Seasonal life-history models for the integrated management of the invasive weed nodding thistle Carduus nutans in Australia

1. It is widely accepted that combining several control options into integrated pest management strategies is the most effective way to provide long-term suppression of pest populations. However, full factorial field trials of all single and integrated control options for a pest species would be prohibitively expensive and time consuming. Methods to allow triage of the huge array of management options would be of great value in streamlining the decision-making process. 2. We present a seasonally structured, individual-based model, specifically designed to compare and rank detailed management strategies for a noxious pasture weed. The model structure is determined in part by the demographic data available, and in part by the management options under consideration. The case study is for nodding thistle Carduus nutans in Australia. Eight years of demographic data, for more than 8000 mapped plants, were used to parameterize the model, which is age-, size- and density-dependent and incorporates individual variation. Management options for this plant include three biocontrol agents, as well as conventional herbicide and grazing management strategies, which can be used alone or in a variety of combinations. Data on management impacts were drawn from multiple studies. 3. The model predicts that the root-crown weevil Trichosirocalus mortadelo will more effectively suppress weed populations than either of the two flowerhead-feeding insect agents Urophora solstitialis and Rhinocyllus conicus. Crash grazing (up to four times the regular grazing pressure) in any single season, or when most effectively applied across spring and summer, is less effective than T. mortadelo, while combinations of crash grazing and biocontrol agents strongly decrease weed population persistence. However, lethal herbicide is the best single strategy, while spring spray-grazing (a combination of non-lethal herbicide and grazing) is the best integrated weed management strategy. 4. Synthesis and applications. The model is structured by, and serves to integrate, available information on demography and management from multiple sources. The subset of strategies that performed well forms the focus for fewer, more thorough, field trials. The decision-making approach illustrated here is also applicable to any species and any array of management options.