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Course Of Action Against Mosquitoes

Expert Group Considers Best Course Of Action Against Mosquitoes

AN expert Technical Advisory Group to the Ministry of Health is currently determining what may be the best option to deal with exotic southern saltmarsh mosquitoes after they were discovered in the Kaipara Harbour.

The southern saltmarsh mosquito, Aedes camptorhynchus, has been declared an unwanted organism in New Zealand. In Australia they are thought to be the main carrier of the Ross River Virus but to date there have been no confirmed cases of Ross River Virus in New Zealand.

Ministry of Health Chief Technical Officer (Biosecurity) Dr Bob Boyd said the Technical Advisory Group (TAG) had considered a range of options for the Ministry to put to the Government. The options range from doing nothing, through to containment, through to continuing to aim for total eradication of the mosquitoes. He said the Minister for Biosecurity and the Minister of Health needed to be briefed and needed to consider the Ministry's recommendations, probably within the next 10 days, before he could discuss a preferred option.

The Technical Advisory Group (TAG) was set up to advise on how to deal with the mosquitoes when they were first identified in Hawke's Bay in December 1998. The first infestation of southern saltmarsh mosquitoes in Napier were sprayed with Bti and s-methoprene as part of an eradication programme. The mosquitoes were later found in both Muriwai in Gisborne and Porongahau in Central Hawke's Bay and spraying has also started in those areas, using Bti and s-methoprene.

Dr Boyd said the southern saltmarsh mosquito is known to be an aggressive daytime biter and advised people to avoid being bitten by mosquitoes by screening open doors and windows, using insect sprays or mosquito coils indoors, and wearing long clothing and insect repellent when outdoors.

Ross River virus disease is a non-fatal viral infection. All cases reported in New Zealand have been acquired overseas. People infected by Ross River virus disease may suffer pain and tenderness in muscles and joints, fever, chills, sweating, headache and tiredness. A rash may also occur on the trunk and limbs for a short time. The symptoms subside eventually and leave few or no after-effects. Symptoms occur most commonly in adults and the disease is usually milder and runs a shorter course in children. The only way people can catch Ross River virus is by being bitten by a virus-carrying mosquito. The virus cannot be spread from person to person.


When and where were the larvae found in the Kaipara area? Sampling was taken in the Kaipara Harbour area on 18 February as part of a routine national monitoring programme. On Tuesday 20 February the Ministry of Health was alerted that seven of the larvae found in the Rodney District of Kaipara Harbour may be southern saltmarsh mosquito larvae. The samples were then sent to Australia for confirmation. Since then, adult mosquitoes have also been found in the area. The Kaipara Harbour has always been considered a possible breeding ground for southern saltmarsh mosquitoes, but none had been found in previous surveys of the area.

Is there any suggestion as to how the southern saltmarsh got to Kaipara Harbour? We do not know when or how they arrived in the area. We do know that the southern saltmarsh species prefers coastal areas due to a preference for saline water habitats, and it is believed that the species has a 5 kilometre flight range.

How widespread is the mosquito in the Kaipara Harbour area? It is not possible at this time to say how widespread it may be, or how it arrived or when it established here. However, this area has been surveyed for some time, with enhanced surveillance being undertaken following the finding of southern saltmarsh mosquitoes in the Gisborne area in October 2000. No evidence of establishment has been detected until now.

What happens now? An intensive survey of the area is underway to confirm the extent of breeding sites, and identification of other potential breeding areas. The Ministry is working on recommendations to the Government about what is the best option for dealing with the mosquitoes. The recommendations are expected to go to the Government within 10 days.

What advice is being given to people in Kaipara Harbour area? The public have an important role in the containment programme. They are asked to tell their local Public Health Service of any places where they suspect the presence of southern saltmarsh mosquitoes, which are an "aggressive day-time biter", whereas the endemic mosquito is more commonly active during the evening.

Who is on the Technical Advisory Group? Members of the Technical Advisory Group to the Ministry of Health are: Dr Virginia Hope (Medical Officer of Health, Auckland Healthcare); Ruud Kleinpaste, (consulting entomologist); Professor Phil Weinstein (Wellington School of Medicine / National Health Council); Associate Professor Brian Kay, (Queensland Institute of Medical Research); Dr Mark Hearnden, (Wellington School of Medicine). Members unable to be present include: Ruth Frampton (Ministry of Agriculture and Forestry expert on biosecurity issues); Graham Mackereth, (Ministry of Agriculture and Forestry expert in veterinary epidemiology).

What is Bti ? Bacillus thuringiensis (Bt) strains and varieties are pathogenic to a number of insect pests. The discovery of B. thuringiensis israelensis (Bti), a variety specific to Diptera (especially mosquitoes and blackflies) in Israel in 1978, has led to the development of many products based on this bacterium. These products have been used extensively in mosquito and biting fly control programmes, especially in Australia, Africa, USA and Germany. There is a well document history of environment safety of Bt strains used in pest control. The environmental safety of Bt, coupled with the nature of toxicity and level of specificity for target hosts, has led to the use of Bt in many pest control programmes in environmentally sensitive areas, including the eradication of tussock moth in New Zealand (using Btk). Bti is relatively specific to mosquitoes and blackflies (known in New Zealand as sandflies). It has also been shown to be pathogenic to some species of midges (Chironomidae) and Tipulidae, although usually to a lesser extent than mosquitoes and biting flies. Bti has not been reported to affect a large number of other invertebrate species including most aquatic fauna. It is not toxic to bees. Fish are not affected, either in the laboratory or after field application. Bti is considered to pose little threat to mammals. 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. The persistence of Bti after application is dependent on the type of formulation/product used, with some formulations (pellets/briquettes) designed specifically to enhance residual activity.

What is s-methoprene S-methoprene is being used to supplement Bti, the control agent already being used. These two products are the same ones used already in the Napier eradication programme, both of which have undergone full health impact assessment. Bti, which is a biological spray, leaves no long-term residue and has no other impact on the environment or people. S-methoprene is an insect growth regulator that stops the mosquito pupae hatching into adults. It has been used extensively overseas to control mosquitoes. S-methoprene breaks down quickly in the environment and is believed to be environmentally safe for use in New Zealand. Studies of the impact on non-target species where it has been applied in the Hawke's Bay have shown no impact.

For more information contact: Angus Barclay, Media Advisor, ph: 04-496-2067 Selina Gentry, Media Advisor, ph: 04-496-2483 or 025-277-5411 Internet address:

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