The Environment

Egeria guide

Controlling Egeria

What is being done to control Egeria?

The Christchurch City Council has trialled a number of different control methods, including mechanical harvesting, suction dredging, bottom lining and hand weeding. None of these is capable of achieving a high level of control, although they may all be useful as additional methods to keep Egeria under control.

It has therefore been decided to trial the use of the herbicide diquat, which is used successfully to control Egeria in the North Island. It is hoped that a mixture of methods will enable us to keep the Egeria to a minimum level and thereby reduce its spread to other sites.

Why is the Christchurch City Council using herbicide to control Egeria?

The Christchurch City Council chose to use diquat herbicide following investigation into the impacts of the weed if it was left uncontrolled. Other control methods were tested unsuccessfully and now diquat is the next tool available to the Council to evaluate.

What is the risk to my pets?

At herbicidal rates diquat poses no risks to pets. This includes swimming in or drinking the water. However, the river will be closed for 24 hours following application as a precautionary measure, and it is recommended to keep your pets out of the water during this time.

What is the risk to fish and wildlife?

If the weed is left to grow unchecked it poses a greater risk to wildlife than the discharge of diquat at herbicidal rates. Diquat has been used in the Rotorua lakes for the control of aquatic weed for over 10 years and no effect on the trout, ducks or native fish has been recorded. It is also worth noting that diquat is used in salmon fisheries for the control of fish-gill rot.

Can diquat herbicide kill people?

Caffeine and nicotine are more toxic than diquat at herbicidal rates. An adult would need to drink 1500 litres of water treated at maximum allowable herbicidal rates (2ppm) to take in enough diquat to cause toxic effects.

What will happen to the diquat herbicide following application to the watercourse?

The active ingredients of diquat herbicide are very short lived in natural waters; the herbicide is deactivated or broken down into harmless products.

Can I eat fish or shellfish from the river following application?

Following the 24-hour closure, any fish harvested from the river will be safe to eat.

Why doesn’t the Council just use the cutter more?

Egeria can grow from very small pieces, so using the weed cutter potentially increases its rate of spread.