Investigations into the Efficacy of the Biocontrol Programme of Australian Myrtle, Leptospermum laevigatum, in South Africa: Lessons Learnt and Where to Now?

Despite decades of biological control (biocontrol) endeavours against Leptospermum laevigatum (Gaertn.) F.Muell. (Myrtaceae), the weed remains a significant problem in areas within the Western Cape Province and parts of the Eastern Cape Province. Being predominantly invasive within coastal dune fynbos, the species is notoriously difficult to manage. Whilst the past 10-year period has seen limited focus and funding for this specific project, this review addresses possible reasons for the lack of success of the biocontrol programme over the past three decades, which has relied on two agents, a leaf-mining moth, Aristea thalassias (Meyrick) (Lepidoptera: Gracillariidae) and a flower-galling midge, Dasineura strobila (Dorchin) (Diptera: Cecidomyiidae). Although both agents are widespread and occur across the range of L. laevigatum, the plant continues to grow aggressively despite their presence. Findings to date suggest that mortality resulting from parasitism or predation of the agents' immature stages is not the underlying problem for the limited control of the plant and that other factors must be at play. However, further investigation to better understand the dynamics of the insect populations and ecology of this weed, is needed to help to clarify the questions that, to date, remain unanswered. Other potential agents are also currently under consideration, notably a scale insect, Callococcus leptospermi (Maskell) (Homoptera: Coccoidea). In its native range (Australia) C. leptospermi, has a patchy distribution and variable levels of abundance, reasons for which are still under investigation. With no foreseeable biocontrol breakthrough in the near future, an integrated approach to managing L. laevigatum is probably the only short to medium-term solution for this species.