Biocontrol introduction
Target pest: Carduus nutans (Asterales: Asteraceae), nodding thistle
Agent introduced: Rhinocyllus conicus (Coleoptera: Curculionidae), nodding thistle receptacle weevil
Imported:
1972, later importations pre-1975
Import source:
Europe via Canada
Import notes:
Cameron et al. (1989) - in 1972, 119 adults were received from Canada. There were two subsequent importations, prior to 1975, from the same source, totalling 3,000 adults.
Released:
1973
Release details:
Jessep (1975) - R. conicus has now been released at 88 Valley and Kokorua (Nelson) and Winchmore (Ashburton) in 1973 and Taneatua (Whakatane) and Hakataramea (South Canterbury) in 1974.
Cameron et al. (1989) - progeny from the 1972 importation were released at two sites in Nelson (260 adults at one and 26 at the other) and Winchmore (215 adults). Weevils from the two later importations were used for releases near Whakatane.
Establishment:
Jessep (1975) - overwintering and successful establishment has taken place at the three 1973 release sites. Summer survival and successful reproduction indicate a similar level of establishment can be expected at Taneatua and Hakataramea.
Cameron et al. (1989) - natural dispersal, aided by a major effort to redistribute adults has resulted in R. conicus now being present in most C. nutans areas throughout New Zealand.
Groenteman (2008a) - as a result of redistribution efforts in the late 1970s and early 1980s the weevil is now common throughout the country.
Landcare Research (2014c) - common on several thistles.
Impacts on target:
Reduces seed production by about 36% in primary flowers but not secondary or tertiary flowers. The reduction in seed production is insufficient on it's own to cause a decline in thistle populations.
Cameron et al. (1989) - populations are now sufficiently large that a significant proportion of seeds are prevented from developing, although many secondary inflorescences escape damage. It is not known if R. conicus has reached its population potential; if not, then the impact on secondary inflorescences can be expected to increase.
Kelly et al. (1990) - at Argyll in Hawkes Bay R. conicus destroyed 34% and 49% of the C. nutans seed crop in 1984-85 and 1985-86, respectively. In Canterbury, in 1988-89 only 3% of seed was lost to R. conicus, possibly due to later flowering in Canterbury. Even at the higher rates of seed predation, many seeds survive to maintain the nodding thistle population. Although R. conicus attacks many inflorescences produced early in the season, most thistle seeds are produced in January and February, when the majority of inflorescences escape attack.
Groenteman (2008a) - study in 1980s at three sites (Ashburton, Whakatane, Rotorua) showed the weevils destroyed most seed produced by primary flowers (99%), but not secondary (72%) or tertiary (64%) flowers. A more recent study in Canterbury showed the weevil is not well synchronised with nodding thistle and may reduce overall seed production by around 15%. It is likely the impact of this agent will vary from place-to-place and year-to-year.
Landcare Research (2014c) - can help to provide 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:
R. conicus will oviposit on Carduus tenuiflourus and C. pycnocephalus but these are not preferred hosts and the weevil has minimal impact on the these species and low rates (8%) of successful development when using them as hosts. It may lessen the impact of the gallfly Urophora soltitialis on nodding thistle control.
Kelly et al. (1990) – at Argyll in Hawkes Bay R. conicus destroyed 24% of the Carduus pycnocephalus (slender-winged thistle) seed crop in both 1984-85 and 1985-86, though actual losses may be higher (up to 65%) due to the mode of feeding of this weevil. Hence, R. conicus seed predation may be having a useful effect on populations of C. pycnocephalus, an important weed in some areas. Unfortunately, no data is available about changes in density of this thistle since the introduction of R. conicus.
Paynter et al. (2004) - surveys of globe artichoke (Cynara scolymus), the only valued thistle in New Zealand, record no R. conicus feeding, as predicted by lab tests.
Groenteman (2008a) - although the weevil prefers nodding thistle, it will attack 5 other thistles to varying degrees: plumeless (Carduus acanthoides), winged (Carduus tenuiflorus), slender-winged (Carduus pycnocephalus), Californian (Cirsium arvense) and Scotch (Cirsium vulgare) thistles. It has been reported in the literature to attack variegated thistle (Silybum marianum) but this has not been observed in New Zealand.
Cripps et al. (2011) - field surveys in Europe and host specificity studies have shown that this weevil will attack a wide range of thistle species. Studies have found it feeding on 24 percent of inflorescenes in Californian thistle (Cirsium arvense) populations in the North Island, but absent from South Island populations. However, reducing seed production is not likely to be an effective strategy for controlling C. arvense since established populations reproduce primarily by vegetative means.
Cripps et al. (2020) - in Feb/Mar 2018, a survey of thistle seedhead-feeding biocontrol agents was undertaken in 18 pastures under sheep and/or beef production across the North and South Islands. In addition to C. nutans, R. conicus was recorded on Cirsium vulgare (four locations) and Cirsium arvense (one location). In Dec 2019 an opportunistic collection of thistle seedheads from three locations in the Gisborne region found R. conicus on C. arvense at two locations, and on Carduus tenuiflorus at one location.
Cripps & Mills (2024) - a survey of phytophagous insects associated with 16 populations of variegated thistle (Silybum marianum) in New Zealand showed that 27.4% of such insects were categorised as ‘specialists’, and that this category was almost entirely comprised of R. conicus. However, while R. conicus adults were commonly collected on S. marianum, only 6.5% of seedheads contained larvae of the weevil, indicating that S. marianum is not a common developmental host.
References
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.
Cripps M, Mills J (2024). Potential for biocontrol of Silybum marianum (variegated thistle): 1. Survey of natural enemies in New Zealand. New Zealand Entomologist, published online 1 Feb 2024 https://doi.org/10.1080/00779962.2024.2307997
Cripps M, Navukula J, Kaltenbach B, van Koten C, Casonato S, Gourlay H. (2020). Spill-over attack by the gall fly, Urophora stylata, on congeners of its target weed, Cirsium vulgare. New Zealand Plant Protection 73: 24–32 https://doi.org/10.30843/nzpp.2020.73.11718
Cripps MG, Gassmann A, Fowler SV, Bourdôt GW, McClay AS, Edwards GR. (2011). Classical biological control of Cirsium arvense: Lessons from the past. Biological Control 57: 165–174
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. (2008a). Nodding Thistle Receptacle Weevil, Rhinocyllus conicus. 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/
Jessep, C.T. (1975). Introduction of a weevil for biological control of nodding thistle. Proc. 28th N.Z. Weed and Pest Control Conf. 205-206 https://nzpps.org/_journal/index.php/pnzwpcc/article/view/9156/8988
Kelly D, McCallum K, Schmidt CJ, Scanlan PM. (1990). Seed predation in nodding and slender winged thistles by nodding thistle receptacle weevil. Proceedings of the Forty-Third New Zealand Weed and Pest Control Conference 43: 212-215 https://journal.nzpps.org/index.php/nzpp/past_volumes
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
