New Zealand EPA approves Horsetail weevil as a biological control agent to curb horsetail

The New Zealand Environmental Protection Authority (EPA) has approved the horsetail weevil (Grypus equiseti) as a biological control agent to help curb the weed field horsetail (Equisetum arvense).

Field horsetail is an invasive species with green fern-like fronds that grow up to 80cm 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 that 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 5–8 mm long that 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.

“Biological control agents, like the horsetail weevil, are used as natural enemies to reduce the populations of pests such as insects and weeds. The aim of this biological control agent is to limit the effects of field horsetail, and to reduce the rate and strength of invasion,” says Ray McMillan, EPA’s Acting General Manager of Hazardous Substances and New Organisms.

“Using biological control agents to manage weeds can be 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 EPA has approved horsetail weevil as the first agent for control of this weed. The applicant may consider applying for other agents for this weed in the future,” said Mr McMillan.

The application to introduce the new organism Grypus equiseti, from the Rangitikei Horsetail Group, was made under the Hazardous Substances and New Organisms (HSNO) Act 1996. The EPA received 19 submissions on the application, with 18 in support and one opposed.