1. Investigations into the potential of classical biological control of the invasive aquatic weed, Crassula helmsii
- Author
-
Varia, Sonal
- Subjects
biological control ,Crassula helmsii ,Aculus crassulae ,invasive plant ,host specificity testing ,risk assessment - Abstract
Crassula helmsii, a semi-aquatic plant native to Australia and New Zealand, was first introduced to the UK over a hundred years ago. Since then, it has spread across much of the UK and is present in parts of Western Europe. Crassula helmsii can dominate aquatic habitats and there are limited options for its management, particularly in protected habitats. Consequently, alternative methods of control have been encouraged by the UK government. The potential for using classical biological control for C. helmsii was investigated and the results are presented in this thesis. Assessments of the plant and its associated natural enemy complex in south-eastern Australia were made and several natural enemies with high potential as biological control agents were selected and underwent further investigation to assess their suitability. The gall-forming mite, Aculus crassulae (Eriophyidae) was found to be the most promising natural enemy based on its high host specificity and the damage inflicted on C. helmsii. Further laboratory-based assessments predicted its ability to tolerate the low temperatures experienced during winters in the UK confirming that there was potential for the mites to survive in the UK. Field experimentation with A. crassulae was also undertaken and mite populations and C. helmsii growth were monitored, verifying predictions that the mites were able to survive natural conditions in the winter and develop robust populations in the spring and summer. Furthermore, the effect of shade was assessed and it was found that winter survival of mites increased under these conditions. With herbicide application being the main control measure employed against C. helmsii, the impact of the use of a low dose of glyphosate in combination with the mites was also investigated. Findings were varied although the trend for increased plant decay when both were applied together warrants further study into the interactions between the mite and the herbicide.
- Published
- 2020