Resurveying for Southern saltmarsh mosquito
13 November 2000
Resurveying for Southern saltmarsh mosquito after recent rainfall
Intensive surveying of the coast of the North Island for the presence of the southern saltmarsh mosquito began today after recent heavy rainfall has created ideal hatching conditions for the larvae.
Health Protection Officers from Hutt Valley Health, Choice Health Wairarapa and Mid-Central Health are combing the coastline from Cape Kidnappers in Hawke's Bay to Cape Palliser in Wairarapa for any potential habitats of Aedes camptorhynchus and looking for larvae and adults.
The 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.
"If there are any larvae or adults we would expect to find them in such conditions, " Ministry of Health Deputy Chief Technical Officer (Health) Sally Gilbert said.
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.
A national surveillance programme has been in place since the mosquitoes where first identified in Hawke's Bay bay in December 1998. Surveillance measures were enhanced last month following the discovery of mosquitos in Gisborne.
The surveillance programme had been based on the fact that the southern saltmarsh mosquito has a five kilometre flight range and its potential locality to ports and airports. However the find in Porongahau, which is 85 kilometres south of the most southern positive site in Hawke's Bay and the fact that Porongahau has no port or airport, raised questions about how it may have arrived. The surveillance area has been broadened.
Porongahau was resurveyed last week and surveying is underway in the Whakaki Lagoon north of Wairoa this week.
The Gisborne area is also being resurveyed this week after recent rainfall in the Tairawhiti region.
Ms Gilbert said the attempted eradication programme in Napier is progressing well and no larvae or adults have been found since April.
Mosquitos breed in water. The public can help discourage the breeding of mosquitos for example by checking gutters and drains around their house and ensuring they are clear of leaves and blockages, getting risk of old tyres, and also drill holes in tyre swings to allow water to drain free. Swimming pools should be well chlorinated, and boats and canoes on land overturned so they don't accumulate water.
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: http://www.moh.govt.nz/media.html
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.