A retrospective evaluation of the host range of four Aphidius species introduced to New Zealand for the biological control of pest aphids

Abstract Four species of Aphidius (Hymenoptera: Braconidae) were deliberately established in New Zealand in the period 1977–1994 for the biological control of pest aphids. Biological control practice and its regulation evolved over this period, so that whereas the evidence required for the 1977 introductions was based on literature records, by 1994 additional experimental or observational information was required. This paper describes the use of no-choice and choice tests conducted in 1996 and 1997 to retrospectively evaluate parasitoid host ranges and it considers if this information would alter the original regulatory decisions. The test species included several native aphids that were unknown at the time of the original parasitoid introductions. Consistent with literature records, the experiments confirmed that Aphidius eadyi Stary, Gonzalez and Hall was specific to its target host, and Aphidius sonchi Marshall was largely specific. Aphidius ervi Haliday and Aphidius rhopalosiphi De Stephani-Perez parasitized several test aphid species including cosmopolitan pest species already recorded in the literature, and some native test species. The patterns of mummification of individual test aphids varied greatly. Test aphids in the subfamilies Saltusaphidinae, Calaphidinae and Neophyllaphidinae appeared not to be at risk at all . The native Aphidini species Paradoxaphis plagianthi Eastop appeared to be more susceptible to attack by both A. ervi and A. rhopalosiphi than were P. aristoteliae Sunde or Aphis spp., suggesting that further investigations of P. plagianthi would be a priority if the introduction of these parasitoids were reconsidered now. Because P. plagianthi was not known when A. ervi and A. rhopalosiphi were introduced, prediction of their subsequent host range was limited by knowledge of the aphid fauna at that time. Therefore, reassessments of any decisions to release particular parasitoids would not be altered significantly by tests using knowledge available at the time of their introduction. However, if introduction of these parasitoids were to be considered today there would need to be a greater emphasis on determining their impact on (the recently documented) native aphid species.