Relative risk scores generated from laboratory specificity tests predict non-target impacts of Microctonus spp. parasitoids in the field

A ‘relative risk score’ approach to help determine the potential impact of biological control agents on non-target species, previously proposed for weed biocontrol agents and aphid parasitoids, was examined in a retrospective study for Microctonus spp. parasitoids introduced to control pest weevils in New Zealand. ‘Relative risk scores’ (% parasitism for non-target weevil species/% parasitism for target weevil species) were calculated for 20 weevil (New Zealand adventive and indigenous) species and 3 introduced (New Zealand) Microctonus biocontrol taxa (Microctonus aethiopoides Moroccan biotype, Microctonus aethiopoides Irish biotype, and Microctonus hyperodae), based on previously published no-choice host range laboratory testing. Field hosts for each parasitoid and host combination were also examined through an extensive literature review and targeted field surveys. Strong correlations between ‘relative risk scores’ for no-choice laboratory assays and field host binary values (host: yes or no) were obtained for each Microctonus/weevil dataset, but there was no threshold score indicating that the approach cannot be relied upon to predict parasitoid field host ranges with a high degree of certainty. A second analysis that categorized weevil hosts as ‘major field hosts’, if literature records indicated high (≥10%) levels of parasitism in the field, showed a clearer relationship between relative risk score and the probability of a weevil being a major field host. There were no observations of a weevil being a major field host when the relative risk score value was <0.26. There was only one observation of a weevil not being a major field host where the relative risk score was >0.33. This nevertheless, indicates that the approach can mistakenly exclude some acceptable agents that would be host specific in the field. We conclude that this approach could potentially be used to assist risk assessment when introducing Microctonus spp. biocontrol agents, but other factors need to be assessed concurrently to reliably assess risk.