Predicting the economic benefits and costs of introducing new biological control agents for Scotch broom Cytisus scoparius into New Zealand

We estimated the potential benefits and costs of introducing three candidate biological control agents against the invasive weed Scotch broom Cytisus scoparius into New Zealand. Farming, forestry, the Department of Conservation, district and regional councils should benefit from a successful biological control programme against broom. However, costs may be incurred by beekeepers who benefit from broom pollen, and due to potential non-target attack to Chamaecytisus palmensis, an exotic relative of broom, which is a minor fodder crop and is occasionally planted for erosion control on marginal hill country. Due to uncertainties regarding the costs, we adopted a risk-averse approach by assuming a worse-case scenario where levels of non-target damage to C. palmensis are similar to those on C. scoparius and that no pollen sources suitable for beekeeping will replace broom stands. Due to uncertainties regarding the level of control that will be achieved, we calculate a single best estimate for overall benefits of the proposed programme using an average of four potential biocontrol scenarios: no control, and 25, 50, or 95% reductions in broom, weighted according to the success rate of past biological control programmes against legume weeds worldwide. We estimated that benefits would outweigh costs by 2.9:1 (the weighted ratio of benefits to costs), with an annual net benefit to New Zealand of $5.99 million, once biological control agents are fully established nationwide. Using discounting techniques over 50 years, we calculate an internal rate of return of 52%, and a net present value of $61.49 million at the 5% discount rate and $21.95 million at the 10% discount rate. These returns are robust to significant overestimation of benefits and to significant underestimation of costs. The potential costs of successful biological control of Scotch broom should not prevent the introduction of additional biological control agents into New Zealand.