T.E.R:R.A.I.N - Taranaki Educational Resource: Research, Analysis and Information Network

Weevil (Horsetail weevil) Grypus equiseti

Kingdom: Animalia
Phylum: Arthropoda
Subphylum:  Hexapoda s
Class: Insecta
Subclass: Pterygota
Order: Coleoptera
Suborder: Polyphaga
Infraorder: Cucujiformia
Superfamily: Curculionoidea
Family: Curculionidae
Genus: Grypus
Species: G. equiseti
Scientific name: Grypus equiseti
Common name: Horsetail weevil

The Grypus equiseti is a type of beetle about 5–8 mm long that feeds on Equisetum arvense (field horsetail) or related species, laying its eggs in 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 killing the fronds.

The Environmental Protection Authority (EPA) has approved (May 2016) the horsetail weevil (Grypus equiseti) as a biological control agent to help curb the weed field horsetail (Equisetum arvense).which It is now classed as an unwanted organism under the Biosecurity Act 1993. This pervasive weed not responsive to herbicides. 

During January and February releases of the weevil, Grypus equiseti
were made. In March 2018 about 50 adult weevils were released in Rangitīkei to try to control the invasive, devastating weed ‘Field horsetail’ Equisetum arvense, and its devastating impact on pasture. Field horsetail is found extensively in the lower Rangitīkei region. The release was managed by the Rangitikei Horsetail Group with Horizons Regional Council, NZ Landcare Trust and Landcare Research. The weevils were released near Bulls, with the help of pupils from Clifton School in the small Rangitīkei town.

Equisetum arvense (field horsetail) click image for more details on this invasive weed

Equisetum arvense 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’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.

“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. 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 Ray McMillan, EPA’s Acting General Manager of Hazardous Substances and New Organisms.