Keywords: biological control, biological agents, invasive species, stochastic optimisation, invasive plants, Californian thistle, New Zealand, optimisation, dynamic modelling, optimal control, environmentally friendly control
On the economics of controlling an invasive plant: a stochastic analysis of a biological control agent
Invasive plants can cause significant problems in natural and agricultural ecosystems. It is recognised that biological agents may assist in controlling invasions, but due to stochastic effects of biological control, the biological agent may not be effective. In this article, we analyse to what extent the stochastic effects of a biological control agent affect the optimal choice of control strategies to deal with the invasion of the Californian thistle in New Zealand. A stochastic dynamic optimisation model is set up that derives the path and combination of control options that maximise the expected net present value of returns from a pasture. The analysis focuses on two situations: a deterministic case and a second case in which the effect of introducing the insect Apion onopordi to reduce thistle density is stochastic. Although one would expect that the stochastic specification would lead to different results, we show that the stochasticity of the efficacy of the insect in this specific setting does not affect the optimal control measure adopted compared to the deterministic case. It is also shown that chemicals can be replaced as a control option by more environmentally friendly control options at relatively low costs.