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

An ex-post economic evaluation of biocontrol of St John’s wort (SJW), <i>Hypericum perforatum</i>, 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, <i>Chrysolina hyperici</i> and <i>C. quadrigemina</i>, and the cecidomyiid gall midge, <i>Zeuxodiplosis giardi</i>. 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.本研究针对新西兰（New Zealand, NZ）圣约翰草（St John’s wort, SJW，学名贯叶连翘Hypericum perforatum）的生物防治开展事后经济评估，采用生态位建模（ecological niche modelling）、地理信息系统（GIS）绘制的土地利用脆弱性分布图及逻辑斯蒂方程（logistic equation），预测圣约翰草曾侵染66万公顷低价值山地牧场。为核算生产力损失，本研究假设圣约翰草严重侵染会使每公顷农场收入降低30%。1943年至1992年间，为开展圣约翰草生物防治，研究团队先后释放了叶甲科甲虫（chrysomelid beetles）*Chrysolina hyperici*、*C. quadrigemina*，以及瘿蚊科瘿蚊（cecidomyiid gall midge）*Zeuxodiplosis giardi*；该项目总投入达28万新西兰元（按2022年货币计价）。模型显示，生物防治效力自1943年（首次出现局部落叶现象）起呈线性增长，至1993年全国范围内防治效果达99%，此时圣约翰草已不再被认定为重大农业杂草。经估算，新西兰圣约翰草生物防治项目在2022年为全国带来1550万新西兰元的收益，历史投入产出比达6254:1。敏感性测试结果表明，上述两项指标均保持较高水平。目前仍存在若干不确定性：无法确认圣约翰草种群的全部减少均由生物防治导致，亦无法排除其侵染区域被其他杂草部分替代的可能性。尽管存在上述局限性，圣约翰草生物防治对新西兰的综合收益仍堪称巨大且可持续。