Weevil introduced to whip horsetails

Field horsetail (equisetum arvense) is an invasive species.

A weevil will be brought into New Zealand to rein in field horsetail.

The horsetail weevil should give field horsetail a fright.

The Environmental Protection Authority has approved the horsetail weevil (Grypus equiseti) as a biological control agent for the weed following an application by the Rangitikei Horsetail Group.

Field horsetail (Equisetum arvense) is an invasive species with green fern-like fronds which grow up to 80centimetres tall.

Though it dies back in winter, it has a large underground root system that makes it difficult to control.

It also produces large quantities of spores which can germinate on bare ground, threatening native plants in sensitive habitats, such as wetlands and on the banks of waterways.

It is classed as an unwanted organism under the Biosecurity Act 1993.

It's found in Whanganui, Rangitikei, Taranaki, parts of Greater Wellington and the west coast of the South Island and has also been recorded on the east coast in Havelock North, Marlborough, Canterbury and Otago.

The horsetail weevil is a type of beetle about 5-8mm long which feeds on field horsetail or related species, laying its eggs into the stems of the weed.

These larvae feed on the stem while larger larvae consume and break up the roots, reducing the ability of the plant to produce new fronds in spring.

Adult weevils also feed on the stems, often causing fronds to die.

The EPA's acting general manager of hazardous substances and new organisms Ray McMillan said biological control agents were a cost-effective way of targeting and reducing the impact of weeds without resorting to chemicals.

"Although small infestations of the weed can sometimes be managed by ongoing removal of fronds or by repeated herbicide application, this requires long-term persistent effort and is often unsuccessful as well as uneconomic," said Mr McMillan.

"There are no native plants or valued exotic plants in New Zealand that are closely related to field horsetail. The closest relatives are ferns, but these are only distantly related.

The weevil is well established in Europe and has only been recorded on horsetails," said Mr McMillan.

The application to introduce Grypus equiseti was made under the Hazardous Substances and New Organisms Act 1996, and drew 19 submissions, with 18 in support and one opposed.