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The Highs and Lows of Cost–Benefit Analyses

New Zealand is one of only a handful of countries in the world which have been historically very active in classical biological control of weeds. Since the 1920’s New Zealand has released 69 biocontrol agents against 28 weed species. Notable successes that have been analysed economically include the suppression of pasture weeds such as ragwort (Jacobaeae vulgaris) and St. John’s wort (Hypericum perforatum). Such economic analyses are still rather rare globally for weed biocontrol.

Cost-benefit analyses are a tricky beast to wrangle, but they are a necessary exercise in self-reflection that is not explored enough in the weed biocontrol space. There is a plethora of difficulties with analysing cost-benefits for any weed biocontrol project. Economic losses caused by the weed and records of weed control costs are either spotty or non-existent.  This is particularly hard when environmental weeds targeted for biocontrol are often not being extensively (and expensively) controlled currently: so we lack baseline economic cost data for them. Also with biocontrol programmes, the limited financial resources available to fund programmes are unsurprisingly pushed toward activities that lead to successful agent release, whilst monitoring and evaluation after release, which are needed to quantify outcomes and assign value, are neglected.

We asked Dr Simon Fowler, weed biocontrol researcher at MWLR why so few economic studies have been completed in New Zealand. “Cost-benefit analyses can be relatively straightforward when you have easily quantifiable, market-based measurements. Weeds that heavily impact the agricultural sector such as ragwort and St John’s wort are ideal for this since we can work out the economic losses and costs involved in controlling them from historic records. Environmental weeds, on the other hand, are much harder to evaluate. To begin with, environmental weeds are often seen as beyond control over much of their ranges, so money is not invested in them, giving us no starting point for weed control costs. This is the first problem we have with cost-benefit analyses of environmental weeds” explained Simon. “The second problem is that there are issues with determining how to put a value on environmental benefits e.g. how do we monetarise reductions in biodiversity losses and improvements to ecosystem services, which often comprise the bulk of the benefits from environmental weed biocontrol (and are the primary reason for undertaking the programmes in the first place).

Image: a nodding thistle infestation.

With these limitations, it is not surprising that cost benefit analyses are few and far between when so many of the weeds the team and its sponsors choose to target have primarily environmental impacts.

There are currently no universally agreed criteria for cost-benefit analyses of productive sector or environmental weeds. This is an area of research that really needs expanding. The language of the economy is money, and weed biocontrol researchers need to translate their work into this language if they want to be heard.

With this in mind, Simon and the team recently decided to undertake their most comprehensive weed biocontrol cost benefit analysis yet. The team was inspired by a pioneering study by Australian researchers A.R Page and K.L Lacey from the AEC group who, in an extensive analysis, collated data across all weed biological control programs in Australia (past and current) to determine the economic benefit in investing in biocontrol to control pest plants.

The New Zealand analysis collated data on 6 historical projects where we are sure that biocontrol has suppressed the target weeds to a substantial extent and data were extensive enough to analyse. These included:

  • three agricultural weeds – ragwort, St John’s wort, nodding thistle (Carduus nutans)
  • two environmental weeds – mist flower (Ageratina riparia) and heather (Calluna vulgaris)
  • one weed that was both an agricultural and an environmental weed – alligator weed (Alternanthera philoxeroides).

Quite a few more recent programmes were judged as ongoing or too recent to assess economically such as biocontrol of tradescantia, broom, Californian thistle and buddleja.   

For the analysis, we compiled costs associated with these 6 weed species in the absence of biocontrol. We then estimated the impact of biocontrol on reducing these weed costs. Finally, we estimated the total costs of the classical biological control programmes against weeds in NZ (for all weed targets, whether any success was achieved or not).  

Image: a ragwort infestation.

“The results were staggering” says Simon. “Overall, New Zealand spends only a little more than $1 million on weed biocontrol per year: the figure was NZ$1.34 million for 2022 for example. The three most economically successful weed biocontrol programmes in New Zealand (ragwort, SJW and nodding thistle) yielded an annual saving of $$84.7 million in 2022.

But not everything came up roses. The analysis also highlighted differences in cost-benefit ratios between agricultural vs. environmental weed programmes. Overall, environmental weed biocontrol programmes provided a negative return on investment of between 0.88:1 - 0.46:1. With hindsight, this was not surprising: the main reason is that it remains a challenge to monetarise reductions in biodiversity losses, improvements to ecosystems services or benefits to human/animal health. This is disappointing, since benefits in these usually non-market values are the main purposes for controlling environmental weeds. “Unfortunately, until we can find acceptable and appropriate methods to monetarise benefits to biodiversity, ecosystems services or animal/human health, there is little value in continuing to run traditional economic analyses on environmental weeds that are based only on relatively easily quantified market-based evaluations” said Simon.

Fortunately, the analysis produced has demonstrated some very convincing evidence that there is a highly positive net gain in weed biocontrol for productive sector weeds. The three  weed biocontrol projects in New Zealand that show the greatest economic gains produce enough ongoing annual dollar-benefit to dwarf the costs of the entire weed biocontrol programme year on year. The analysis also highlights the need to investigate ways to value the suppression of environmental weeds. Cost-benefit analyses are used to translate what we as weed biocontrol researchers know and see, to the wider economy in order to justify its investment in our work. Without meaningful valuation criteria we are a bit stuck. Alternative methods of assessing benefits versus costs do exist such as multi-criteria analysis, and are worth exploring for weed biocontrol. But for now, in NZ, a reliance on cost-benefit analyses in the regional pest management strategies of many of our major sponsors is required by the Biosecurity Act (1993).


This project was funded by Manaaki Whenua – Landcare Research’s Beating Weeds Programme funded by the Ministry for Business, Innovation and Employment Strategic Science Investment Fund MBIE- SSIF).