Biological control of weeds in New Zealand
New Zealand is regarded as one of the weediest countries in the world[1] and classical biological control (biocontrol) has long been used as one method to combat New Zealand's most widespread and damaging weeds.[2] Classical weed biocontrol is where an agent specialised to feed on a weed species, typically a species of herbivorous insect or plant pathogen, is introduced into a country or region in an attempt partially or fully control the weed population. New Zealand is recognised as one of the five countries/regions of the world that has been most active in weed biological control (the others being Australia, Hawaii, North America, and South Africa).[3] Since weed biocontrol began in NZ in the 1920s, 69 biocontrol agent species have been released against 28 weed species.[4] Recently New Zealand has been releasing biocontrol agents faster than any other country, which has been partially attributed to the magnitude of the country's weed problem and its broad public support for weed control.[2]
Good weed control in New Zealand has been achieved using biocontrol for mist flower (Ageratina riparia), St John's wort (Hypericum perforatum), and ragwort (Jacobaea vulgaris) (the latter most successfully controlled on NZ's drier east coast).[2] Partial control has been achieved for Mexican devil weed (Ageratina adenophora), alligator weed (Alternanthera philoxeroides), heather (Calluna vulgaris), nodding thistle (Carduus nutans), and Scotch broom (Cytisus scoparius).[2] Other weeds, notably gorse (Ulex europaeus), have remained widespread and abundant despite several introductions of biological control agents.
Economic assessments suggest that weed biocontrol in New Zealand overall has been strongly cost beneficial. For example, the 2022 investment in weed biocontrol for production landscapes was NZ$0.69 million and yielded an estimated annual benefit of NZ$85 million.[4] The calculated economic savings to New Zealand from the successful biocontrol of St John's wort, by itself, is greater than the country's total investment in all weed biocontrol programmes.[2][5] St John's wort used to be one of NZ's worst pasture weeds until being successfully biocontrolled by two now widespread introduced beetles (Chrysolina hyperici and Chrysolina quadrigemina) with assistance in the upper South Island from an introduced gall-forming fly (Zeuxidiplosis giardi).[2][6]
It is too early to judge the success of some programmes as it can takes years to decades for agents to reach densities that have significant impact on weed populations.[2] A notable example of a delay that can sometimes occur in agent success has been the biocontrol of heather (Calluna vulgaris) using the heather beetle (Lochmaea suturalis).[7] Heather had spread widely on the central volcanic plataeu in the North Island.[8] The Department of Conservation initiated a heather biocontrol programme that led to L. suturalis being released at Tongariro National Park in 1990.[7] It was over a decade later, in 2001, that heather beetle outbreaks began killing patches of heather.[7] By 2007, the heather beetle outbreak was spreading quickly and it was demonstrated that heather beetles were more effective at killing heather than herbicide use, and led to an increase in cover of native dicot plants while these decreased in cover and species richness with herbicide application.[7]
With so many weeds wild in New Zealand, there had been the concern that biologically controlling one weed species on conservation land would only lead to replacement by another weed instead of benefiting the native flora.[9] This was shown not to be the case with the successful biocontrol of mist flower, which had become a dense and widespread forest riparian weed in northern New Zealand, and then was successfully biologically controlled by the release of a white smut fungus (Entyloma ageratinae) and a gall fly (Procecidochares alani).[9] Over five years of monitoring, the species richness and percentage cover of other introduced plants was unaltered while the species richness and percentage cover of native plants increased.[9]
History
[edit]
Weed biocontrol research has been done by successive generations of New Zealand scientists, beginning in 1925 at the Cawthron Institute in Nelson.[10] The first weeds targeted were blackberry (Rubus fruticosus agg.), foxglove (Digitalis purpurea), gorse (Ulex europaeus), ragwort (Jacobaea vulgaris), and one endemic native NZ plant, piripiri (Acaena anserinifolia).[10] Of the 17 insects imported for study, three were released and of these two established (the ragwort cinnabar moth (Tyria jacobaeae and the gorse seed weevil (Exapion ulicis),) while the Chilean piripiri sawfly (Ucona acaenae) failed.[11]
In the 1940s through to the 1980s, New Zealand weed biocontrol work was led by the Department of Scientific and Industrial Research, followed by the Crown Research Institutes from 1992.[2] There was a wane in biocontrol investment between 1931 and 1965 due to a focus on new-generation herbicides, followed by a resurgence in interest in weed biocontrol following increased awareness of herbicides' non-target impacts and issues with resistance.[2] To date, the decades with the most weed biocontrol introductions into New Zealand have been the 1990s and 2000s (with 15 and 13 agents introduced, respectively).[2]
Weed biocontrol programmes in NZ have typically been initiated by, and often partially funded by, primary production industries or agencies responsible for environmental management like regional councils and the Department of Conservation.[2]
Agents used
[edit]New Zealand's Better Border Biosecurity (B3) maintains a searchable database of all biocontrol releases into New Zealand, including for weed control.[12] iNaturalist NZ maintains a checklist of weed biocontrol agents[14] and a project for public observations of weed biocontrol agents in the wild.[15]
Active research is ongoing into agents for the biocontrol of the following weeds: Acacia longifolia, Araujia sericifera, Berberis darwinii, Cirsium arvense, Clematis vitalba, Cytisus scoparius, Equisetum arvense, Iris pseudacorus, Lonicera japonica, Marrubium vulgare, Nassella neesiana, Passiflora spp. (banana passionfruit), Solanum mauritianum, Tropaeolum speciosum, Ulex europaeus.[13]
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Safety
[edit]Weed biocontrol programmes need to identify potential agents, research their biology and impacts on their host plants, and then trial potential agents for potential non-target effects, before applying for permission to release into the wild.[2] Over the decades this work been done under increasingly stringent safety checks and balances prior to agents being released.[2] Weed biocontrol has been found to have a generally excellent safety record in New Zealand.[16] Extensive surveys have found cases of minor damage to some native plants, and those were from early introductions and were predictable from retrospective host range testing.[2] The New Zealand Environmental Protection Authority is now responsible for independently reviewing and approving agents before release, a process that includes public consultation.
Retrospective host range testing of the two Chrysolina beetles that successfully controlled St. Johns wort concluded that the more rigorous host range testing requirements of modern NZ weed biocontrol programmes would likely have prevented the introduction of these agents in NZ, due to feeding on related native Hypercium plants.[17]
Not all of New Zealand's weed biocontrol agents were intentionally introduced.[2] Some agents introduced into Australia have made their way across the Tasman to New Zealand. At least four weed biocontrol agents have reached New Zealand unintentionally: the broom twig miner (Leucoptera spartifoliella), hemlock moth (Agonopterix alstromeriana), blackberry rust (Phragmidium violaceum), and bridal creeper rust (Puccinia myrsiphylli).[2] Broom twig miners, native to Europe, now cause extensive damage to Scotch broom in parts of New Zealand, but were accidentally introduced, presumably on imported ornamental broom plants, and first seen wild in New Zealand in 1950.[18]
See also
[edit]References
[edit]- ^ Hulme, Philip E. (May 2020). "Plant invasions in New Zealand: global lessons in prevention, eradication and control". Biological Invasions. 22 (5): 1539–1562. doi:10.1007/s10530-020-02224-6.
- ^ a b c d e f g h i j k l m n o p q Hayes, L.; Fowler, S. V.; Paynter, Q.; Groenteman, R.; Peterson, P.; Dodd, S.; Bellgard, S. (2013). "Biocontrol of weeds: achievements to date and future outlook". In Dymond, J. R. (ed.). Ecosystem services in New Zealand – conditions and trends. Lincoln, New Zealand: Manaaki Whenua Press. pp. 375–385. ISBN 978-0-478-34736-4.
- ^ Schwarzländer, M.; Hinz, Hariet L.; Winston, R. L.; Day, M. D. (June 2018). "Biological control of weeds: an analysis of introductions, rates of establishment and estimates of success, worldwide". BioControl. 63 (3): 319–331. doi:10.1007/s10526-018-9890-8.
- ^ a b Fowler, Simon V.; Groenteman, Ronny; Paynter, Quentin (June 2024). "The highs and the lows: a cost benefit analysis of classical weed biocontrol in New Zealand". BioControl. 69 (3): 253–267. doi:10.1007/s10526-023-10225-2.
- ^ Fowler, Simon V.; Barringer, James; Groenteman, Ronny; Humphries, Grant (4 March 2025). "Biocontrol of St John's wort (Hypericum perforatum) provides huge ongoing benefits to New Zealand agriculture". New Zealand Journal of Agricultural Research. 68 (2): 308–320. doi:10.1080/00288233.2023.2232762.
- ^ Syrett, Pauline (1997). "Biological control of St. John's wort in New Zealand". Plant Protection Quarterly. 12 (2): 88–90.
- ^ a b c d Peterson, Paul G.; Merrett, Merilyn F.; Fowler, Simon V.; Barrett, Douglas Paul; Paynter, Quentin (October 2020). "Comparing biocontrol and herbicide for managing an invasive non‐native plant species: Efficacy, non‐target effects and secondary invasion". Journal of Applied Ecology. 57 (10): 1876–1884. doi:10.1111/1365-2664.13691.
- ^ Chapman, Hazel M.; Bannister, Peter (1990). "The spread of heather, Calluna vulgaris (L.) Hull, into indigenous plant communities of Tongariro National Park". New Zealand Journal of Ecology. 14: 7–16.
- ^ a b c Barton, Jane; Fowler, Simon V.; Gianotti, Alison F.; Winks, Chris J.; de Beurs, Maarten; Arnold, Greg C.; Forrester, Guy (March 2007). "Successful biological control of mist flower (Ageratina riparia) in New Zealand: Agent establishment, impact and benefits to the native flora". Biological Control. 40 (3): 370–385. doi:10.1016/j.biocontrol.2006.09.010.
- ^ a b Cameron, P. J.; Hill, R. L.; Bain, J.; Thomas, W. P. (1989). A review of biological control of invertebrate pests and weeds in New Zealand 1874 to 1987. Wallingford, UK: CAB International. ISBN 9780851986456.
- ^ Miller, David (1970). Biological control of weeds in New Zealand, 1927–48. New Zealand: Information Series No. 74, Department of Scientific and Industrial Research.
- ^ a b Philip, B.; Ferguson, C. M.; Moeed, A.; Barratt, B. I. P.; Hill, R. L.; Kean, J. M. "BCANZ—Biological Control Agents introduced to New Zealand". B3 · Better Border Biosecurity, New Zealand. Retrieved 9 November 2025.
- ^ a b "Meet the weeds". Manaaki Whenua – Landcare Research Research Group, Bioeconomy Science Institute, New Zealand. Retrieved 9 November 2025.
- ^ "Weed biocontrol agents of NZ". iNaturalist NZ. Retrieved 13 November 2025.
- ^ "Biocontrol of weeds in NZ". iNaturalist NZ. Retrieved 13 November 2025.
- ^ Fowler, S. V.; Gourlay, A. H.; Hill, R. H.; Withers, T. (2004). Cullen, J. M.; Briese, D. T.; Kriticos, D. J.; Lonsdale, W. M.; Morin, L.; Scott, J. K. (eds.). Safety in New Zealand weed biocontrol: a retrospective analysis of host-specificity testing and the predictability of impacts on non-target plants. Proceedings of the XI International Symposium on Biological Control of Weeds. Canberra, Australia: CSIRO Entomology. pp. 265–270.
- ^ Groenteman, R.; Fowler, S.V.; Sullivan, J.J. (April 2011). "St. John's wort beetles would not have been introduced to New Zealand now: A retrospective host range test of New Zealand's most successful weed biocontrol agents". Biological Control. 57 (1): 50–58. doi:10.1016/j.biocontrol.2011.01.005.
- ^ Hayes, L. Broom Twig Miner Leucoptera spartifoliella. Lincoln, New Zealand: Manaaki Whenua–Landcare Research. ISBN 0-478-09306-3. Retrieved 13 November 2025.