Biocontrol introduction
Target pest: Equisetum arvense (Equisetales: Equisetaceae), field horsetail
Agent introduced: Grypus equiseti (Coleoptera: Curculionidae), horsetail weevil
Imported:
2013-17
Import source:
United Kingdom
Import notes:
Landcare Research (2014g) - four potential insect biocontrol agents for E. arvense from the UK, including G. equiseti, are currently undergoing host-range testing at the Lincoln, Canterbury containment facility.
Landcare Research (2016n) - potential biocontrol agents for E. arvense sourced from the UK, including G. equiseti, were host-range tested at the Lincoln, Canterbury containment facility during 2013-15. Now that EPA approval to release G. equiseti has been granted [see EPA (2016b) entry in ‘EPA applications’ section below], the challenge is to rear sufficient numbers to allow field releases to begin. Currently there are only of small numbers of larvae in containment. Some additional adult weevils recently shipped from the UK to help boost the population are producing plenty of eggs in containment, but their offspring will need to be rephased to southern hemisphere seasons before they can be released.
Landcare Research (2017a) - the weevil has proved difficult to rear in containment; another importation from the UK has just been received [this report was published May 2017] with the aim to build up the population for field releases.
Landcare Research (2020d) - Grypus equiseti was first imported into containment in New Zealand for host specificity testing in 2013. The containment 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.
Landcare Research (2026b) - mass-rearing G. equiseti was at first constrained by the physical stature of field horsetail stock plants. However, significant improvements to the mass-rearing protocol have resulted in the release of thousands of weevils [see Landcare Research (2026b) entry in ‘Release details’ section].
Released:
2017
Release details:
Landcare Research (2016i) - first field release planned for spring 2016.
Landcare Research (2017a) - the weevil has proved difficult to rear in containment; another importation from UK has just been received [this report was published May 2017] with the aim to build up the population so the first field release can be made in spring 2017.
Landcare Research (2018a) - in early-November 2017 the first adult horsetail weevils were released at two sites near Bulls in the Manawatu (lower North Island). Further releases were made in late-December at the first site on the edge of a wetland on a Parewanui dairy farm, and in January 2018 at the nearby second site along a fenced riparian strip. At both sites releases were made onto one of a paired set of plots, to enable the assessment of impacts as the weevils become established.
Landcare Research (2018h, 2019j) - 7 releases made in the year Sep 2017 - Aug 2018, 2 in the year Sep 2018 - Aug 2019.
Landcare Research (2020d) - as a result of rearing difficulties, only 545 weevils have been released since 2017. A new rearing system is being trailed with the hope that large releases of adult weevils can be made this coming (2020) spring.
Landcare Research (2021e) - 7 releases made in the year Sep 2020 - Aug 2021.
Landcare Research (2021g) - since approval to release the field horsetail weevil in 2017 it has been a struggle to establish field populations, probably due low numbers available for release. Following the success of a new rearing technique in 2020, 2,670 weevils have now been released; over 80% of these since rearing methods were improved. Releases have been made at six sites, all in Rangitikei District of the lower North Island. Assessment plots have been set up at five of these sites so impacts of the weevil on the target weed can be evaluated.
Landcare Research (2022h, 2023f, 2025f) - six releases made in the year Sep 2021 - Aug 2022, six in the year Sep 2022 - Aug 2023, five in the year Sep 2024 - Aug 2025.
Landcare Research (2026b) - significant improvements to the mass-rearing protocol has resulted in more than 6,500 G. equiseti being released across multiple sites since 2017.
Establishment:
Landcare Research (2020d) - establishment has yet to be confirmed. To date, no weevils have been recovered from the field, although the release sites have not been visited this year.
Landcare Research (2021f) - establishment is looking likely.
Landcare Research (2021g) - since the increase in number of beetles released (due to improved rearing methods in 2020) weevil damage has been recorded at three of the six release sites, strongly suggesting the weevils have established there.
Landcare Research (2026b) - confirming establishment of G. equiseti has proved difficult; they are not easy to find due to host plant density and weevil defensive immobility behaviour. In summer 2022-23, all release sites were visited, and although some damage looked consistent with weevil activity, there were no sightings of any life stage. However, when several sites were revisited in November 2025, adult G. equiseti were observed feeding and dead E. arvense stems containing G. equiseti eggs were found, indicating beyond doubt that the weevil has successfully established.
Impacts on target:
Landcare Research (2026b) - although it is still too early to know the full impact the weevil will have on E. arvense, there is optimism that if G. equiseti populations continue to build it will result in significant damage to field horsetail over time that could really help reduce the size and vigour of infestations.
Impacts on non-targets:
Landcare Research (2015k) - there are no New Zealand native genera in the same class as Equisetum; the most closely related native plants are currently believed to be the Marattioid ferns. Due to the extreme taxonomic isolation of E. arvense from native or valued exotics, a short test plant list was adequate to demonstrate environmental safety. Larval development tests confirmed that New Zealand test plant species are not hosts of G. equiseti and that it is unable to form populations on native New Zealand ferns and even minor spill-over attack is highly unlikely.
Smith (2016) - in its native range the horsetail weevil attacks a range of horsetail species and host-range testing has indicated that the other horsetail species also present in New Zealand, Equisetum hymale, is also a potential host.
EPA Applications:
EPA (2016b) - 20 Jan 2016: application by the Rangitikei Horsetail Group to introduce Grypus equiseti, a weevil for the control of field horsetail (Equisetum arvense). EPA Application # APP202712, approved without controls 19 May 2016.
References
EPA (2016b). EPA application APP202712: to introduce Grypus equiseti, a weevil for the control of field horsetail (Equisetum arvense). Environmental Protection Authority website https://www.epa.govt.nz/database-search/hsno-application-register/view/APP202712
Landcare Research (2014g). Field horsetail project forges ahead. What’s new in biological control of weeds? November 2014, 70: 2-3 https://www.landcareresearch.co.nz/assets/Publications/Weed-biocontrol/biocontrol-of-weeds-70.pdf
Landcare Research (2015k). The host-range of Grypus equiseti (F.) (Erirhinidae), a potential control agent for field horsetail Equisetum arvense L. Landcare Research website. https://www.landcareresearch.co.nz/__data/assets/pdf_file/0017/107504/host_range_tests_Grypus_equiseti.pdf
Landcare Research (2016i). Who's who in biological control of weeds? Weed Biocontrol: What's New? 77: 10-11 http://www.landcareresearch.co.nz/publications/newsletters/biological-control-of-weeds/issue-77
Landcare Research (2016n). Three new agents approved for two weeds. Weed Biocontrol: What's New? August 2016, 77: 6 https://www.landcareresearch.co.nz/assets/Publications/Weed-biocontrol/Issue-77.pdf
Landcare Research (2017a). Three successes with Sustainable Farming Fund. Weed Biocontrol: What's New? 80: 2 https://www.landcareresearch.co.nz/assets/Publications/Weed-biocontrol/Issue-80.pdf
Landcare Research (2018a). First release of new agents. Weed Biocontrol: What's New? 83, February 2018 https://www.landcareresearch.co.nz/publications/newsletters/biological-control-of-weeds/issue-83/first-release-of-new-agents
Landcare Research (2018h). Biocontrol agents released in 2017/18. Weed Biocontrol: What's New? 85, August 2018 https://www.landcareresearch.co.nz/publications/newsletters/biological-control-of-weeds/issue-85/biocontrol-agents-released-in-201718
Landcare Research (2019j). Biocontrol agents released in 2018/19. Weed Biocontrol: What's New? 89, August 2019 https://www.landcareresearch.co.nz/publications/newsletters/biological-control-of-weeds/weed-biocontrol-issue-89/biocontrol-agents-released-in-201819
Landcare Research (2020d). Clearing a hurdle for field horsetail weevils. Weed Biocontrol: What's New? 92, May 2020. https://www.landcareresearch.co.nz/news/clearing-a-hurdle-for-field-horsetail-weevils/
Landcare Research (2021e). Further reading and agents released. Weed Biocontrol: What's New? 97, August 2021 https://www.landcareresearch.co.nz/publications/weed-biocontrol/weed-biocontrol-articles/further-reading
Landcare Research (2021f). Who's who in biological control of weeds? Weed Biocontrol: What's New? 97, August 2021 https://www.landcareresearch.co.nz/publications/weed-biocontrol/weed-biocontrol-articles/whos-who-in-biological-control-of-weeds
Landcare Research (2021g). It is a numbers game for the field horsetail weevil. Weed Biocontrol: What's New? 98, November 2021 https://www.landcareresearch.co.nz/publications/weed-biocontrol/weed-biocontrol-articles/it-is-a-numbers-game-for-the-field-horsetail-weevil/
Landcare Research (2022h). Further reading and agents released 2021/22. Weed Biocontrol: What's New? 101, August 2022 https://www.landcareresearch.co.nz/publications/weed-biocontrol/weed-biocontrol-articles/further-reading-and-agents-released-202122/
Landcare Research (2023f). Further reading and biocontrol agents released in 2022/23. Weed Biocontrol: What's New? 105, August 2023. https://www.landcareresearch.co.nz/publications/weed-biocontrol/weed-biocontrol-articles/further-reading-and-agents-released
Landcare Research (2025f). Biocontrol agents released in 2024/2025. Weed Biocontrol: What's New? August 2025, 113: 12 https://www.landcareresearch.co.nz/assets/Publications/Weed-biocontrol/Issue-113/weed-biocontrol-issue-113.pdf
Landcare Research (2026b). Finding the weevil in the horsetail-stack. Weed Biocontrol: What's New? February 2026, 115: 5 https://www.landcareresearch.co.nz/assets/Publications/Weed-biocontrol/Issue-115/weed-biocontrol-issue-115.pdf
Smith L (2016). Field horsetail weevil: Grypus equiseti The biological control of weeds handbook (Landcare Research). http://www.landcareresearch.co.nz/__data/assets/pdf_file/0004/124708/Field-horsetail-weevil.pdf
