Salvinia

30 August

Salvinia treatment update

A salvinia survey and treatment in the Old Reservoir were completed on 30 August 2023 in perfect conditions – calm and dry.

No salvinia plants were detected downstream. Also, many salvinia plants on the banks surrounding the dam are above the current lowered water level and have dried out. However, there were still live plants present.

The team sprayed the reservoir edges from the water and also walked the flats and spot-sprayed all salvinia found.

Update: Due to the bad weather last Friday the salvinia survey was postponed to Wednesday 26 July.

20 July

Salvinia monitoring in the old reservoir at Waihī Beach

On Friday 21 July Biosecurity NZ contractors will carry out a survey for presence of the aquatic weed Salvinia molesta in the old reservoir and downstream from its outlet.

Contractors will be working with local property owners to inspect the downstream waterway on foot, and will be using kayaks within the old reservoir.

Long-term monitoring is being undertaken as part of the eradication programme, after this invasive weed was detected in the reservoir in March 2023.

13 June

Treatment update

A second treatment occurred on 25 May. Some days later a deluge of rain caused flooding which flushed some salvinia plants downstream. However, the days between the application and the rainfall were enough for most plants to take up the herbicide. There will be a further assessment in late June.

10 May

Salvinia at Waihi reservoir - second treatment, late May

Biosecurity operations staff report the first treatment has made a good impact on the salvinia infestation.

They will follow up with a second treatment to deal with remaining live salvinia plants in late May.

20 April

Treatment update

The treatment of the salvinia plants at the reservoir has had a good effect. There are already large areas of dead salvinia. Some survival from the first treatment can be expected and so a second one may be necessary in some weeks.

In mid April a number of small salvinia plants were manually removed from the area downstream of the reservoir, during a survey. This area will continue to be surveyed regularly.

3 April 2023

Notice: Treatment of the salvinia weed

Treatment of the infestation of salvinia will commence in the first week of April. The area, including the carpark, will be closed during the treatment operation.

Staff with chemical application approvals will apply herbicide at a low concentration directly to individual plants.

More than one application may be necessary, followed by manual removal, over an expected treatment period of a few months. The fence is expected to remain in place for the duration.

The product being used is Diquat, at the rate recommended for aquatic weed control use. Diquat breaks down very rapidly in water. It will not harm people, fish or other non-target aquatic organisms. Also, this method of application will not cause spray drift or movement downstream from the reservoir outflow.

Also, a temporary weir – a floating barrier – will be put across the outflow to prevent salvinia plants moving downstream.

If you want further information, please call Biosecurity New Zealand (a business unit of the Ministry for Primary Industries) on 0800 00 83 33 then press 0 (general inquiries).

17 March 2023

Salvinia weed find at old Waihī Beach reservoir

An infestation of the invasive aquatic weed salvinia (Salvinia molesta) has been detected at the old Waihī Beach reservoir. Biosecurity New Zealand is working on this with Western Bay of Plenty District Council, Bay of Plenty Regional Council, AsureQuality, Ngāti Tara Tokanui and Te Whānau a Tauwhao.

The reservoir is part of the Western Bay of Plenty District Council stormwater system, and if salvinia spread further it could cause problems for the wider drainage system. To prevent spread, we are asking people to stay out of the waters and to keep pets out of the waters.

Control of Salvinia

Compared with other sites, this infestation is quite large and complex, which means it will be difficult to access and remove all plant material.

We are currently surveying the area to assess the extent of the infestation at this site and in connected waterways. Work will then begin to remove all existing plants, which could take a few months. Manual removal is the preferred method, however careful use of a low-concentration herbicide may be necessary. A fence will be in place while removal is taking place.

Given how serious salvinia is, once all the existing plants are gone there will be a monitoring period for at least two years before this site can be declared free of salvinia.