host plant tests. These demonstrated that A. crassulae is host specific to C. helmsii, with only minor exploratory feeding observed on the native congener, C. aquatica, under no-choice tests, and no damage recorded in choice tests. The impact of A. crassulae on the growth of C. helmsii was also investigated by applying two different mite densities to plants. Both densities, especially the higher mite density, caused a significant reduction in plant growth by the end of the experimental period. Secondary shoot number and length, number of nodes and new leaf pairs were reduced by the mites during the course of the experiment, but the largest impact was on the reduction of plant height. The host specific nature of A. crassulae and the observed reduction in these significant growth parameters demonstrate the potential A. crassulae has as a biological control agent for C. helmsii in the UK and Europe.