Endothall helping in the fight against invasive aquatic weeds

New research on the effectiveness of the herbicide endothall shows favourable results in the battle to rid lakes and rivers of New Zealand's most invasive aquatic weeds, including hydrilla, hornwort and lagarosiphon (an oxygen weed).

Until 2004, the herbicide diquat dibromide was the only product registered and used in New Zealand for submerged aquatic weed control. Though effective in controlling most aquatic weeds, diquat has had a limited effect on some of our worst aquatic weeds, especially hydrilla. It's also largely ineffective in turbid waters because it binds strongly to suspended sediments in the water, rendering it inactive.

Endothall was approved for use by ERMA in 2004 as an alternative herbicide. A contact-type herbicide that causes defoliation and stem die-off in aquatic plants, endothall has been used in the United States for aquatic weed control for over 40 years.

NIWA aquatic plants specialists have been testing endothall's effectiveness in New Zealand-specific conditions. Initial results are proving very promising.

Endothall has been used to effectively control New Zealand's worst aquatic weed, hydrilla, as part of a MAF Biosecurity New Zealand eradication programme centred on four Hawke's Bay lakes. Subsequent studies have shown the herbicide can eradicate hornwort and oxygen weed as well.

"Using endothall to remove lagarosiphon hasn't been trialled before, because it doesn't grow in the US. The suppliers of endothall and water managers were keen to see how effective endothall was on lagarosiphon in South Island lakes before they look at large scale applications," says NIWA Principal Scientist, Aquatic Plants, Dr John Clayton.

"What we've found is that endothall can potentially eradicate this pest species, even at very low doses. It also targets the root crown of the plants, preventing regeneration. That's something that diquat can't do because it's deactivated by the sediments. Better yet, it has very little effect on our native flora."