Management strategies for an invasive weed: A dynamic programming approach for Californian thistle in New Zealand

Invasive plants can cause significant problems in natural and agricultural ecosystems. Although research has already been conducted on the economics of a single-control option for some invasive weeds, we extended the analysis by developing a dynamic optimisation model that evaluates the net benefits of a range of possible control options simultaneously in order to identify the optimal strategy (mix of control options). This paper focuses on Californian thistle (Cirsium arvense) in pasture in New Zealand. The net benefit is maximised by considering the costs and efficacy of control options, and the monetary value of animal production. Trajectories of shoot density are developed and the optimal strategies are found. Our results suggest that the introduction of a biological control agent (Apion onopordi), in combination with one or more control options, is the optimal strategy when the initial density of the thistle population exceeds 1.0 shoot m(-2). Results show that in the setting of the model excluding MCPA, MCPB and a Sclerotinia sclerotiorum-based mycoherbicide reduces the net present value (NPV) by less than 2%.