Landcare Research - Manaaki Whenua

Landcare-Research -Manaaki Whenua

How Cost-Effective Is Successful Weed Biocontrol in New Zealand?

Lessons from Three Programmes

A recent economic analysis by Simon Harris (Harris Consulting) and Simon Fowler looked at whether it has been cost-effective to release biocontrol agents against weeds in New Zealand. In Australia the economic benefits of using biocontrol to control weeds have been well studied, but until now this kind of information has not been available for New Zealand programmes. There are several reasons for this. As Simon Fowler explains, “there isn’t much demand for retrospective studies as sponsors would rather spend money finding new biocontrol agents, and it has not been considered a priority by funding agencies. Also the long-term nature of biocontrol can mean that it may not be appropriate to undertake an economic analysis for several decades after agents are released”.

To address this important knowledge gap, Landcare Research has made a start on gathering data, beginning with successful agents such as the St John’s wort beetles (Chrysolina spp.). At the time that beetles were first released in the 1940s, St John’s wort (Hypericum perforatum) was rapidly expanding in distribution, particularly in high country pastures. By the 1980s, the beetles were successfully controlling the weed. To assess the economic gains, it was first necessary to look at results from ecoclimatic models. These were produced by Grant Humphries (University of Otago) using data provided by Landcare Research. Only data from the South Island were used, as the plant is not such an issue in the North Island.

The models were used to predict where the weed is capable of invading and to determine what its potential range would have been in the absence of any control. Various filters were added into the model to create a realistic scenario. For example, only areas of pasture used for sheep, beef and deer farming and where there was a high probability of St John’s wort infestation were used. The model suggested that 660,000 ha of the South Island would have been badly infested if St John’s wort had been allowed to grow unfettered until 2042. The negative impact from this level of infestation (based on loss of pasture and grazing to farmers) was calculated to be $109/ha with a smaller cost of $6/ha for manual weed control. The final figures suggested that the Net Present Value (NPV) of the introduction of the beetles is between $140 million (given a conservatively slow rate of spread) and $1,490 million (with a faster rate of spread) over 70 years. The respective benefit-to-cost ratios are therefore an impressive 10:1 and 100:1. Note that the savings provided by the St John’s wort biocontrol programme, even at the lower end, more than pay for all the other weed biocontrol programmes undertaken in New Zealand to date!

Another approach to gaining data was taken by a project supported by the Sustainable Farming Fund (administered by the former Ministry of Agriculture and Forestry, now the Ministry for Primary Industries). The West Coast Ragwort Control Group surveyed farmers on the West Coast of the South Island to find what the average cost was of controlling ragwort (Jacobaea vulgaris) on dairy farms. They estimated this to be $980 per farm per year. If you multiply this by the 12,000 dairy farms in New Zealand you reach a total cost of $12 million per year to control ragwort in the absence of biocontrol agents. If you do the assessment based on areas where the ragwort flea beetle (Longitarsus jacobaeae) is successful there is a net saving of $7 million per year. It is anticipated that now, with the establishment of the ragwort plume moth (Platyptilia isodactyla) in wetter areas of New Zealand such as the West Coast where the flea beetle was not effective, the benefits could be as high as an additional $5 million per year.

It is more difficult to determine the financial gains from controlling environmental weeds such as mist flower (Ageratina riparia) since the intrinsic benefits to native flora and fauna are not always measurable in dollar terms. However, a preliminary analysis of the financial savings from no longer needing to control mist flower in the upper North Island suggests a cost reduction of $80,000–90,000 per year. The NPV for this is more than $3 million with a benefit-to-cost ratio of 2.5:1, which is still considered very good over a 13-year period.

These three examples give a good indication of the substantial economic benefits to New Zealand already provided by weed biocontrol. We are now planning to collect economic data for other biocontrol targets, e.g. alligator weed (Alternanthera philoxeroides) and heather (Calluna vulgaris).

Funding for this project was provided by the former Ministry of Science and Innovation (now the Science and Innovation Group of the Ministry of Business, Innovation and Employment) to the Beating Weeds Programme and Landcare Research’s Discretionary Capability Fund.