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• Braconid parasitoids in the genus Microctonus were found to successfully parasitise endemic weevils in natural tussock grassland areas in southern New Zealand at altitudes higher than the pastures in which they were introduced. • The target hosts of these weevils are also found in these tussock grasslands, providing an opportunity for "spillover" parasitism, but it was also found that some cycling within native weevil communities occurs. • Nine endemic weevil species, mainly in the subfamily Entiminae were found to be parasitised mostly at levels below 10% although the highest level recorded was 36%. • Most non-target parasitism was caused by an Irish ecotype of Microctonus aethiopoides introduced to control Sitona obsoletus , a pest of clover. The parasitoids Microctonus aethiopoides Loan (Moroccan and Irish ecotypes) and M. hyperodae Loan (Hymenoptera: Braconidae) have been introduced to New Zealand as biological control agents for agricultural pest weevils Sitona discoideus Gyllenhal, S. obsoletus Gmelin and Listronotus bonariensis (Kuschel). These parasitoids are also present in native tussock grassland habitats and a survey carried out over two years at nine tussock grassland sites demonstrated that several endemic weevil species were parasitized by these biological control agents within these habitats. New records of non-target parasitism were found for the endemic species of Chalepistes, Nicaeana , and Eugnomus. It was found that M. aethiopoides is established and cycling within weevil communities in these environments, using endemic weevil hosts in part, but pathways also exist via three pest weevil species for spillover from agricultural environments. Parasitism of endemic weevils was not found to be ubiquitous in these environments and parasitism levels were mostly below 10%. [ABSTRACT FROM AUTHOR]

Listronotus bonariensis (Argentine stem weevil) is a stem-boring weevil that has become a major pasture pest in New Zealand, and cool climate turf grass in Australia. This species is also frequently found in native tussock grassland in New Zealand. Laboratory and field trials were established to determine the risk posed to both seedlings and established plants of three native grass species compared to what happens with a common host of this species, hybrid ryegrass (L. perenne X L. multiflorum). Adult weevil feeding damage scores were higher on Poa colensoi and Festuca novae-zelandiae than Chionochloa rigida. Oviposition was lower on P. colensoi than hybrid ryegrass, and no eggs were laid on F. novae-zelandiae. In field trials using the same four species established as spaced plants L. bonariensis laid more eggs per tiller in ryegrass in a low altitude pasture site than in ryegrass in a higher altitude site. No eggs were found on the three native grass species at the tussock sites, and only low numbers were found on other grasses at the low altitude pasture site. Despite this, numbers of adult weevils were extracted from the plants in the field trials. These may have comprised survivors of the original weevils added to the plants, together with new generation weevils that had emerged during the experiment. Irrespective, higher numbers were recovered from the tussock site plants than from those from the pasture site. It was concluded that L. bonariensis is likely to have little overall impact, but a greater impact on native grass seedling survival than on established plants. [ABSTRACT FROM AUTHOR]

[Display omitted] • We calculated relative performance of Microctonus parasitoids on non-target v. target weevils. • Field host use correlated to relative performance in specificity tests. • Relative performance could help assess risks in future biocontrol programs. 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. [ABSTRACT FROM AUTHOR]