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Ministry Set To Eradicate Saltmarsh Mosquito

Ministry to begin treating infested habitat at Wairau

The Ministry of Health is preparing to treat areas of the Wairau estuary in an initial attempt to eradicate Southern Saltmarsh Mosquito (Ochlerotatus camptorhynchus) that has been found in Wairau Lagoon.

The Southern Saltmarsh Mosquito (SSM) is an aggressive daytime biter and in Australia it is known to spread disease although there is no evidence to date of this happening in New Zealand.

The mosquito was found after duck shooters reported being bitten by aggressive mosquitoes on the opening day of the duck season.

New Zealand Biosecure and Nelson Marlborough Public Health Service staff completed a delimiting survey last week. The mosquito was in the estuary area south of the Wairau and in one small outlying site at Lake Grassmere.

This initial eradication programme will be a combination of aerial and ground treatments using an organic larvacide called Bacillus thuringiensis israelensis (Bti). In some drains and other suitable sites S-methoprene, a slow-release larvacide, will be used to supplement the Bti treatments.

The Ministry scientific advisors are developing the range of options for the government to enable it to make a decision regarding the final strategy that will be adopted to deal with this incursion.

Background Information Ochlerotatus (ochlerotatus) camptorhynchus (the Southern Saltmarsh mosquito) Introduction

Ochlerotatus (ochlerotatus camptorhynchus formerly Aedes camptorhynchus is commonly referred to as the Southern Saltmarsh Mosquito (SSM) in Australia due to its distribution throughout the southern coastal portion of Australia. This mosquito was discovered in Napier in December 1998, Gisborne Mahia and Porangahau in October and November 2000 and the Kaipara and Mangawhai harbours in February and April 2001 respectively. Two outlier sites were discovered at Whitford and Whangaporoaoa in 2002 and 2004 respectively.

Government determined that the mosquito should be eradicated from the East Coast (Muriwai) and Hawke’s Bay and more recently at the Kaipara and Mangawhai harbours.

The East Coast and Hawke’s Bay treatment programmes have all been completed. Napier and Mahia now meet the definition of eradication (2 years of sampling with no detections of larvae or adult SSM mosquitoes). Surveillance will be completed in Porangahau and Muriwai programmes in September this year.

The treatment phase of the programme at the Kaipara harbour will be completed in June this year. The last adult SSM was trapped in September 2003 and the last larve detected in March 2004. The Mangawhai and Whitford treatment programmes have been completed and surveillance will continue until December 2004. The Whangaporaoa site will be treated until April 2005.

In May 2004 the mosquito was discovered at Wairau lagoon near Blenheim. A delimiting survey also detected a single larvae and adult SSM on farmland adjacent to Lake Grassmere. Further positive sites and potential habitat were discovered along the Wairau Bar, low lying land adjacent to the Wairau Lagoon and at low lying sites adjacent to tidal drains, the Opawa River and the Wairau Diversion. The Mosquito The mosquito is an aggressive biter with the potential to cause significant nuisance effects for people, livestock and birds and is a confirmed vector of Ross River Virus (RRV) in Southwest Western Australia. If the SSM mosquito population is left unchecked a future risk exists for the introduction of RRV to susceptible human populations with the potential for an epidemic of the disease.

Whilst a cost benefit analysis has yet to be completed for the Blenheim Hawke’s Bay figures for the economic cost of disease were set at between $230,000 and 2.3 million dollars per annum.

If left unchecked and under favourable weather conditions the mosquito will colonise further areas of habitat with the potential for spread to other neighbouring harbours around the Malborough Sounds and to other parts of New Zealand. Eradication phase one – containment Under this phase the use of the larvicide Bti and insect Growth Regulator S-methoprene can be used to reduce the SSM population whilst Government consider the feasibility of undertaking a full eradication programme. Bti Bacillus thuringiensis (Bt) strains and varieties are pathogenic to a number of insect pests. The discovery of Bacillus thuringiensis israelensis (Bti), a variety specific to Diptera (especially mosquitoes and blackflies (sandflies) in Israel in 1978 has led to the development of many products based on this bacterium. These products have been extensively used in mosquito and biting fly control programmes, especially in Africa, USA and Germany and also in recent times New Zealand in Napier and Gisborne.

There is a well documented history of environmental safety of Bt strains used in pest control. The environmental safety of Bt, coupled with the nature of the toxicity and specificity for target hosts, has led to the use of Bti in many pest control programmes in environmentally sensitive areas including Napier, Gisborne and the Kaipara Harbour.

A review of the literature on the host range and effect of non target organisms indicates that Bti is relatively specific to mosquitoes and blackflies (called sandflies in New Zealand). It has also been shown to be pathogenic to some species of midges (chironomidae) and Tipulidae. However these species are less susceptible than mosquitoes and blackflies. In addition only midges are likely to be sharing a proportion of the same habitat as the Southern Saltmarsh mosquito (SSM) Ocherlotatus camptorhynchus.

Bti has not been reported to affect a large range of invertebrate species including most aquatic fauna. It is not toxic to bees. Fish are not affected either in the laboratory or after field applications. Shellfish including oysters and mussels are not affected by this product (Glare & O’Callaghan 1998). No direct applications of Bti over habitat of fish or shellfish are likely to occur as these areas are tidally flushed and are not good habitat for the SSM.

Bti is considered to pose little threat to mammals. Per os inoculations of animals and humans have not resulted in clinical symptoms. No direct applications over stock or people are proposed.

Bti does not persist in the environment after application. Generally reports of activity after application show a decline in efficacy within days and little residual activity after several weeks. In mosquito control terms no useful efficacy occurs after 48 hours of application In addition Bt occurs naturally and ubiquitously in the environment as a common component of the soil micro flora. In New Zealand Chilcott and Wigley (1993) found between 60-100% of soils sampled contained Bt. Organisms. Persistence is also related to the type and formulation of the product used. The liquid formulation proposed to be used has a low persistence profile.

S-methoprene Methoprene was first registered in 1975 in the USA and so an extensive safety data base has been established. In June 2001 the US Environmental Protection Agency completed a re-registration of S-methoprene products. The product is now registered as a Biochemical product rather than as a pesticide.

Methoprene degrades rapidly in sunlight, metabolises rapidly in soil and doesn’t leach into ground water. At label rates there are no toxic effects on terrestrial species and the report notes that methoprene has minimal acute toxicity for fish, freshwater invertebrates, and estuarine species

Studies completed on aquatic macro invertebrates determined LC50’s of >900ppb-1000ppb.

The S-methoprene products being used were assessed by The New Zealand Institue of Scientific Research (ESR) in 1999. These studies showed that the products we use release between <0.1ppb and 1ppb so the products used have a 900-9000 times margin of safety against test organisms.

A NIWA study 2002 (unpublished data) assessed S-methoprene as a control agent for chironomidae (midge larvae) in New Zealand. In the Laboratory Methoprene was found to begin to be effective at between 1-10 ppb. Field trials are being carried out to assess methoprene as a control agent at 10ppb – almost 10 times our current application rate.

The USEPA (2001) determined that as formulated and used at label rates there are no human health effects and no risks from occupational exposure to S-methoprene. There are no stock or crop withholding periods. The product does not effect plants.

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