Diquat is the herbicide used to combat Lagarosiphon in waterways of New Zealand. You have probably seen the helicopters whizzing around the lake and had to avoid going into the water for 24 hours afterwards. But do you want to know more?
Diquat is the herbicide currently used to kill the aquatic weed Lagarosiphon. Diquat is fast acting and has been used for many years in NZ to combat invasive aquatic weeds. This is a contact herbicide that desiccates (removes moisture) and defoliates (removes leaves) the plants causing it to die. Diquat is best suited to dense weed beds as the herbicide will be retained for longer in the area, increasing contact with the targeted plant. Application of diquat is dependent on the turbidity of the water; organic matter and silt quickly deactivate the chemical meaning it can no longer take effect.
Herbicides can cause a substantial reduction in the biomass of the weed beds for up to one year before the need for reapplication. Application is done by both helicopter and boat using surface booms or subsurface injections. Helicopters are ideal for large areas and those further out in the lake whereas boats are ideal for shoreline and specific location applications. Delaying removal of dead weeds can further slow recolonisation in the hope that native weeds can instead occupy the area.
There has been no evidence of any negative impact on native aquatic ecosystems in the 60 years Diquat has been used in NZ. Diquat is deactivated when it comes into contact with sediment and it therefore does not bioaccumulate in the water or soil. The low concentration applied to the water means there is no risk to humans, pets or agriculture. LINZ however momentarily restricts access (24 hours) to the water as a precaution to reduce any unforeseen risk to human health and agriculture.
Click on the links below to learn more about Diquat.