Mimosa pigra L. – mimosa

Mimosa pigra is a serious weed of agriculture and the environment in northern Australia, particularly the NT. It is a Weed of National Significance and is listed in the Global Invasive Species Database as one of the One Hundred of the World’s Worst Invasive Alien Species. It is a prickly, perennial, woody shrub which forms monocultures on floodplains and around wetlands. M. pigra has been the subject of a biological control program in Australia from 1979. Projects were also conducted in several south-east Asian countries. A large suite of agents were assessed but rejected for various reasons, and a total of 13 insects and two fungi were released in Australia to control M. pigra of which 10 insects and one fungus are known to have established. Two agents, Carmenta mimosa and Neurostrota gunniella, are currently inflicting severe damage, reducing both seed production and seed banks, and defoliating plants, which favours competing vegetation and leads to lower seedling survival and increasingly senescent stands. The impact of more recently released agents, particularly Malacorhinus irregularis and Macaria pallidata, appears to be substantial but has not been measured. Some agents were also released in Asia but their fate has barely been evaluated. The biological control program against this weed is characterised by the selection of challenging agents that might have been rejected for practical reasons in a less well-funded project. The experience gained in overcoming these challenges, for the flower-feeding agents and the green seed-feeders in particular, provides a model for the use of these types of agents in future programs. The M. pigra program also provides a leading example of how to measure the impact of biocontrol agents.