Report: TR06/03

Authors: John Clayton, Charlotte Severne (NIWA)

Abstract

Environment Waikato contracted NIWA to review several investigative reports dealing with the impacts of the aquatic herbicide diquat in the environment when used for aquatic weed control. The purpose of the review was to determine whether the results from these reports were reliable and consistent with other information on the use of diquat. Comment was also requested on the impact of this particular control method on iwi values, particularly the eel fishery.

Three references were supplied by Environment Waikato for this review, including Tremblay (2001), Landward Management (2005), and a pesticide information profile on diquat from the EXTOXNET website. Additional references were also reviewed.

Reglone® product undiluted contains 20 per cent of the active ingredient, diquat dibromide. Reglone® is diluted 100,000 times by water to attain the concentration required for control of waterweeds (1 mg.l-1 of diquat dibromide). This is equal to only one drop of Reglone® product in 10 litres of water. Diquat treatment of aquatic weed beds first began in the Rotorua Lakes in 1960 and has been used there regularly for 45 years. The concentration of diquat in water rapidly declines after application as a result of dispersion, plant uptake and adsorption to organic and inorganic (negatively charged) particles.

When applied to weed beds in an open waterbody the concentration of diquat often falls below detection limits within an hour or so of being applied. No evidence exists for toxic accumulations of diquat residues in bottom sediments following repeated usage. Diquat concentrate is a toxic substance, but at diluted rates for control of nuisance submerged weeds it is so diluted that it is less toxic than other common household products such as chlorine as used in swimming pools.

A review of reports addressing the impact of diquat usage in New Zealand found no evidence of toxicity and no significant changes in sensitive physiological biochemical markers in short-finned eels. Other field studies have shown no significant changes in mussels, shrimps, inunga, bullies, and koura presence inside and outside areas treated for weed control.

Targeted control of nuisance weed beds can provide habitat diversity by minimising specific problems arising from excessive weed growth in target areas, while maintaining the benefits of vegetation presence in uncontrolled areas. Extensive, uncontrolled dense weed beds can be more detrimental to the mauri through the decline in water quality and associated biota than from any potential impacts arising from diquat usage for the control weed beds. With respect to Waikato iwi values, such as the mauri and freshwater kaimoana, and for consistency with the iwi overall strategy for the awa, it would be advised that weed management be viewed in terms of a long-term management goal or strategy for the awa. This type of strategy should consider protecting and enhancing iwi values in conjunction with wider stakeholder activities and will require further discussion and development.

Review of Diquat Reports of Relevance to Iwi Values in Lake Karapiro [PDF, 156 KB]