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.
Groenteman (2008a) - Rhinocyllis conicus is native to Europe and western Asia, but the New Zealand population came from Canada where it was introduced as a biocontrol agent.
Released:
1973
Release details:
Jessep (1975) - R. conicus has now been released at 88 Valley and Kokorua (Nelson) and Winchmore (Ashburton) in the South Island in 1973 and Taneatua (Whakatane, Bay of Plenty, North Island) and Hakataramea (South Canterbury, South Island) 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.
Groenteman (2008a) - Rhinocyllus conicus was redistributed throughout the country from established sites the late-1970s and early-1980s.
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:
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) - over four years (1982-83 to 1985-86) at Argyll in Hawke’s Bay (North Island) R. conicus destruction of the potential C. nutans seed crop increased from 22 to 49%, possibly reflecting the fact that R. conicus was first released in the area in 1981-82 and populations may still have been increasing. In contrast, in Canterbury (South Island) in 1988-89, 12 years after the release of R. conicus, only 3% of seeds were destroyed, possibly due to later flowering in Canterbury, so that a much smaller proportion of the seed crop was set in December when R. conicus was most common. At both sites, the proportion of seeds destroyed by R. conicus was low in mid-summer (January and February) when most seeds are produced, with the majority of inflorescences escaping attack, but higher earlier and later in the season. Even at the higher rates of seed predation, many seeds survive to maintain the nodding thistle population.
Kelly & Wood (1991) - previous work [see Kelly et al. (1990) entry above in this section] showed C. nutans seed predation by R. conicus of 49% at Argyll in Hawke’s Bay, four years after release of the weevil there in 1981, but only 2.8% in Canterbury in 1988-89, 12 years after release in that region. In the 1990-91 season, R. conicus removed 38.6% of seed at the Hawke’s Bay site (i.e. losses have levelled out and not increased beyond levels seen before 1986), while <9% of seed was lost at three Canterbury sites. On this evidence, seed losses at Argyll are unlikely to exceed 50% with any regularity; the effect of repeated losses of 30-50% of seeds in unknown, but many seeds still survive to germinate. The seasonal pattern of seed loss showed high losses early in the season (with essentially all early season seeds destroyed), with a dip in January, followed at some sites (particularly Argyll) by another increase late in the season. The density of R. conicus peaked in December at all sites. The relative timing of R. conicus attack and C. nutans seed production is crucial to determining levels of seed loss. Later timing of flowering (by three weeks) and lower densities of larvae per inflorescence in Canterbury are jointly responsible for the limited effectiveness of R. conicus there.
Cameron et al. (1993) - Rhinocyllus conicus has a large impact on one life stage of C. nutans (seeds) but has not had a clear impact on populations of the weed and cannot be classified as achieving even "partial" control (defined as “additional control remains commonly necessary but…pest outbreaks occur less frequently”) of its target species.
Kelly & McCullum (1995) - reductions in C. nutans seed output caused by R. conicus reached a peak of 49% only four years after release at Argyll (Hawke’s Bay, North Island) and have since settled down to around 40%. Further south, in Canterbury (South Island), <9% of seed was lost at three sites over two years, apparently because of worse synchrony between insect and plant life cycles and lower densities of the weevil [see Kelly et al. (1990) and Kelly & Wood (1991) entries above in this section]. However, even with the 40% loss of seed at Argyll, C. nutans seems as common as it ever was. Measured density-dependent survival of C. nutans from germination to flowering at Argyll would ameliorate the 40% decrease in seedling density to about a 13% reduction in flowering plant density, which would be scarcely detectable. Therefore, R. conicus is providing little control of C. nutans in New Zealand. However, this is not to say that R. conicus is of no value in this country; by reducing seed output at Argyll by 40% it must be reducing dispersal, making containment of populations easier, and other agents in combination with R. conicus (two - Trichosirocalus horridus and a seed-feeding gall fly [Urophora solstitialis] - have recently been released) may provide effective control.
Shea & Kelly (1998) - modelling using field data from 8,000 C. nutans plants at two sites in the North Island of New Zealand (Manawatu on the east side, Hawke’s Bay on the west side) confirm that both populations of the weed were increasing in number, as is expected of a noxious weed in its invasion phase. Simulations of attack by R. conicus indicate that it provides some measure of control by decreasing population growth rates of C.nutans, but that seed losses of 69% would be required to make the populations decrease in size, far more than the observed losses of 30-40% in New Zealand.
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.
Marchetto et al. (2014) - wind tunnel trials using C. nutans capitula (flower heads) with a natural range of attack by R. conicus, followed by capitula dissections, shows that R. conicus reduces the dispersal of C. nutans by lowering seed production, reducing the proportion of intact seeds that release from capitula and damaging seed dispersal structures so that seeds drop more quickly in still air. Past assessments of biocontrol agent impacts have labelled R. conicus a failure in New Zealand because weevil attack has not reduced C. nutans fecundity enough to cause population declines. However, floral feeders such as R. conicus may be providing benefits by reducing dispersal, because preventing invasive species from spreading to new areas is the most cost-effective strategy for long-term control.
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.
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 R. conicus, a seed-feeding beetle, have not been reported in its native range, it does have not a New Zealand native ecological analogue and its impact in New Zealand is assessed as ‘slight’.
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) - although C. nutans is the preferred host for R. conicus, Carduus pycnocephalus (slender winged thistle) is also attacked early in the season. At Argyll in Hawke’s Bay (North Island) C. pycnocephalus seed losses were estimated at 24% in both 1984-85 and 1985-86 seasons (a previous study at the same site estimated 22% and 40% in 1982-83 and 1983-84 respectively), but this may be an underestimate due to the mode of feeding of R. conicus, with actual losses potentially up to 65%. Hence, R. conicus may be having a useful effect on C. pycnocehphalus, which is 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.
Cameron PJ, Hill RL, Bain J, Thomas WP (1993). Analysis of importations for biological control of insect pests and weeds in New Zealand. Biocontrol Science and Technology 3(4): 387-404 https://doi.org/10.1080/09583159309355294
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 - Te Whakapau Taru: A New Zealand Guide (Landcare Research) [Updated 2021] https://www.landcareresearch.co.nz/discover-our-research/biodiversity-biosecurity/weed-biocontrol/projects-agents/biocontrol-agents/californian-thistle-gall-fly-2/
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 New Zealand Weed and Pest Control Conference 43: 212-215 https://journal.nzpps.org/index.php/pnzwpcc/article/view/10859/10691
Kelly D, McCullum K (1995). Evaluating the impact of Rhinocyllus conicus on Carduus nutans in New Zealand. Proceedings of the Eighth International Symposium on Biological Control of Weeds. 2-7 February 1992, Lincoln University, Canterbury, New Zealand. Delfosse ES & Scott RR (eds). DSIR/CSIRO, Melbourne, pp. 205-211 https://www.invasive.org/proceedings/pdfs/8_205-212.pdf
Kelly D, Wood R (1991). Why nodding thistle receptacle weevil destroys so little nodding thistle seed in Canterbury. Proceedings of the New Zealand Weed and Pest Control Conference 44: 280-283 https://journal.nzpps.org/index.php/pnzwpcc/article/view/10790/10622
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, November 2023 https://www.landcareresearch.co.nz/publications/weed-biocontrol/weed-biocontrol-articles/the-highs-and-lows-of-costbenefit-analyses/
Marchetto KM, Shea K, Kelly D, Groenteman R, Sezen Z, Jongejans E (2014). Unrecognized impact of a biocontrol agent on the spread rate of an invasive thistle. Ecological Applications 24(5): 1178-1187 https://doi.org/10.1890/13-1309.1
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Shea K, Kelly D (1998). Estimating biocontrol agent impact with matrix models: Carduus nutans in New Zealand. Ecological Applications 8(3): 824-832 https://doi.org/10.1890/1051-0761(1998)008[0824:EBAIWM]2.0.CO;2
