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Southern saltmarsh mosquito suspected in Mahia

20 November 2000

Southern saltmarsh mosquito suspected in Mahia

Mosquitoes, suspected to be the exotic southern saltmarsh mosquito, Aedes camptorhynchus, have been found at Maungawhio Lagoon near Mahia on the East Cape. This is the fourth area where mosquitoes have been detected, since they were first found identified in New Zealand in December 1998.

The southern saltmarsh mosquitoes are a potential vector for Ross River virus disease.

The mosquito larvae and adult were detected at Mahia, in an area covering 27 hectares, during intensive surveying of the North Island coast from Gisborne to Cape Palliser in Wairarapa over the past fortnight.

Recent heavy rainfall, creating flooding of the estaurine areas in which the mosquitos breed, followed by a dry spell and then further rain and reflooding has created ideal hatching conditions for the larvae.

"We have refined our surveying methods and times, and the conditions were such that we expected to find larvae or adults if they were present," Ministry of Health Deputy Chief Technical Officer (Health) Sally Gilbert said.

"The results show that our surveying methods are working," Sally Gilbert said.

A national surveillance programme has been in place since the mosquitoes were first identified in Hawke's Bay in December 1998. Surveillance for the mosquitoes was intensified last month after they were identified from a site in Muriwai near Gisborne, and, as a result of this surveillance, were subsequently found in Porongahau in southern Hawke's Bay. Porongahau and Gisborne were resurveyed a fortnight ago and again last week to confirm the extent of these infestations.

The results of surveillance along the east coast of the lower North Island last week are being awaited, although health protection officers undertaking the survey noted that some areas of potential saltmarsh mosquito habitat had dried out and would need to be rechecked after the next heavy rainfall.

The surveillance programme had previously been based on the belief that the southern saltmarsh mosquito had a five kilometre flight range and would therefore only be found in close proximity to ports and airports. However the find in Porongahau, which is 85 kilometres south of the most southern site in Hawke's Bay and the fact that the town has no port or airport, raised questions about how the mosquito may have arrived. The surveillance area was then broadened.

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.

Ms Gilbert said the attempted eradication programme in Napier is progressing well and no larvae or adults have been found for some time. She said that the eradication programme is also working to ensure there is no re-infestation of the areas that have been the subject of on-going treatment to eradicate the mosquito.

Ms Gilbert said the southern saltmarsh mosquito was known to be an "aggressive" day-time biter. She advised that people should avoid being bitten by any mosquitos. By screening open doors and windows, using insect sprays or mosquito coils indoors, wearing long clothing and repellent when outdoors, the possibility of being bitten can be reduced.


For more information contact: Annie Coughlan, Media Advisor, ph: 04-496 2067 or 025-495 989 Internet address:


An eradication programme is currently underway in Napier, Hawke's Bay. It is estimated that this will cost approximately $6-million over four years. A containment programme is underway in Muriwai and Sponge Bay in Gisborne and in Porongahau, southern Hawke's Bay. This involves ground spraying any sites returning positive samples, as well as disinsection of all aircraft departing Gisborne.

Sites that do return positive samples are being treated directly with Bti and S-methoprene.

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 mosquitos 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 documented 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.

Annie Coughlan Media Advisor Communications Communications DDI: 496 2182 Fax: 496 2010 Ministry of Health

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