Horsetail Weevil

A weevil has been brought into New Zealand and bred at Landcare Research as a biological control agent to rein in field horsetail. 

Field horsetail, (Equisetum arvense), has become a serious invasive pest of pasture, crop and riparian areas in the Rangitikei and has shown signs of spreading into crops in Gisborne and Hawkes Bay. It’s a significant threat to dairy pastures. 

Field horsetail has green fern-like fronds that grow up to 80 centimetres tall. The weed was imported into New Zealand in the 1970’s as a herbal remedy from Europe, although Lindsay Smith (a research technician at Landcare Research) suggests it may have been here much earlier. Lindsay says early herbarium records of E. arvense date back to the 1920’s and 30’s in the Wanganui River system and over subsequent years has appeared in Westland, Marlborough and much of the lower North Island.

The stems contain silica, which is not digestible, but a more serious effect of the weed is that it is toxic to horses and cattle, leading to acute thiamine deficiency in horses and cattle.

Though field horsetail dies off in winter, it has a large underground root system consisting of rhizomes that give it the ability to spread easily and regenerate, making it extremely difficult to contain and control.

The weed also disperses via root fragments and after the Rangitikei-Manawatu floods of 2004, it spread much more widely. As a result there is now the potential for cropping and pastoral districts to be inundated, as a result of fragments attached to machinery travelling between farms. 

An additional dispersal issue is when river gravels are harvested for roading, as the roots of the plant (which are brittle and stick-like) are picked up by machinery and spread with the gravel. It is also easily spread through cultivation. As well, horsetail material in hay bales (which can contain as much as 15-20% horsetail) when rotten, promotes fungal growth and makes the rest of the hay inedible. 

Traditional weed control measures are costly, only minimally effective and struggle to control or even reduce the spread of this weed. 

In 2013, the Rangitikei Horsetail Group (earlier known as Lower Rangitikei Horsetail Control Group) was set up and obtained $316,000 from the Sustainable Farming Fund and $160,000 from farmers and local organizations to take on this issue. They approached Landcare Research who then became the science advisor to the RHG.  Chaired by farmer Alistair Robertson, the group consists of farmers and concerned community members from around the Rangitikei area, supported by district and regional councils and the New Zealand Landcare Trust, which manages the project. 

The first lump of Sustainable Farming Fund money allowed Landcare Research to import four potential bio-control agents from the UK: a flea beetle (adults and larvae attack the aerial plant parts); 2 x sawflies (the larvae feed on the aerial plant parts); and the horsetail weevil (whose adults feed on aerial plant parts and larvae mine down the stems into the roots). 

Initial host tests had to be undertaken to determine the insect’s host range and hence safety to release from containment. Lindsay Smith says they eventually focused on the weevil, as it was very damaging, being the only bio-control agent to attack the both the stem and the roots of horsetail plant.

Adult weevils lay their eggs near the top of the plant and the larvae then bore into and down the central stem, killing all above-ground material. Larvae continue to bore down into the root system, causing complete collapse of the stem. Larvae remain within the roots over winter, pupating the following spring to emerge as adult weevils and repeat the cycle.

Horsetail is the weevil’s only source of food and they have no known specialist predators in New Zealand.  Lindsay says, “Initial host-testing results are promising and the weevil is so effective that we are beginning to wonder whether any other agents would be needed; however, it is good to have the other species as a back-up”.

Environmental Protection Authority approval to release the weevil was granted in May 2016.  Landcare Research is currently building weevil numbers outside of containment.    

The Rangitikei Horsetail Group was successful in gaining a second tranche of SFF funding which will allow Landcare Research to rear and release the weevil over the next 3 years. 

Lindsay says they are scheduled to make the first field release in spring of 2017. He adds however, that biological control is not a quick fix solution, and while the weevils are extremely effectual in attacking individual plants, it will be some years before the weevils are widely dispersed and truly effective across the regions.