Mosquito eradication programme approved

The Cabinet has approved a $10 million programme to attempt to eradicate the southern saltmarsh mosquito around the Wairau and Lake Grassmere, Associate Biosecurity Minister Marian Hobbs announced today.

"This exotic mosquito, a known vector of the Ross River virus, is a real nuisance and is particularly vicious and aggressive, biting during the day," Marian Hobbs said. "We have yet to have a case of New Zealand-acquired, mosquito-borne disease in this country and this eradication programme will help to keep it that way.

"We're encouraged by successfully eradicating SSM from the North Island's east coast and the positive results we are seeing so far in the Kaipara eradication programme. We expect a similar outcome from the Wairau/Grassmere programme."

Around $10 million will be spent over the next four years developing and implementing a plan to eradicate the exotic mosquito from the Wairau and Lake Grassmere areas, which cover a potential habitat of 900 hectares.

The programme will involve aerial and ground spraying, using S-methoprene, an insect growth regulator that stops the mosquito pupae hatching into adults, and the biological spray Bti.

The mosquito lays its eggs on vegetation just above the waterline but wetting is vital for the eggs to hatch. High tides, heavy rain and wind that increase the size of waves can all encourage hatching. Media contact: Trevor Henry, press secretary, tel 04 471 9131; 025 477 994 Annie Coughlan, Ministry of Health media liaison, tel 04 495 4376; 0274 343 222

BACKGROUND Why is the southern saltmarsh mosquito a problem? The SSM is a known vector for the debilitating RRV. RRV is known as epidemic polyarthritis (inflammation of the joints). Being an aggressive day-biting species, the mosquito also has a high nuisance value and would significantly impact on the use and amenity value of the area. Both Wairau Estuary and Lake Grassmere are sites of public interest, visited by tourists, bird enthusiasts, duck hunters and the general public. Local Mäori have plans for tourism around the archaeological sites in the area, and the outdoor lifestyle of the region, for example the outdoor vineyard dining in the immediate vicinity. Further, workers (such as DoC staff) regularly operating in these areas would be at significant risk.

In Australia, the vicious day biting habit of these mosquitoes has resulted in the displacement of wading birds from areas where habitat range overlap occurred. Both the Wairau Estuary and Lake Grassmere are stopover sites for migratory waders such as oystercatchers, turnstones and red necked stints. These sites also host endemic waterfowl species. During treatment at these sites, helicopters will not fly over wading bird roosting sites at high tide or in the early morning when the majority of the birds are roosting. This will then minimise the disturbance of these birds.

What is Ross River virus? Ross River virus disease is known as epidemic polyarthritis (inflammation of the joints). Symptoms can be wide ranging, from pain and tenderness in the muscles and joints to flu-like symptoms of chills and fevers. Most people fully recover within a month of the onset of symptoms. No locally acquired cases of Ross River Virus disease have been reported.

How is Ross River virus spread? RRV can only be transmitted by mosquitoes; it cannot spread from person to person. Transmission of RRV into the human population requires the presence of the virus, its mosquito vector, a susceptible human population and suitable climatic conditions before the likelihood of an epidemic dramatically increases. These conditions exist within New Zealand. People carrying Ross River Virus will regularly be present in New Zealand (e.g. tourists or travellers returning from Australian states where Ross River Virus is endemic). Marlborough is a tourist destination as well as being a major transport corridor for tourists travelling between the North and South islands by ferry. SSM is an efficient vector of the disease both because of its susceptibility to the virus and the readiness with which the mosquito bites reservoir as well as human hosts. The New Zealand human population is non-immune and susceptible to RRV. There are animals in New Zealand that could act as ‘bridge hosts’ for the disease, for example possums and horses. Climate conditions within New Zealand are suitable for the SSM to establish and breed readily.

What is S-methoprene? S-methoprene is an insect growth regulator that stops the mosquito pupae hatching into adults. It has been used extensively overseas to control mosquitoes and the Ministry of Health commissioned an environmental and health impact assessment before deciding to use the product. S-methoprene breaks down quickly in the environment and is believed to be environmentally safe for use in New Zealand. Studies of the effect on 'non-target' species in Hawke's Bay have shown no impact.

What is Bti? Sites are being treated with the biological spray Bti -- already being used in eradication programmes in Hawke's Bay and Tairawhiti. This product has also been used extensively in control programmes in Australia, Africa, the United States and Germany. The Ministry of Health commissioned an environmental and health impact assessment before deciding to use the product. Bti is not allergenic. It leaves no long-term residue but is not considered adequate to achieve eradication.

Are there any effects on non-target species? The environmental impacts on non-target species of an eradication programme are considered to be very low. There has been sufficient use of Bti and S-methoprene and reporting on their relative environmental safety to consider that these treatment agents are very unlikely to cause any longterm or irreversible impacts to damage to non-target species. Short term eradication programmes, like that envisaged for the Wairau, are generally accepted as having less impact than long-term containment and control programmes. In addition, the areas to be treated are small and spaced apart, allowing for recolonisation even in the unlikely event of 100 percent of a species being eradicated from the areas subjected to treatment agents. Whilst there may be some initial reduction in susceptible native invertebrates, this is likely to be short term. Technical advice indicates that populations will rapidly recover as a result of recruitment from nearby untreated areas.

Chironomids (midges) have been recognised as a potential non-target species that may be affected by Bti, as these are considered a pest in many parts of the world and are controlled by various control agents, including Bti. Even so it is unlikely that Bti treatments in the Wairau area would eradicate Chironomid population as Bti is only effective on Chironomid populations at high rates of application, at least 10 times the rates needed for mosquitoes.

When and where were mosquito larvae first found in the Wairau/Grassmere area? In May 2004, duck shooters in the Wairau Lagoons reported nuisance biting from mosquitoes. Specimens were collected and were identified as SSM. The detection was made after two dry years followed by stimulation of hatching through unusually heavy rain in February and May 2004.

How big is the area of infestation in the Wairau/Grassmere area? A delimiting survey identified infestations throughout the estuarine area and at Lake Grassmere. Apart from two small sites at Station Creek and Blind River (in the prevailing wind path between Wairau and Grassmere) other habitat in the region has not been found to be infested. An upper estimate of the potential, but not necessarily infested, habitat is approximately 800-900 hectares.

What is happening with the other eradication programmes? The eradication programmes have, to date, been completed within the planned timeframes. Officials remain cautiously optimistic that eradication will be successful at all sites.

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