Biological control 'silver bullet'

Biological control is more often associated with pest insects but there have been a number of examples where beneficial insects and pathogens have been deployed against weed species. In India, a rust pathogen, Puccinia spegazzinii, is being tested in a classical biological control (CBC) programme against the invasive weed Mikania micrantha, which originates from tropical America.

credit: Carol Ellison

Mikania is an invasive alien weed that can smother both agroforestry and natural forest ecosystems, leading to invasions of home-gardens and plantations in many regions of tropical Asia. It was introduced to India as a cover crop and as camouflage for airfields in the 1940s, but its impact has escalated in recent decades due to widespread cutting and degradation of natural forests; Mikania, having colonised the degraded forestland, then invades adjoining tea gardens and village vegetable plots. The weed has also become a problem throughout the moist forest zones of South-east Asia.

Control of Mikania has relied on slashing and herbicides but these methods are expensive yet not effective, as well as being environmentally damaging. Indeed, herbicides have been used at such high levels that residues have prejudiced sales of the tea crop. Therefore, a project to investigate an Integrated Pest Management (IPM) approach to Mikania control has been funded by DFID. This involves CABI Bioscience, UK and three Indian organisations: Kerala Forest Research Institute, Directorate of Biological Control, and Assam Agricultural University. The project has been to assess the socio-economic impact of Mikania on the basis of mapping its distribution and monitoring its spread, and to evaluate potential biological control agents. Since no effective natural enemies of Mikania have been found in India, attention was turned to potential pathogens in the weed's native range in the Americas.

credit: Carol Ellison

Fungal natural enemies
A broad range of fungal pathogens occurs on Mikania in its neotropical range. An assessment of these led to the selection of the rust Puccinia spegazzinii, as the most suitable for introduction to India as a classical biological control agent. Eleven samples of the rust from six countries-Argentina, Brazil, Costa Rica, Ecuador, Peru and Trinidad-were evaluated by CABI Bioscience UK in its quarantine glasshouse. An isolate from Trinidad proved to be virulent against a wide range of Mikania present in India, including all the weed populations that were screened from the Western Ghats. This Puccinia strain was then tested against 55 non-target species, including crops, and proved to be totally specific against Mikania, to which it is highly damaging, resulting in leaf, petiole and stem cankering and death of the whole plant.

Although a suite of natural enemies may often be required to achieve significant control of invasive alien weeds, it appears that in the case of Mikania, Puccinia spegazzinii may prove to be the 'silver bullet'. There are promising indications that the introduction of this rust in the moist forest region of the Western Ghats in Southern India and Assam in the north, will prove effective as a CBC agent, and that the growth and spread of Mikania will be significantly reduced. The weed's control will have a major beneficial effect in reducing time spent weeding, increasing agricultural productivity and consequently alleviating poverty of subsistence farmers. Equally important, control of Mikania will benefit conservation of biodiversity of natural forest ecosystems by reducing the current adverse impact of this invasive alien in these habitats. The rust is currently being held under quarantine in New Delhi, where final assessment and screening are taking place, before potential field release in 2004. A similar project funded under the DEFRA Darwin Initiative has just begun for China, where Mikania is also a serious weed.