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We describe a bio-economic model for Nassella neesiana (Chilean needle grass) that estimates the net benefit of a containment programme for the weed in Canterbury as the difference between the cost of containment and the costs incurred over time should the weed spread within sheep and beef pastoral systems. Logistic spread is assumed with the maximum area that could be invaded (772,080 ha) determined by constraining a climate niche model for the weed to susceptible farm system types within productive land use capability classes. The current size of the invaded area (80 ha) was determined by field observations in Canterbury in 2008, and the spread rate (201 years to 90% saturation) was derived from observations in the adjacent Marlborough region. With these assumptions, and discounting at 8% per year over 100 years, the net benefit is negative NZ$173,178 ($83,900–$257,078) and containment would not be economically worthwhile. However, the net benefit is positive over a range of lower discount rates and higher spread rates, revealing a need for robust estimates of these parameters. The model presented here provides a generic blueprint for meeting the requirements of the Biosecurity Act with respect to evaluating proposed regional weed management programmes.
