Endothall trial on hornwort in the Wairarapa

Endothall spraying
Contractor spraying a drain
choked with hornwort with
liquid endothall from a gun
and hose.
Photo: Greater Wellington
Regional Council

Suitable herbicide control in New Zealand for the invasive aquatic pest plant hornwort (Ceratophyllum demersum) could soon be available following recent trials in the Wairarapa. The trials were the result of collaboration between regional councils, district councils, Biosecurity New Zealand (BNZ), the Department of Conservation and NIWA.

These agencies have, over the last five years, been involved in a national consortium that has been successful in gaining ERMA approval for the importation of endothall, a herbicide that successfully controls hornwort in the United States. Greater Wellington Regional Council was also successful in gaining resource consent to apply it in the Wellington region.

Control options limited

Hornwort is a submerged freshwater perennial aquatic pest plant that has the ability to form dense mats that smother other species and impede hydro-electricity generation and recreational activities. Plants, or stem fragments of plants, can easily be dislodged and float to other areas to establish new colonies. It can flourish in still or flowing water to depths of up to 16 metres.

Hornwort is reasonably widespread in the North Island but spread is very limited in the South Island. Control options for large water bodies are restricted to mechanical harvesting (which only provides short-term control), Chinese grass carp, and the herbicide diquat.

Diquat can be effective against hornwort and is used in some New Zealand lakes for control. The herbicide can be largely ineffective in turbid water, however, because it clings to suspended sediments in the water body, and on epiphytes on plant surfaces, which render it inactive.

The current control options are unsuitable for hornwort in the Wairarapa and in many other parts of New Zealand.

Endothall trials encouraging

In recent years, NIWA has done extensive research on herbicides registered overseas for possible use in New Zealand aquatic environments, including those that work on hornwort. Previous trials on hornwort using endothall in liquid form have proved encouraging. Unlike diquat, endothall is not inactivated by suspended sediments.

Since consent was granted to import endothall into New Zealand, an application trial site has been arranged on farmland on the north shore of Lake Wairarapa. The area has a range of potential control situations with drains, wetlands and streams choked with hornwort.

The site was surveyed to find out the extent of the hornwort within the drainage and stream system. This was to gauge whether any hornwort that remained after control was due to reinfestation or regrowth from plants unaffected by the endothall.

There are two forms of endothall registered for use in New Zealand: the liquid form (Aquathol K®) and the granular form (Aquathol Super K®). Both formulations were used in the Wairarapa trial. This was the first time the granular form – Aquathol Super K – has been used in New Zealand.

Endothall in granule form was applied to the stream mechanically from a boat. Diluted liquid Aquathol K® was sprayed over the vegetation within the drains using a gun and hose method. Both formulations were applied to give a concentration of endothall of 4 parts per million in the receiving water body.

Water quality monitored

NIWA staff took water samples from the stream at referenced points prior to and immediately after treatment. Monitoring will now take place to ascertain what effect the herbicide and decomposition of hornwort is having on the water quality in terms of dissolved oxygen, temperature, and pH. Regular site visits will also be undertaken to seewhat effect endothall is having on controlling hornwort and the effects on other aquatic vegetation.

If results are favourable, this will then mean a control method will be available to use on hornwort in turbid water. At the time of going to press, results were still inconclusive, as it will take some time for water samples to be processed fully. However, the liquid endothall appeared to be giving good initial control, while the granular form was having little effect.

Endothall also has potential to be used in New Zealand as a control method for other submerged aquatic weed species, such as Lagarosiphon major and Hydrilla verticillata.

For an evaluation of endothall for control use in New Zealand:

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Page last updated: 30 April 2008