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Biocontrol introduction

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

Agent introduced: Bruchidius villosus (Coleoptera: Chrysomelidae) = Bruchidius ater, Bruchidius fasciatus, broom seed beetle


1985, 1986, 2002

Import source:


Import notes:

Cameron et al. (1989) - Bruchidius villosus was imported from Britain into quarantine at Lincoln, Canterbury, in September 1985. Following host specificity testing the beetles were released from quarantine in November 1986 for laboratory rearing at Lincoln.

Landcare Research (2007a) - specimens of B. ater [a synonym of B. villosus] from England, dated 1985 and 1986, and B. villosus from England, dated 2002, are present in the Biological Control Voucher Collection of the New Zealand Arthropod Collection [indicating importations in those years].

Released: 1987

Release details:

Harman et al. (1996) - Bruchidius villosus was first released in the field in 1987; releases have now been made throughout New Zealand.

Gourlay (2007c) - released widely in the 1990s.


Harman et al. (1996) - has established at sites in mid-Canterbury, North Canterbury, and the Central North Island.

Gourlay (2007c) - widespread and common in North and South Islands.

Impacts on target:

Gourlay (2007c), Gourlay (2009) - some limited studies have shown broom seed beetle can destroy up to 90% of seed at some sites. Impact on broom population dynamics still uncertain.

Landcare Research (2014c) - beetle estimated to destroy about 75% of seeds in New Zealand now (and up to 84% at older release sites); models suggest this alone is unlikely to be sufficient to reduce broom populations.

Gourlay (2021e) - studies have shown that B. villosus can destroy up to 90% of seed. Population models indicate that B. villosus impacts depend on plant pollination rates, which, in turn, depends on the local abundance of honeybees and bumblebees. In areas where pollination is high, seed set is high, and 90% seed destruction will not affect broom populations. In areas where seed set is low, then B. villosus should cause a decline in broom populations.

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 B. villosus, a seed-feeding beetle, have not 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 ‘slight’.

Impacts on non-targets:

Cameron et al. (1989) - following importation of B. villosus into quarantine at Lincoln, Canterbury, in September 1985, host specificity tests were completed showing that oviposition occurred only on Cytisus species.

Paynter et al. (2004), Fowler et al. (2004) - B. villosus is recorded attacking tagasaste (Cytisus proliferus), an introduced minor fodder crop. In 'choice tests' B. villosus laid eggs on broom in preference to tagasaste, but the latter produces pods before the former in New Zealand, presenting a 'no-choice' scenario in the field. Extensive surveys indicate no native legumes are attacked by B. villosus.

Sheppard et al. (2006) - the original B. villosus host-specificity tests conducted for New Zealand in the 1980s was carried out in the UK using laboratory choice tests. Test plants included New Zealand natives and three species in the same subtribe as C. scoparius, and also exotic in New Zealand: Cytisus proliferus (tagasaste, a forage species), C. multiflorus (white Spanish broom, an invasive species) and Genista monspessulana (cape broom, an invasive species). Test results indicated B. villosus would only oviposit on C. scoparius. Thirteen years after release in New Zealand adults were found ovipositing and developing on C. proliferus growing adjacent to C. scoparius infested by B. villosus. Subsequent field-cage choice testing showed that, in contrast to the earlier testing, oviposition was observed on all species tested in the known host genera Cytisus, Genista and Spartium. (While B. villosus is only known from C. scoparius in the field in the UK, other field hosts are known in southern Europe.) It appears that the discrepancy between the original test results and the post-release field host specificity was not due to host range expansion but differing host flowering phenology, with the beetle congregating on the first host pollen source available in spring, which in New Zealand was C. proliferus, which flowered earlier than (though overlapped with) the target C. scoparius.

Paynter et al. (2018) - minor impacts of B. villosus have been observed on Cytisus proliferus.


Cameron PJ, Hill RL, Bain J, Thomas WP (1989). A Review of Biological Control of Invertebrate Pests and Weeds in New Zealand 1874-1987. Technical Communication No 10. CAB International Institute of Biological Control. DSIR Entomology Division. 424p.

Fowler, S.V., Gourlay, A.H., Rill, R.H. and Withers, T. (2004). 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. Cullen, J.M., Briese, D.T., Kriticos, D.J., Lonsdale, W.M., Morin, L. and Scott, J.K. (Eds). Canberra, Australia, CSIRO Entomology, 27 April - 2 May 2003.

Gourlay H (2007c). Broom seed beetle: Bruchidius villosus. The Biological Control of Weeds Book - Te Whakapau Taru: A New Zealand Guide (Landcare Research) [Updated 2021 - see Gourlay (2021e)] https://www.landcareresearch.co.nz/discover-our-research/biodiversity-biosecurity/weed-biocontrol/projects-agents/biocontrol-agents/broom-seed-beetle/

Gourlay H (2021e). Broom seed beetle: Bruchidius villosus. The Biological Control of Weeds Book - Te Whakapau Taru: A New Zealand Guide (Landcare Research) [Update of Gourlay (2007c)] https://www.landcareresearch.co.nz/discover-our-research/biodiversity-biosecurity/weed-biocontrol/projects-agents/biocontrol-agents/broom-seed-beetle/

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

Harman HM, Syrett P, Hill RL, Jessep CT. (1996). Arthropod introductions for biological control of weeds in New Zealand, 1929 - 1995. New Zealand Entomologist, 19(1): 71-80

Landcare Research (2007a). New Zealand Arthropod Collection (NZAC) Biological Control Voucher Collection. Landcare Research website [Updated 2020] https://www.landcareresearch.co.nz/tools-and-resources/collections/new-zealand-arthropod-collection-nzac/databases-and-holdings/new-t2-landing-page/

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

Paynter QE, Fowler AH, Gourlay AH, Haines ML, Harman HM, Hona SR, Peterson PG, Smith LA, Wilson-Davey JRA, Winks CJ, Withers TM (2004). Safety in New Zealand weed biocontrol: A nationwide survey for impacts on non-target plants. New Zealand Plant Protection 57: 102-107 https://journal.nzpps.org/index.php/nzpp/issue/view/vol57

Sheppard A, Haines M, Thomann T (2006). Native-range research assists risk analysis for non-targets in weed biological control: the cautionary tale of the broom seed beetle. Australian Journal of Entomology 45: 292-297 https://doi.org/10.1111/j.1440-6055.2006.00553.x