Biosecurity surveillance of mosquitoes in New Zealand with a case example of methods used for the eradication of the Australian southern saltmarsh mosquito Ochlerotatus camptorhyncus

Abstract

This dissertation describes the science necessary to undertake a regional eradication of the established introduced mosquito species Ochlerotatus camptorhynchus. The science includes the establishment of a robust monitoring programme, the development of techniques to detect, identify and monitor New Zealand’s mosquitoes, and the testing of mosquito control products and their application that do not impacting non target species. A new dichotomous taxonomic key for the differentiation of all 16 mosquito larvae found in New Zealand is presented. Also new methods for preserving, transporting, and rearing of mosquitoes have been developed. A national survey established New Zealand has relatively few mosquito species (16 in total) and of these four are exotics (Culex quinquefasciatus, Oc. australis, Oc. camptorhynchus, and Oc. notoscriptus). Cx. pervigilans is the most abundant species and is widely distributed throughout New Zealand. Oc. notoscriptus is the most widespread nuisance mosquito. Notable records of Oc. notoscriptus and Cx. quinquefasciatus are found in Nelson and Christchurch. It is concluded that larval habitats are underutilised leaving New Zealand exposed for further exotic mosquito establishments. Oc. camptorhynchus possibly arrived in Napier some time prior to its discovery in the summer of 1998 covering an area up to 50 km2. Oc. camptorhynchus is a multivoltine saltmarsh mosquito species that does not exhibit larval diapause. Different approaches to mosquito surveillance were assessed. Larval dipping and CO2 adult trapping with the addition of 1-octen-3-ol (octenol) were the best methods for sampling Oc. camptorhynchus. Key environmental factors leading to Oc. camptorhynchus egg hatching were rainfall greater than 23 mm and tidal influences greater than 1.7 m. Often heavy rainfall events were associated with low pressure systems that heightened tidal events. The efficacy of methoprene against Oc. camptorhynchus and non-target species was assessed. In laboratory bioassays Oc. camptorhynchus mortality was 99.7% when treated with methoprene rates equivalent to 4 kg/ha of XRG granules. Laboratory reared Oc. camptorhynchus that were not acutely affected by the treatment showed sublethal effects with male genitalia not undergoing the normal rotation post emergence, effectively leaving males sterile. There was no acute impact on the survivorship of trout fry (Onchorhynchus mykiss), and no discernible effect on the notonectid Anisops assimilis and a dipteran Ephydrella species at rates of XRG up to 60 kg/ha. No significant difference in mortality was recorded between Chironomus zealandicus larvae exposed to rates equivalent to 6 kg/ha of methoprene and the control larvae, however, a significant increase in mortality was observed at a rate equivalent to 60 kg/ha. In the field rates of 6 kg/ha XRG granules extended egg to fourth instar larval development times from 8 to as much as 20 days before pupal mortality. Vegetation reduces the effectiveness of XRG granule applications by 20 - 30%. Based on the above experiments the eradication programme used methoprene as XRG granules and pellets at a rate of 6 kg/ha, supplemented by BTI at 1.5 L/ha and environmental modifications including vegetation removal and water management practises. Oc. camptorhynchus was declared successfully eradicated from the Napier region by the Associate Minister for Biosecurity, Hon. Marian Hobbs on 27 November 2003.