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Mosquito Containment Programme Recommended

13 October 2000

Mosquito Containment Programme Recommended

An expert Technical Advisory Group to the Ministry of Health has recommended a programme to contain the exotic southern saltmarsh mosquito, Aedes camptorhynchus begin as soon as possible, while a decision is made about future management of the mosquito in the Gisborne area.

The Technical Advisory Group, TAG, which was initially set up to advise on steps to be taken after the mosquito was first identified in Hawke's Bay in December 1998, includes various New Zealand experts as well as Australian mosquito expert, Associate Professor Brian Kay.

The recommendation comes after an intensive survey of 9000 hectares of land in the Gisborne area identified 15 areas where Aedes camptorhynchus larvae were found and four sites where adult Aedes camptorhynchus were captured.

Deputy Chief Technical Officer (Health) Sally Gilbert said the results of surveys to date suggest the southern saltmarsh mosquito is limited to a 36 hectare area around the Wherowhero Lagoon at Muriwai, 20 kilometres south of Gisborne, plus a very small area of 0.6 ha directly opposite at Sponge Bay.

She said a containment programme, as recommended by TAG would include treatment of any site returning positive samples, as well as disinsection of all aircraft departing Gisborne.

"Although the survey has confirmed that there are limited potential breeding sites in the Gisborne area, and the existing establishment is a long way from the airport, we don't want to take any chances. While the mosquito is largely limited to the Wherowhero Lagoon area, we have a good chance of containing the spread of Aedes camptorhynchus."

"The containment approach was also used in Hawke's Bay using the biological spray Bti, until the decision was made to progress with eradication. Bti, which is a biological spray, has undergone a full health impact assessment. It is specific to mosquito, blackfly (known as sandflies in New Zealand) and gnat larvae. The spray leaves no long term residue and has no known impact on the environment or people."

Associate Professor Brian Kay said the discovery of the southern saltmarsh larvae in Gisborne through routine surveillance was clear evidence that the national surveillance programme was working well.

"It was pleasing to see that an action plan was put in place, and surveys began within 72 hours of the Ministry of Health being advised on the find.

"It's also encouraging to know that no adult mosquitos have been found in Napier since April, and tests have shown that eight out of the nine mosquitoes captured prior to April were infertile as a result of the treatment programme in Hawke's Bay."

TAG advised the Ministry that information at this time suggested the eradication programme was achieving results and recommended it continue pending a long term decision about the action required in Gisborne.

Ms Gilbert said while the containment programme was underway in Gisborne, surveys and sampling would continue north to Hicks Bay, as well as coastal lagoons as far south as Wairoa. A health impact assessment and cost benefit analysis would also be conducted to provide further information before a recommendation is made to Government on longer term management in the Gisborne area.

Ms Gilbert said the public also had an important role in the containment programme.

"We would ask that, if possible, the public take any mosquito specimens to their Public Health Service, and also report any suspected sightings of southern saltmarsh mosquito." The southern saltmarsh mosquito is known to be an "aggressive biter".

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


Members of the Technical Advisory Group to the Ministry of Health present at today's meeting were: 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).

The southern saltmarsh mosquito has been declared an unwanted organism in New Zealand. In Australia it is thought to be the main carrier of the Ross River Virus. To date there have been no confirmed cases of Ross River Virus in Napier or Gisborne.

An eradication programme is currently underway in Napier, Hawke's Bay. It is estimated that this will cost approximately $6-million over four years. The biological spray, Bti was initially used for containment in Hawke's Bay and continues to be used for spot treatment.

The southern saltmarsh mosquito is known to be an "aggressive" biter. The Ministry of Health advises people in the Gisborne area to avoid being bitten by any mosquitoes by screening open doors and windows, using insect sprays or mosquito coils indoors, wearing long clothing and repellent when outdoors.

Background Information - 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).

A review of the literature on host range and effect on non-target organisms indicates that 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.

Over 40 tons of Bti were applied in west Africa alone, without any reports of safety or non-target concerns. The environmental threat posed by Bti would appear to be significantly less than that posed by most other forms of mosquito control which have a similar level of efficacy.

Selina Gentry Media Liason Communications DDI: 496 2483 Fax: 496 2010 Ministry of Health

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