Image: yellow flag iris.
Background
The application to release a leaf-feeding chrysomelid beetle, Aphthona nonstriata (Goeze) as a biocontrol agent for yellow flag iris, Iris pseudacorus L. will be made by Waikato Regional Council , representing the National Biocontrol Collective; a consortium of regional councils, unitary authorities and the Department of Conservation. Manaaki Whenua - Landcare Research is the science advisor to the Collective for this application.
Yellow flag iris is a perennial wetland herb of the iris family Iridaceae. It is native to Europe, North Africa, the Mediterranean, and western Asia. Yellow flag iris has a wide distribution in New Zealand and is particularly weedy in the lower Waikato River catchment and along the Avon River in Christchurch. Yellow flag iris has not yet reached its full potential distribution across New Zealand. Flooding events can spread its seeds and root fragments over long distances, enabling new infestations in vulnerable wetland and riparian areas.
Yellow flag iris threatens native flora in wetland and riparian habitats and is a problematic weed of wet pastures. It can alter habitats through the formation of dense rhizome mats, which accumulate sediment and cause wetland areas to become drier. These rhizome mats are also responsible for the displacement of native plant species. Yellow flag iris can clog waterways and irrigation ditches, which not only disrupts farming and recreational activities, but also increases the risks of flooding by obstructing water flow and reducing channel capacity.
Identification and assessment of risks, costs and benefits
The potential risks, costs and benefits associated with the proposed introduction of a root-feeding chrysomelid flea beetle, Aphthona nonstriata to New Zealand and the possible reduction in the abundance and vigour of yellow flag iris have been identified by literature review and by consultation with stakeholders.
The application will address in detail the potential significant adverse and the beneficial effects identified (highlighted in bold in the above linked report). The potential benefits of biological control will be the mitigation of the adverse effects of yellow flag iris on the New Zealand environment. The key potential adverse effects relate to impacts on native flora and fauna, and on exotic ornamental irises. These will be addressed fully in the application:
- potential for direct damage to native plants
- potential for direct damage to ornamentals plants
- potential indirect effects on flora and fauna as a result of disruption of ecological relationships
Yellow flag iris (Iris pseudacorus) belongs to the well-known plant family Iridaceae in the order Asparagales (Kamra et al. 2023). The family Iridaceae comprises more than 2,000 species, including numerous economically significant taxa that are grown for ornamental and horticultural purposes (Kamra et al. 2023). There are no native Iris species in New Zealand and the closest related species are in the genus Libertia (Breitwieser et al. 2010-2025; Allan Herbarium 2025). There are eight Libertia species (and 1 hybrid) endemic to New Zealand (Breitwieser et al. 2010-2025), and their conservation status ranges from ‘Not Threatened’ to ‘Nationally Critical’ (de Lange et al. 2024). There are also two exotic species of Libertia naturalised in New Zealand (Allan Herbarium 2025) and one species known from cultivation (Manaaki Whenua - Landcare Research 2025).
At least 20 exotic genera in the plant family Iridaceae are represented in New Zealand’s introduced flora, either as naturalised species or casual aliens (Allan Herbarium 2025). Exotic genera in the Iridaceae with high species occurrence in the New Zealand flora include Gladiolus L., Iris L., Sisyrinchium L. and Watsonia Mill. (Allan Herbarium 2025). Several Iris species and cultivars are widely used as ornamentals in New Zealand, including (but not limited to) I. germanica, I. innominata, I. orientalis and I. spuria. These are all rhizomatous species. All except I. germanica belong to the subgenus Limniris, which also includes yellow flag iris. Iris germanica, by contrast, is placed in the closely related subgenus Iris. One Iris species, I. foetidissima L. is regarded as an environmental weed and found throughout New Zealand (McAlpine & Howell 2024).
Because the development of the yellow flag iris flea beetle is limited to species within specific subgenera of the genus Iris, the risk of direct harm to native New Zealand flora including native Libertia species, is expected to be negligible. Any potential impact on exotic ornamental irises cultivated in New Zealand can be effectively managed using standard pest control measures, such as insecticide application.
Significant population densities of the yellow flag iris flea beetle will only be found in close proximity to yellow flag iris infestations and other closely related species (none of which are native). Consequently, opportunities for interaction with other plants or animals will be minimal. As a result, no significant disturbance of ecological relationships is expected in New Zealand from the introduction of this biocontrol agent.
Pre-application consultation with Iwi
The members of Te Herenga, the EPA's national network, will be contacted and invited to enter dialogue on plans to apply to the EPA to introduce a biocontrol agent for yellow flag iris.
The message to Te Herenga will describe how the applicant intends to assess the risks, costs and benefits associated with the proposed introduction. Members will be invited to identify any issues that they would like to be addressed in the application. Further consultation will be led by Waikato Regional Council and Manaaki Whenua - Landcare Research.
At the request of the applicant, the EPA convened a reference group from Te Herenga to meet and discuss the issues surrounding the proposed applications. The application will address principles identified in that report.
Pre-application consultation with other organisations
Other organisations that will be consulted in the course of preparing this application include:
- Regional Councils
- Royal Forest and Bird Protection Society of New Zealand
- Department of Conservation
- QEII National Trust
- NZ Landcare Trust
- NZ Entomological Society
- NZ Plant Protection Society
- Iris growing societies (e.g., NZ Iris Society https://nziris.org.nz/),
- Iris nurseries (e.g. Amazing iris garden https://irisgarden.co.nz/).
Key documents
A survey of New Zealand Yellow flag iris associated invertebrate fauna and fungi
A survey of the invertebrate fauna and fungi associated with yellow flag iris (Iris pseudacorus L.) in New Zealand was carried out during the autumn of 2021 and the summer of 2022 by Manaaki Whenua - Landcare Research for the National Biocontrol Collective and the Ministry for Primary Industries.
Report on host-range determination
Safety issues are paramount to the development of biocontrol for weeds. Researchers rigorously test all proposed agents to assess the risk of damage to non-target plants. A set of procedures helps researchers choose a suitable shortlist of test plants, and this methodology is now well-accepted internationally (Wapshere, 1974). The technique is under constant review to update best practice (e.g. Sheppard et al., 2005; Briese, 2005).
- Host range determination - Report (report coming soon)
Cited references
Allan Herbarium (2025) Checklist of the New Zealand Flora – Seed Plants. Lincoln, Manaaki Whenua-Landcare Research. http://dx.doi.org/10.26065/py9d-ch68.
Breitwieser I, Heenan PJ, Nelson WA, Wilton AD 2010-2025. Flora of New Zealand Online - Taxon Profiles.
Briese D 2005. Translating host-specificity test results into the real world: The need to harmonize the yin and yang of current testing procedures. Biological Control 35: 208–214.
de Lange P, Gosden J, Courtney S, Fergus A, Barkla J, Beadel S, Champion P, Hindmarsh-Walls R, Makan T, Michel P 2024. Conservation status of vascular plants in Aotearoa New Zealand, 2023.
Kamra K, Jung J, Kim J-H 2023. A phylogenomic study of Iridaceae Juss. based on complete plastid genome sequences. Frontiers in Plant Science Volume 14 - 2023.
Manaaki Whenua — Landcare Research (2025) NZ Plant Names Database. Accessed https://biotaNZ.landcareresearch.co.nz. 2025-08-07.
McAlpine K, Howell C 2024. List of environmental weeds in New Zealand 2024. Science for Conservation 340: 37.
Sheppard AW, Heard TA, van Klinken RD 2005. Scientific advances in the analysis of direct risks of weed biological control agents to non-target plants. Biological Control 35: 215–226.
Wapshere AJ. 1974. A strategy for evaluating the safety or organisms for biological weed control. Annals of Applied Biology 77: 201–211.
