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St John’s wort beetles more than pay off: A cost benefit analysis
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An ex-post economic evaluation of the impact of St John’s wort (Hypericum perforatum) biocontrol programme shows New Zealand low-value hill-country farming sector is saving $15.5 million on control costs and production losses, year on year.
Furthermore, the programme can report a benefit-cost ratio of 6254:1 when looking at the total savings from 1943 to 2022 vs the upfront programme cost (in 2022 value).
One of NZ’s worst pasture weeds in the early 20th century, biocontrol has rendered St John’s wort rare on the NZ landscape, making calculating what it would have cost today to control it in the absence of biocontrol a challenge. We estimated 660,000 ha of low-value hill-country pasture could have been infested today had the biocontrol agents not been introduced. Biocontrol effectiveness has increased consistently from 1943 (when initial local defoliation occurred) to reach 99% control nationwide by 1993 – when St John’s wort was no longer considered a significant agricultural weed.
The St John’s wort biocontrol programme ran from 1943–1992 and cost NZ$0.28 million (2022 value). During this time three biocontrol agents were released: two chrysomelid beetles (Chrysolina hyperici and C. quadrigemina), and the cecidomyiid gall midge (Zeuxodiplosis giardi). Uncertainties remain, concerning whether biocontrol caused all reductions in St John’s wort, or whether St John’s wort infestations were partially replaced by other weeds. Despite such caveats, benefits of St John’s wort biocontrol to New Zealand appear huge and sustainable.