MPI moves to rid significant waterway of pest water weed

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The Ministry for Primary Industries (MPI) is taking action to contain an infestation of the aggressive aquatic weed Salvinia molesta in the nationally-significant Te Henga Wetland near Bethells Beach.

This rapidly spreading water weed was first found in Te Henga back in 2011 and treated with herbicide. MPI has been monitoring the area closely since then and recently found the salvinia was present again in three areas of the privately-owned wetland.

“Salvinia is known world-wide for its fast spread and difficulty to control,” says Dr Edwin Massey, Manager, Biosecurity Response for MPI.  “It forms dense mats on the water surface and has the potential to destroy habitats for our native plants and animals, including native aquatic bird life. It attracts breeding mosquitoes, removes oxygen from the water, affects recreational activities and creates a drowning risk for people and animals.

“It is unfortunate that it has reappeared in this unique wetland and we are now taking further actions to get rid of it for good.”

In a move to stop any further spread of the weed, a sign has been installed at the key entry point to the waterway off Bethells Road asking people not to remove water plants from the wetland and also to avoid fishing or using kayaks or other boats in the waters.

The Ministry and partners in the salvinia control, the Department of Conservation and Auckland Council, are this week installing floating booms around the infestations to help contain any further spread.

“These have been custom-made for Te Henga and are similar to the booms used to contain oil spills,” Dr Massey explains.

“They will form a floating barrier to stop salvinia plants and fragments drifting further within the wetland.”

Then the Ministry plans to re-spray by hand the three areas of weed, accessing the infestations by foot and kayak. The plan is to use a treatment called diquat – an aquatic herbicide that’s been safely used in New Zealand lakes for almost 50 years.

Dr Massey says it is possible there may be a need to use targeted helicopter based spraying of the affected parts of the wetland in future.

“This is due to the dense matting of the salvinia, the fact that even the tiniest fragment can re-seed a new infestation, and the difficulty getting full access by foot or boat to the affected areas.

“It is really important we talk to the local community about eradication options as we realise some members of the community may have concerns about targeted helicopter based herbicide spraying.  This is a proposal only at this time and we are scheduling a meeting to talk with the community and consider any feedback before finalising plans.

“MPI has comprehensive information about the situation and any spraying, including the dilution and safety of the product, the relatively small area of the total wetland affected, the accuracy of helicopter spray application and why we believe it is vital to get rid of the salvinia,” Dr Massey says.

MPI will advise a venue and date/time for the meeting in the near future. In addition, a letter and information sheet will be distributed through all letterboxes in the Te Henga/Bethells Beach area.

Dr Massey says ultimately it is important that people are well informed about the situation – both the threat salvinia poses to the wetland environment and the possible actions to deal with it. 

General information about salvinia is at:  

People with who believe they have seen salvinia can report it to MPI’s Pest Hotline – 0800 80 99 66.

Photos of salvinia are available on request.

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