In November 2020, Land Information New Zealand (LINZ) undertook weed spraying of the invasive waterweed, lagarosiphon on a 55 hectare area in Lake Ōkataina.

This work was managed by Boffa Miskell Limited on LINZ’s behalf to support the Te Arawa lakes restoration programme, working with Te Arawa Lakes Trust and the Bay of Plenty Regional Council. 

Follow-up monitoring by the Bay of Plenty Regional Council shows the amount of weed in the lake has significantly reduced and we are now seeing the regeneration of native underwater plants, particularly around Otangimoana Bay. This is great news for Lake Ōkataina and everyone who enjoys the area.

Building on this success and previous restoration work, LINZ will be doing further spraying in a different area of Lake Ōkataina in April. This operation will use a helicopter to provide greater accuracy and coverage.

LINZ will also be carrying out boat-based spraying applications to tackle lagarosiphon and other invasive weeds in Lake Tarawera, Lake Ōkāreka, Lake Rotorua and Lake Rotoiti between 7 April and 28 May as weather, weed and water conditions permit.

The helicopter and boat-based spraying operations will be managed by Boffa Miskell Ltd on contract to LINZ.

More information about this work is below.  If you have any questions please contact Boffa Miskell Ltd on 0800 638 943 directly.       

When will LINZ be carrying out this work?

Work will be carried out when conditions are favourable for spraying between 7 April and 28 May. Signs will be installed at lake entry points with further information. You can also see the schedule of operations on LINZ website here

What type of spray will be used for these operations? 

The spray that will be used in both the helicopter and boat spray operations is Diquat, a fast-acting herbicide that has been used in New Zealand for more than 50 years to help control underwater weeds on waterways and agricultural crops. It is registered for freshwater use in New Zealand and overseas.

Is it safe to use the lake for swimming, fishing and taking food and water when spraying is taking place?

Diquat poses little risk to human health at the concentrations used for underwater weed control. Despite this, LINZ takes a careful approach to its use and advises people not to swim, fish, drink or use water from the lake for 24 hours after spraying.

How will Diquat be applied in Lake Ōkataina?

Diquat will be applied in a gel form to around 120 hectares of Lake Ōkataina by helicopter. Using a helicopter means we can cover a much wider area in a more cost effective and efficient way. The spraying is expected to take around half a day.

Contractors are using a helicopter in this lake due to the large size of the weed beds and the extent of the area to be treated. 

Why do we need to control the weeds?

Invasive submerged weeds in New Zealand freshwaters are not native to our country and grow taller and more densely than our native vegetation, preventing native species from thriving in their natural environments.

These weeds are responsible for disrupting recreational use of our lakes and rivers. They overtake and replace native plant life and can interfere with the use of water for hydro power generation and irrigation.

Without control measures, these weeds will completely overrun our waterbodies.

Will Diquat hurt aquatic life?

Diquat use for submerged weed control has negligible (very low) risk to desirable native plant and animal life. Diquat has been registered for use in freshwater for more than 50 years with no identified impacts on aquatic life.

But isn’t Diquat described as toxic to aquatic life?

Like many products used around the home every day, Diquat is toxic in its concentrated form. What is most important in assessing toxicity is the dose of, or exposure to a substance. An effective Diquat concentration for weed control is less than one part in one million parts of water, resulting in a very low concentration when applied to treat submerged weed. Medications need to be given in the correct dose to do their job, similarly diquat needs to be applied in the right dose to do its job without causing harm to the environment, animals or people.

Why is LINZ carrying out this work and how is Te Arawa Lakes Trust involved?

Te Arawa Lakes Trust, Bay of Plenty Regional Council, LINZ and the Aquatic Pests Coordination Group work collaboratively together to ensure the lakes are sustainable, healthy and can be enjoyed for generations to come.

If we don’t take steps to control invasive water weeds, our lakes will end up smothered impacting the water quality and associated native plant and animal life.

Why is Diquat used instead of other weed control methods?

There are a handful of control methods for aquatic weeds and each has advantages depending on the situation. Herbicide use is currently the only method that can achieve weed control over large areas in a short space of time. Other methods are slower to apply and may not keep pace with the growth of weed. Herbicide is generally the most cost-effective method for large scale control.

How is Diquat applied to waterbodies?

There are strict instructions (on the label) for Diquat use in New Zealand aquatic systems that must be followed. Many regions also require that a resource consent is granted when using an aquatic herbicide. The label and resource consent ensure diquat is applied consistently and safely. A resource consent is in ace for this work in our lakes.

Diquat is applied directly to weed beds in the water at a rate of 30 litres of herbicide (containing 20% diquat) per hectare, resulting in a very low in-water concentration. Generally, one or two applications of diquat are made in a year to target weeds in New Zealand.