Biocontrol of St John’s wort ( <i>Hypericum perforatum</i> ) provides huge ongoing benefits to New Zealand agriculture

ABSTRACT An ex‐post economic evaluation of biocontrol of St John’s wort (SJW), Hypericum perforatum , in New Zealand (NZ), used ecological niche modelling, GIS‐mapped land‐use vulnerabilities and a logistic equation to predict that SJW could have infested 660,000 ha of low‐value hill‐country pasture. To calculate productivity losses, we assumed serious infestations of SJW caused 30% decreases in farm income per hectare. Investment in SJW biocontrol totalled NZ$0.28 million (2022 rates) associated with the 1943–1992 releases of the chrysomelid beetles, Chrysolina hyperici and C. quadrigemina , and the cecidomyiid gall midge, Zeuxodiplosis giardi . Biocontrol effectiveness increased linearly in our model from 1943 (when initial local defoliation occurred) to reach 99% control nationwide by 1993 (when SJW was no longer considered a significant agricultural weed). We estimate that SJW biocontrol in NZ provided a national benefit of NZ$15.5 million in 2022, with a historical benefit–cost ratio of 6254:1. Both figures remained large in sensitivity testing. Uncertainties remain concerning whether biocontrol caused all reductions in SJW, or whether SJW infestations were partially replaced by other weeds. Despite such caveats, benefits of SJW biocontrol to NZ appear huge and sustainable.