Eradicating the world’s worst aquatic weed

Four decades after its accidental introduction, hydrilla – known as the world’s worst submerged weed – has been almost completely eradicated from New Zealand. This highly invasive weed has been 99-percent cleared from four small lakes in Hawkes Bay, thanks to research and advice from NIWA’s freshwater biosecurity team. The risk of spread of this weed is now minimal.

NIWA’s research has led to the initiation of national eradication programmes for hydrilla and five other species by MAF Biosecurity New Zealand (MAFBNZ).

Hydrilla (Hydrilla verticillata, or Indian star vine), has earned its super-weed status through its ability to clog up open water in lakes and rivers. It has caused huge problems in Florida, where it escaped from aquariums in the 1950s, costing NZ$270 million per annum to control. In New Zealand, it first appeared in Lake Tutira, an isolated lake in Hawke’s Bay, in the 1960s.

Tubers make NZ hydrilla difficult to control

Contained experiments by NIWA scientists in the 1990s identified it as the most invasive of all the submerged weeds known here. “The New Zealand strain has proved to be very difficult to control,” says NIWA freshwater scientist John Clayton, “because it produces small tubers that can lie dormant in lake sediments for more than a decade.”

NIWA identified the herbicide endothall as effective at controlling large beds of hydrilla while leaving no toxic residues behind. Endothall has since become an important tool in the war on hydrilla and other aquatic weeds. But to prevent new tubers developing, every hydrilla plant had to be removed for at least a decade. “For that, grass carp were the only option, but were not proven,” says Dr Clayton.

Grass carp remove hydrilla from Hawkes Bay lakes

In 1988, NIWA began trialling introduced grass carp in Lake Elands (Hawkes Bay) as a means of long-term hydrilla removal. These plant-eating fish (not to be confused with koi carp, a pest fish currently infesting the lower Waikato River) successfully removed hydrilla from the lake over a 12-year period. The lake has remained hydrilla-free for the past decade.

In the past year, MAFBNZ has used grass carp to reduce the biomass of hydrilla by 99 percent in three other Hawkes Bay lakes (Lakes Tutira, Waikapiro, and Opouahi).

Thanks to this concerted research and eradication effort, a potentially very expensive weed problem has been averted, with minimal risk of hydrilla invading lakes and waterways nationally.

NIWA’s initial research on freshwater weeds was funded by the Foundation for Research, Science & Technology. NIWA continues to provide contracted advice to MAFBNZ on freshwater weed management and control.

What it takes to control a super-weed: timeline of hydrilla management

1982 – Hydrilla nationally banned from sale and distribution under Noxious Plant Act

1986 – proposal to trial grass carp in Lake Elands

1988 – grass carp introduced in Lake Elands

1997 – Proof of concept - competition experiments & report

2001 – successful endothall field trial

2005 – endothall registered for aquatic use in NZ

2005 – Hydrilla reassessed & confirmed as a National Interest Pest

2006 – present eradication plan to MAFBNZ for hydrilla

2008 – MAF eradication response commenced

2010 – Hydrilla reduced to <1% and risk of spread now negligible

Contact: Dr Deborah Hofstra

MAFBNZ information on hydrilla, its impact and control