Mikania micrantha (mile-a-minute weed or South American Climber) is a major invasive alien weed in many of the tropical moist forest regions of Asia. This neotropical vine is able to smother plants in agricultural ecosystems, agroforestry and native habitats. Conventional control methods of manual removal (slashing) or herbicide application, are expensive, ineffective, not sustainable, and can be environmentally damaging. Classical biological control was considered the best option to manage this weed, and CABI was funded by DIFID (UK- Department for International Development) to implement this strategy in India. The research culminated in the release of a coevolved, host specific, rust pathogen (Puccinia spegazzinii), from the South America native range of the weed, into Kerala and Assam. This rust pathogen infects all aerial parts of the plant (leaf, petiole and stem), leading to cankering and whole plant death (see image below).
The anticipated establishment and spread of this pathogen on weed populations in India has not yet been realised. However, further releases of the rust have been made in Papua New Guinea (PNG) and Fiji under an ACIAR (Australian Centre for International Agricultural Research) funded project, lead by Mike Day from DEEDI (Department of Employment, Economic Development and Innovation), Queensland. Mike reported on this work at the recently held 8th IOBC International Workshop on Biological Control and Management of Chromolaena odorata and Other Eupatorieae, at ICRAF (World Agroforestry Centre), Nairobi, Kenya, from 1-5 November 2010. In PNG the rust has been released in over 450 sites in 15 provinces and currently has established at nearly 70 sites in four provinces. The rust has spread naturally over 7km in 13 months at some sites, and is having a significant impact on the growth and density of the weed. Scientist Warea Orapa, of the South Pacific Commission, who is leading the work in Fiji, told me that the rust has been released at 80 sites on four of the main islands (Viti Levu, Vanua Levu, Taveuni and Ovalau) and has established at 20 sites. Mike and his colleagues are currently preparing a paper on the biology, field release and impact of the rust; which I am keenly awaiting.
Rust appears to be mainly establishing in the wetter area. Perhaps in time, it may be worth considering other isolates of the rust in South America that may be more tolerant of dryer conditions. In addition, other biocontrol agents, both arthropod and fungal may be further evaluated. Mikania continues to be a serious weed, and these initial results in PNG suggest that biological control offers a safe and effective control method: helping farmers – without any cost to them – and the environment without any ‘collateral’ damage to other vegetation.