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Biocontrol introduction

Target pest: Carduus nutans (Asterales: Asteraceae), nodding thistle

Agent introduced: Trichosirocalus horridus (Coleoptera: Curculionidae), nodding thistle crown weevil


1977, 1978, 1982-3

Import source:

1977, 1978 Rome, Italy. 1982-3 Germany via Canada

Import notes:

Cameron et al. (1989) - in 1977 and 1978 three consignments of the weevil were received from Rome. Adults from two consignments perished in transit.

Alonso-Zarazaga and Sánchez-Ruiz (2002), Groenteman (2007) - confusion over whether T. horridus was the species introduced to New Zealand [see Alonso-Zarazaga and Sánchez-Ruiz (2002) entry in the 'General comments' section].

De Baise et al. (2016) - evidence that it was in fact T. horridus released in New Zealand [see De Baise et al. (2016) entry in the 'General Comments' section].



Release details:

Cameron et al. (1989) - repairs to the rearing facility forced the release of the progeny of the surviving 1977-78 consignment before numbers were adequate for a satisfactory release. Two hundred adults, progeny of the Canadian imports, were released into a field cage at Winchmore near Ashburton (Mid Canterbury) in April 1984.

Jessep (1989) - in April 1988, 300 adults were collected from the Winchmore site and released at two sites: near Timaru in the South Island and near Whakatane in the North Island. In March 1989, 5,000 adults from the Winchmore site were released at 26 sites throughout the South Island and the lower half of the North Island. A similar number of releases is planned for March 1990.

Groenteman (2008b) - the weevil was mass reared and liberated widely during the late 1980s and early 1990s.


Cameron et al. (1989) - The 1978 release failed to establish. The cage from the 1984 Winchmore release was removed one year later and T. horridus has since become established at this site.

Jessep (1989) - Trichosirocalus horridus has now established at the Ashburton (Winchmore), Timaru and Whakatane sites. It is too early to tell if it is established at the 26 1989 release sites.

Groenteman (2008b) - the weevil has established and is becoming widespread in both the North and South Islands.

Landcare Research (2014c) - becoming common on several thistles.

Impacts on target:

Appears to cause a decline in thistle populations. Its impact may by augmented by that of Urophora solstitialis (nodding thistle gall fly).

Jessep (1989) - in 1987-88 the possible impact of T. horridus on nodding and Scotch (Cirsium vulgare) thistles (both previously present at the site) and five other thistle species (planted at the site for the trial) was examined at Ashburton. All nodding and Scotch (see ‘Impacts on non-targets’ section) thistles in the trial area became infested and severely damaged by T. horridus. Uninfested nearby nodding thistles produced a mean of 49 inflorescences and no lateral stems; by contrast, the crowns of all infested nodding thistles were destroyed and the weak lateral growth produced a mean of only 7.7 inflorescences. This site was managed to increase the weevil population; it remains to be seen how much damage will be caused by T. horridus in more natural conditions.

Harman et al. (1996) - anecdotal reports of the weevil reducing thistle densities are numerous.

Groenteman (2008b) - at many sites thistle populations appear to have declined about 5 years after the weevils were released, but their effectiveness has been variable.

Landcare Research (2014c) - often provides excellent control in conjunction with other thistle agents.

Landcare Research (2022j) - biocontrol agents were introduced against C. nutans in 1972 (Rhinocyllus conicus), 1984 (Trichosirocalus horridus) and 1990 (Urophora solstitialis). Although there were widespread reports of declines in abundance of C. nutans several years after establishment of T. horridus in particular, the thistle seemed to remain a serious pasture weed in some parts of New Zealand, and quantitative, nationwide data have been absent. To provide such data, revisits between 2013 between 2021 were made to 118 release sites across New Zealand where there are good records of nodding thistle density within four years of the release of either T. horridus or U. solstitialis. Results show the average C. nutans density at sites within 3 years of releases (1988-98) was 3.1 plants per square metre, and that this had dropped to 0.65 plants per square metre (a 78.9% reduction) at the 2013-21 revisits. There are still some heavily infested nodding thistle sites, even after biocontrol; there was no apparent geographical variation in this pattern, and no obvious factors to explain it. While the 79.8% reduction in C. nutans density cannot definitively be linked to biocontrol, there has been no change to Californian thistle (Cirsium arvense) densities at these sites, suggesting nodding thistle density has reduced due to biocontrol rather than land management changes. Approximately half the land managers at the revisit sites now spray less (or not at all) for nodding thistle; if these control costs are being achieved on just 10% of New Zealand sheep and beef farms, then the current, ongoing national cost saving is $26 million per year, a huge benefit:cost ratio for the complete nodding thistle biocontrol programme of 580:1.

Fowler et al. (2023), Landcare Research (2023h) - a cost-benefit analysis of all weed biocontrol programmes in New Zealand showed that in 2022 investment in weed biocontrol in the productive sector (targeting agricultural as opposed to environment weeds) was NZ$0.69 million, with the three most economically successful weed biocontrol programmes in New Zealand - against ragwort (Jacobaea vulgaris), St John’s wort (Hypericum perforatum) and nodding thistle (Carduus nutans) - yielding a combined annual benefit of NZ$84.7 million.

Impacts on non-targets:

Attacks plumless thistle (Carduus acanthoides), winged thistle (C. tenuiflorus), slender-winged thistle (C. pycnocephalus) Scotch thistle (Cirsium vulgare), marsh thistle (C. palustre) and cotton thistle (Onopordum acanthium). Reported to locally reduce the density of O. acanthium in Central Otago.

Jessep (1989) - in 1987-88 the possible impact of T. horridus on nodding and Scotch (Cirsium vulgare) thistles (both previously present at the site) and five other thistle species (planted at the site for the trial) was examined at Ashburton. All nodding (see ‘Impacts on target’ section) and Scotch thistles in the trial area became infested and severely damaged by T. horridus. Uninfested nearby Scotch thistles produced a mean of 83.5 inflorescences while infested Scotch thistles yielded only 7.7 inflorescences. Winged (Carduus tenuiflorus), slender-winged (Carduus pycnocephalus) and marsh (Cirsium palustre) thistles were also acceptable hosts in this trial with all rosettes becoming infested; these will almost certainly be alternative hosts for this weevil in New Zealand. Cotton (Onopordum acanthium), variegated (Silybum marianum) and Californian (Cirsium arvense) thistles were not attacked by T. horridus in this trial. This site was managed to increase the weevil population; it remains to be seen how much damage will be caused by T. horridus in more natural conditions.

Groenteman (2008b) - although the crown weevil prefers nodding thistle, they will also attack other thistles, especially plumeless (Carduus acanthoides), winged (Carduus tenuiflorus), slender-winged (Carduus pycnocephalus), Scotch (Cirsium vulgare) and cotton (Onopordum acanthium) thistles, and also marsh thistle (Cirsium palustre) to a lesser extent. Other plants are not thought to be at risk.

Paynter et al. (2004) - surveys of globe artichoke (Cynara scolymus), the only valued thistle in New Zealand, record no T. horridus feeding, as predicted by lab tests.

General comments:

Alonso-Zarazaga and Sánchez-Ruiz (2002) - what was considered T. horridus is, based on morphological characters, a species complex of three species: T. horridus, T. mortadelo sp. n. and T. briesei sp. n. This may explain the different food preferences observed between the different populations or strains of the supposed single species T. horridus. It is not clear which species were introduced as biocontrol agents to North America, Australia and New Zealand.

De Baise et al. (2016) - molecular and morphological analyses of specimens from three continents, and host plant associations, clearly indicate that there are only two distinct species, T. horridus and T. briesei, and support the synonymy of T. horridus and T. mortadelo. T. horridus has been established in Canada, USA, New Zealand and Australia and T. briesei in Australia. The former species was collected from Carduus, Cirsium and Onopordum spp. in the field, whereas the latter appears to be specific to Onopordum.


Alonso-Zarazaga MA and Sánchez-Ruiz M. (2002). Revision of the Trichosirocalus horridus (Panzer) species complex, with description of two new species infesting thistles (Coleoptera: Curculionidae, Ceutorhynchinae) Australian Journal of Entomology 41, 199-208

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.

De Biase A, Colonnelli E, Belvedere S, La Marca A, Cristofaro M and Smith L. (2016). Genetic and morphological studies of Trichosirocalus species introduced to North America, Australia and New Zealand for the biological control of thistles. Bulletin of Entomological Research 106: 99-113

Fowler SV, Groenteman R, Paynter Q (2023). The highs and the lows: a cost benefit analysis of classical weed biocontrol in New Zealand. BioControl (2023) https://doi.org/10.1007/s10526-023-10225-2

Groenteman R (2007). Multi-Species Interactions in Weed Biocontrol: Carduus nutans as a case study. PhD Thesis. Unversity of Canterbury, New Zealand

Groenteman R. (2008b). Nodding Thistle Crown Weevil, Trichosirocalus horridus. The Biological Control of Weeds Book (Landcare Research) https://www.landcareresearch.co.nz/discover-our-research/biosecurity/weed-management/using-biocontrol/the-biological-control-of-weeds-book/

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

Hayes L (2007a). Status of weed biocontrol agents in Southland. A report prepared for Environment Southland Sept 2007. Landcare Research Contract Report: LC0708/022

Jessep CT. (1989). Introduction of the crown weevil (Trichosirocalus horridus) as an additional biocontrol agent against nodding thistle. Proc. 42nd N.Z. Weed and Pest Control Conf. 52-54 https://doi.org/10.30843/nzpp.1989.42.10953

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/

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 (2022j). Comparing nodding thistle then and now. Weed Biocontrol: What's New? 102, November 2022 https://www.landcareresearch.co.nz/publications/weed-biocontrol/weed-biocontrol-articles/comparing-nodding-thistle-then-and-now/

Landcare Research (2023h). The highs and lows of cost-benefit analyses. Weed Biocontrol: What's New? 106, August 2023 https://www.landcareresearch.co.nz/publications/weed-biocontrol/weed-biocontrol-articles/the-highs-and-lows-of-costbenefit-analyses/

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