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Biocontrol introduction

Target pest: Cytisus scoparius (Fabales: Fabaceae), Scotch broom, broom

Agent introduced: Aceria genistae (Acariformes: Eriophyidae), broom gall mite


2006 [NB: a strain of Aceria genistae was present in New Zealand prior to this, but is likely to be host-specific on gorse (see 'General comments' section).]

Import source:


Import notes:

Hill (2006) - it is proposed that galls of A. genistae will be imported from France in July 2006 to be reared in containment.



Release details:

Hill (2006) - it is proposed that A. genistae will be initially released (in mature galls tied to broom plants in the field) at at least one Canterbury site in spring 2007.

Strains attacking Cytisus scoparius from France and UK were released in Canterbury in November 2007.

Gourlay (2009) - only six releases have been made so far.

Gourlay (2010a) - widespread releases are now underway.

Landcare Research (2014c, 2015j) - 100 releases 2013-14, 81 releases 2014-15.

Landcare Research (2016a) - 1,382 infested broom stems dropped by helicopter onto broom over a 30km stretch of the Clarence River bed (Canterbury) in 2015. Strategic releases have been made in North Canterbury and Kaikoura since 2012.

Landcare Research (2018h) - 20 releases made in the year Sep 2017 - Aug 2018.


Recovered at one site in Canterbury by November 2008.

Gourlay (2009) - established at one site in Canterbury.

Gourlay (2010a) - the mite appears to be establishing readily at many sites.

Landcare Research (2014c) - establishing well.

Landcare Research (2016a) - while proving slow to establish in some regions the mite is booming in others, such as Canterbury. It is being more readily dispersed by wind than anticipated.

Landcare Research (2017g) - establishing well and becoming widespread in some regions.

Landcare Research (2022f) - after its release in 2008, A. genistae spread rapidly throughout the South Island but was slower to establish and spread in the North Island.

Impacts on target:

Gourlay (2010a) - plants at some of the earliest release sites are already showing signs of severe galling leading to plant death.

Landcare Research (2014c) - severely damaging plants at some sites.

Landcare Research (2015f) - very heavy galling observed at some sites e.g. Hanmer Springs (Canterbury).

Landcare Research (2016a) - at Leslie Hills (North Canterbury) half of 144 marked broom plants killed by mite after 3 years, with the surviving plants galled. At Lincoln, Canterbury, 70 of 72 trial plants dead, most likely due to A. genistae attack. At Waiau River (North Canterbury) there was significant damage, with large areas of broom killed, 3km downstream from the 2009 release.

Landcare Research (2017g) - showing considerable promise by beginning to cause extensive damage to broom at many sites.

Paynter et al. (2018) - Aceria genistae is an incipient success. Despite being preyed on by native and introduced predatory mites (mainly Typhlodromus caudiglan and Zetzellia maori), extremely damaging outbreaks have endured in New Zealand, perhaps because A. genistae mites are smaller than their phytoseiid predators so that there is a refuge from predation within the highly convoluted gall-like leaf curls.

Landcare Research (2018c) - observations, and reports of, heavily galled Scotch broom plants dying.

Landcare Research (2021f) - causing extensive damage to broom at many sites, especially in the South Island.

Landcare Research (2022f) - the impact of A. genistae started to become apparent about 10 years after its introduction, clearly slowing plant growth rates and in some cases killing whole stands of broom, with damage particularly evident in the South Island. In a trial between the summers of 2016-17 and 2021-22, A. genistae was released onto individual broom plants in the lower North Island between Palmerston North and Tongariro National Park, and between Palmerston North and Hastings. Plants that received a mite release were more likely to die and grew more slowly than control plants without mites, although the impact of the mite was lower than expected. Despite variable results and limitations of this study, it is clear the broom gall mite is reducing the growth rate and survival of Scotch broom in New Zealand.

Gourlay (2022a) - major landscape impacts from A. genistae attack have been observed with more than 50% of C. scoparius plants killed; in some localised instances as many as 90% of infested plants have died.

Paynter (2024) - factors influencing the success of weed biocontrol agents released and established in New Zealand were investigated. Each agent’s impact on the target weed in New Zealand was assessed as ‘heavy’, ‘medium’, ‘variable’, ‘slight’ or ‘none’, where a ‘heavy’, ‘medium’ or ‘variable’ impact have all been observed to reduce populations or percentage cover of their target weed in all or part of their respective target weed ranges in New Zealand. Results showed that: (i) agents that are highly damaging in their native range were almost invariably highly damaging in New Zealand; (ii) invertebrate agents with a closely related ‘native analogue’ species are susceptible to parasitism by the parasitoids that attack their native analogues and failed to have an impact on the target weed, and (iii) agent feeding guild helped predict agent impact - in particular, agents that only attack reproductive parts of the plant (e.g., seed and flower-feeders) are unlikely to reduce weed populations. Damaging impacts of A. genistae, a gall-forming mite, have been reported in its native range, it does not have a New Zealand native ecological analogue and its impact in New Zealand is assessed as ‘variable’.

Impacts on non-targets:

Hill (2006) - host range testing for A. genistae had been carried out in Australia against 33 Australian native and other valued plant species belonging to 10 tribes of the family Fabaceae. (Because of the highly specific nature of eriophyid mites testing of plants outside the Fabaceae was deemed unnecessary.) Additional testing was subsequently conducted in Australia against an additional 10 species from New Zealand; two lupin species of possible economic value, and eight legumes representing three of the four New Zealand native genera in the Fabaceae. In both studies, galls only formed on C. scoparius, the target plant, indicating that there is negligible risk that A. genistae would colonise native or economically valued exotic plants in New Zealand.

Landcare Research (2018c) - host range testing undertaken before A. genistae was introduced into New Zealand in 2006 showed convincingly that it does not attack native broom and is unable to complete its life cycle without access to Scotch broom [see Hill (2006) entry above]. Spill-over damage can sometimes occur on the close relative, tree lucerne (Cytisus proliferus), if broom gall mites are present in big numbers nearby, though they are not able to persist solely on this host.

Gourlay (2022a) - it is extremely unlikely that A. genistae will significantly damage anything other than C. scoparius and white broom (Cytisus multiflorus). Galls sometimes occur on tree lucerne (Cytisus proliferus) but this is spill-over damage which occurs when the mites are present in large numbers on C. scoparius nearby, and the mite cannot persist on C. proliferus.

General comments:

Gourlay (2010a) - prior to being introduced against broom Aceria genistae had already been recorded in New Zealand, but on gorse. Recent research shows that A. genistae includes a number of distinct strains, each of which is specific to one species of plant. The mites being introduced against broom are specific to broom and are unlikely to interbreed with the resident strain. [See Xue et al. (2015) entry below for the redescription of the gorse mite as a new species.]

Xue et al. (2015) - the mite causing galls on gorse (Ulex europaeus) in New Zealand was previously considered to be a strain of Aceria genistae [see Gourlay (2010a) entry above], based on an identification by David Manson in 1989. This was a misidentification and the species on gorse is redescribed here as a new species, Aceria davidmansoni.

EPA Applications:

EPA (2005c) - 18 Nov 2005: application by the Canterbury Broom Group to import into containment the broom biological control agents Aceria genistae and Agonopterix assimillela for breeding purposes pending release approval. EPA Application # NOC05012, approved with controls 18 Dec 2005.

EPA (2006) - 13 Mar 2006: application (EPA Application # NOR05003) by the Canterbury Broom Group to release from containment Aceria genistae, Agonopterix assimilella and Gonioctena olivacea for biological control of the weed broom. However, A. genistae was not considered in the decision because it was already present in New Zealand and therefore not regarded as a new organism by EPA. [However, see the Xue et al. (2015) entry in 'General comments' section above indicating that the identification of the mite already present in New Zealand as A. genistae was a misidentification.]


EPA (2005c). EPA Application NOC05012 to import into containment the broom biological control agents Aceria genistae and Agonopterix assimillela for breeding purposes pending release approval. Environmental Protection Authority website https://www.epa.govt.nz/database-search/hsno-application-register/view/NOC05012

EPA (2006). EPA application NOR05003: to conditionally release from containment a mite, Aceria genistae (Eriophyidae), and two insects, Agonopterix assimilella (Lepidoptera, Oecophoridae) and Gonioctena olivacea (Coleoptera, Chrysomelidae), for biological control of the weed broom. Environmental Protection Authority website https://www.epa.govt.nz/database-search/hsno-application-register/view/NOR05003

Gourlay H (2010a). Broom gall mite: Aceria genistae. The Biological Control of Weeds Book - Te Whakapau Taru: A New Zealand Guide (Landcare Research) [Updated 2022] https://www.landcareresearch.co.nz/discover-our-research/biodiversity-biosecurity/weed-biocontrol/projects-agents/biocontrol-agents/broom-gall-mite/

Gourlay H (2022a). Broom gall mite: Aceria genistae. The Biological Control of Weeds Book - Te Whakapau Taru: A New Zealand Guide (Landcare Research) [Update of Gourlay (2010a)] https://www.landcareresearch.co.nz/discover-our-research/biodiversity-biosecurity/weed-biocontrol/projects-agents/biocontrol-agents/broom-gall-mite/

Gourlay H. (2009). The biological control of broom (Cytisus scoparius). IUFRO International Forest Biosecurity Conference, 16-20 March 2009, Rotorua, New Zealand. Popular Summaries. Compiled by Richardson M, Hodgson C and Forbes A. New Zealand Forest Research Institute Limited. 230-232

Hill R (2006b). Application to import for release or release from containment new organisms (Aceria genistae, Agonopterix assimilella and Gonioctena olivacea). EPA Application Number NOR05003 https://www.epa.govt.nz/assets/FileAPI/hsno-ar/NOR05003/c38af9007b/NOR05003.pdf

Landcare Research (2014c). Who's who in biocontrol of weeds? What's new in biological control of weeds? 69: 10-11 https://www.landcareresearch.co.nz/assets/Publications/Weed-biocontrol/WhatsNew69.pdf

Landcare Research (2015f). Autumn activities. What's new in biological control of weeds? 71: 8 http://www.landcareresearch.co.nz/publications/newsletters/biological-control-of-weeds/issue-71

Landcare Research (2015j). Biocontrol agents released in 2014/15. Weed Biocontrol: What's New? 73: 2 http://www.landcareresearch.co.nz/publications/newsletters/biological-control-of-weeds/issue-73

Landcare Research (2016a). Broom gall mite a decade on. Weed Biocontrol: What's New? 75: 3 http://www.landcareresearch.co.nz/publications/newsletters/biological-control-of-weeds/issue-75

Landcare Research (2017g). Who's who in biological control of weeds? Weed Biocontrol: What's New? 81: 10-11 https://www.landcareresearch.co.nz/publications/newsletters/biological-control-of-weeds/issue-81

Landcare Research (2018c). Galls on native broom no cause for concern. Weed Biocontrol: What's New? 84, May 2018 https://www.landcareresearch.co.nz/publications/newsletters/biological-control-of-weeds/issue-84/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 (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 (2022f). How damaging is the broom gall mite? Weed Biocontrol: What's New? 101, August 2022 https://www.landcareresearch.co.nz/publications/weed-biocontrol/weed-biocontrol-articles/how-damaging-is-the-broom-gall-mite/

Paynter Q (2024). Prioritizing candidate agents for the biological control of weeds. Biological Control, Volume 188, January 2024, Article Number 105396 https://doi.org/10.1016/j.biocontrol.2023.105396

Paynter Q, Fowler SV, Groenteman R. (2018). Making weed biological control predictable, safer and more effective: perspectives from New Zealand. BioControl 63: 427–436 (first published online 8 Aug 2017) https://doi.org/10.1007/s10526-017-9837-5 https://link.springer.com/article/10.1007/s10526-017-9837-5

Xue X-F, Han X, Zhang Z-Q (2015). Correct identification and biosecurity decision-making: Two species instead of one in Aceria genistae complex (Acari: Eriophyidae) in New Zealand. Systematic & Applied Acarology 20(1): 71-86 http://dx.doi.org/10.11158/saa.20.1.8