Despite attempts at management using chemical control, its spread continues with its invasion of the Wanganui River described as “phenomenal and unstoppable” by Craig Davey from Horizons Regional Council. Craig, together with AgResearch, has tested various herbicides for field horsetail, but the plant’s persistent underground root system makes high dosages and repeat applications necessary. According to Alistair Robertson, chair of the Rangitikei Horsetail Group, field horsetail can root down to 2 metres in river silts and sands, and it is extremely difficult to get herbicides to penetrate that deep.
With the demand for a more effective and sustainable solution for field horsetail management, biocontrol options were investigated. The field horsetail weevil (Grypus equiseti) was first imported into containment for host specificity testing in 2013, and permission for its release was granted by the Environmental Protection Authority (EPA) in mid-2016. This weevil has not been used as a biocontrol agent anywhere else in the world, and it was hoped it would be the only agent needed to suppress the growth and spread of field horsetail, but the project has had some challenges. The weevil colony was initiated with a fairly small number of adult weevils received in multiple shipments from the United Kingdom. The weevils were difficult to locate in their native range, so some shipments consisted of only a few individuals, which yielded few offspring, attributed to tapering fecundity of aging females. From the low numbers of progeny produced by the imported weevils only small numbers of weevils have been mass-reared for release.