Mimosa pigra, or giant sensitive plant, is a serious and increasing weed problem in wetlands of Southeast Asia (especially Thailand) and northern Australia. Believed to be of Central American origin, it is a tall, prickly, woody, perennial shrub that forms impenetrable thickets in paddy-fields, along watercourses, etc. Mechanical control is totally ineffective; herbicidal methods can achieve partial control for part of the year, but are ineffective overall. The most promising solution appears to be biological control, combined with herbicidal applications to increase pressure on the plant.

Project 8339 instituted Phase 1 of the control program, which ran from January 1984 to 1987. Natural enemies of Mimosa pigra were collected from a variety of climatic areas in Mexico, and tested for biological suitability and host specificity. A number of promising control agents were identified, and some were introduced to Australia under quarantine. Three have now been field-released in Australia and Thailand, with encouraging early results.

Project workers also monitored Mimosa pigra infestations in Australia and Thailand as part of before-and-after studies of control, and carried out experiments on techniques and seasonality of herbicide application to achieve maximum control.

The project was externally reviewed in June 1986. The reviewers commended the achievements of the project highly, and recommended that it continue for a further three year phase. Thw work will include wider collection, introduction and field release of control agents, both insects and plant pathogens, continuation of the herbicide work, and implementation of a number of extra new initiatives.

Major research objectives are to continue collection and evaluation of natural insect enemies from the tropical Americas; and also conduct host specificity studies, introduce promising species to Australia and Thailand, and multiply for field release. Researchers will undertake similar operations on plant pathogens that were identified during Phase I as having very great potential as control agents, and monitor establishment and effect of field-released control agents in Thailand and Australia. They will continue to test herbicides, including new compounds recently released, and new equipment and techniques for application. They will try to reduce rates of application at seasons when plants are stressed.

A mobile field evaluation unit will be established in Thailand. Workers experienced in the control strategies developed by the project, will tour Thailand to test and refine the control technieque in different areas, and to demonstrate them to the local people and train local officers. They would also provide feedback on spread or decline of Mimosa pigra infestations, and on problems arising with control strategies in varying circumstances.

Successful control measures will release land already invaded by this weed, and prevent even more areas in Thailand being colonised. These measures will also assist control in northern Australia and parts of Southeast Asia where the weed is starting to spread.