Integrating herbicide and mechanical control treatments with fire and biological control to manage an invasive wetland shrub, Mimosa pigra.
In Australia, biological control is a promising long-term management strategy for the woody weed mimosa Mimosa pigra but does not yet provide adequate control. Other management techniques, including herbicides and fire, can be ineffective and their impact on biological control agents is unknown. We investigated the potential of integrating control techniques, including biological control, to provide cost-effective management. A large-scale (128-ha) split-plot experiment was performed to measure the impact of single and repeated applications of herbicide and crushing by bulldozer, either alone or in combination, on both mimosa and five introduced biological control agents that were abundant at the site. Herbicides were applied over three seasons (1997-99) and all plots were burned in 2000. The impact of control options on mimosa cover, biomass, number of stems per ha, stand size structure and seedling regeneration was determined by aerial photography and by sampling permanent and random quadrats. Biological control agent abundance was also quantified. In isolation, herbicide, bulldozing and fire were not effective, but several combinations of techniques cleared mimosa thickets and promoted establishment of competing vegetation that inhibited mimosa regeneration from seed. Depending on the species, biological control agent abundance on surviving mimosa plants was either unchanged or increased following herbicide and/or bulldozing treatments. All agents recolonized regenerating mimosa within 1 year of the fire treatment, and Neurostrota gunniella increased dramatically. Carmenta mimosa abundance, however, was reduced by fire. The abundance of N. gunniella increased in response to all treatments, which we attribute to attack by this species being most common along stand edges. Control treatments separated monocultures of mimosa into smaller patches, thereby increasing the ratio of 'edge' to 'thicket' plants. The proportion of plants susceptible to N. gunniella attack increased as a result. Synthesis and applications. We conclude that integrating control techniques can successfully control dense mimosa thickets. Biological control integrates well with other control options and should lead to significant cost reductions for mimosa management. To maximize this benefit, integrated weed management plans should be designed to integrate biological control fully with other methods, rather than separate them spatially or temporarily.