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Biocontrol introduction

Target pest: Ulex europaeus (Fabales: Fabaceae), gorse

Agent introduced: Exapion ulicis (Coleoptera: Curculionidae) = Apion ulicis, gorse seed weevil



Import source:


Import notes:

Cameron et al. (1989) - between 1927 and 1931, 31 shipments totalling 320,280 weevils [imported as Apion ulicis] were sent from Farnham Royal, England to New Zealand; 84,768 were alive on arrival. Host range testing was carried out in quarantine and New Zealand-bred individuals released in 1931. It was subsequently decided to directly release adults collected from seed pods in the laboratory in England; 38,000 such weevils were imported in 1932 and releases [presumably from further importations] continued until 1946.



Release details:

Cameron et al. (1989) - the first releases of E. ulicis were New Zealand-bred individuals, made in February 1931 at Nelson and Alexandra in the South Island. It was subsequently decided to directly release adults imported from England; 38,000 such weevils were imported and released in 1932. At total of 235,000 adults were distributed throughout New Zealand between 1931 and 1946. [Presumably the 1932-46 releases were of imported individuals.]


Cameron et al. (1989) - Exapion ulicis established readily and soon as many as 99% of seed pods were attacked each spring.

Gourlay (2007a), Hill et al. (2007) - established readily within 10 years and has become common and abundant in most areas except the West Coast in the South Island.

Impacts on target:

Cameron et al. (1989) - the role E. ulicis has played in the population dynamics in New Zealand since 1931 is very difficult to determine. Recent studies indicate 85-95% of green pods formed in spring can be attacked; however, seed is also produced in autumn when the weevil is not reproductively active. Despite E. ulicis destroying much gorse seed each year, this has not resulted in adequate gorse control.

Hill et al. (2000) - in New Zealand gorse can form seeds in both spring and autumn, while E. ulicis is only active in spring. Where the bulk of annual seed production is in autumn, infestation of the small number of pods formed in spring often exceeds 90%. However, where most seed develops in spring, production swamps the weevil, and the rate of seed destruction is lower. A 1983 study around Auckland, North Island found that although infestation of pods in spring was high, E. ulicis only reduced the annual seed crop by about 35%. The impact of E. ulicis and the gorse pod moth (Cydia ulicetana) was studied at a site in Mid-Canterbury, South Island, where gorse grows on a hillside spanning 100 m in altitude. At the bottom of the hill, where seed was set only in spring, the agents together destroyed 90% (range 75-100) of the annual seed crop. At the top of the hill, where gorse sets most seed in autumn, C. ulicetana destroyed 10-20% while E. ulicis was inactive. Elsewhere in the site, gorse produces seed in both seasons, and the impact of the two insects on the annual seed crop varied. As the altitudinal patterns of gorse seed production are repeated latitudinally in New Zealand, reduction in the annual seed production in the south of the country may be large, but in more northern areas may not exceed 50%.

Rees & Hill (2001) - modelling indicates that the potential impact of the seed-feeding biocontrol agents E. ulicis and Cydia ulicetana (gorse pod moth) on gorse abundance depends critically on large-scale site disturbance, such as fire and herbicide application, and the effects of disturbance on germination and seed mortality. If seedlings have a high probability of survival, then seed-feeding biocontrol agents have little impact on gorse dynamics. However, if seedling survival is low, e.g. as a result of grazing, then these biocontrol agents can have a dramatic impact on gorse abundance. The models highlight the need to manage seedling recruitment opportunities carefully in order to maximise the effect of the biocontrol agents.

Gourlay et al. (2004) - an insecticide exclusion trial in Canterbury, South Island showed that while gorse pod moth (Cydia ulicetana) has a negative impact on E. ulicis, the combined effects of the two agents was greater than either alone, with 81% of spring seed destroyed by both agents in combination. Modelling suggests a reduction in the annual seed crop of 75-85% would be sufficient to cause long-term decline in gorse cover. In places where autumn seed production contributes little to the annual seed crop, these two agents may already be contributing to a decline in gorse population density.

Gourlay (2007b) - at a site in Canterbury, E. ulicis and the gorse pod moth [Cydia ulicetana] were found to be destroying 90-100% of the spring/summer seed crop (about half each).

Hill et al. (2007) - along with gorse pod moth [Cydia ulicetana] can destroy 85-95% of seeds formed in spring, but as gorse produces seed at times of the year when the weevil is not reproductively active, the reduction in the annual seed crop is not sufficient to control gorse.

Gourlay (2020a) - in some areas E. ulicis destroys up to 99% of the seeds produced during the spring-summer flowering period but in others the impact is minimal. A recent study in Canterbury, South Island at a high-altitude site showed joint destruction between the E. ulicis and Cydia ulicetana (gorse pod moth) of just 56% of the spring/summer seed crop with E. ulicis taking 42%. Recent work in Manawatu, North Island at a low altitude site showed joint destruction of only 21% of the total crop (included autumn/winter seeding) with E. ulicis taking just 5% of spring/summer seed crop. [See Landcare Research (2023d) entry below for further details.]

Landcare Research (2023d) - surveys at two sites, one in Christchurch, Canterbury in the South Island and one in Palmerston North, Manawatu in the North Island, investigated the impacts of the introduced seed-feeding biocontrol agents (E. ulicis and the gorse pod moth, Cydia ulicetana) on gorse populations. Total seed production in a season was 6,908 seeds/m2 at Palmerston North, 1,072 seeds/m2 at Christchurch; seed predation by biocontrol agents was 24% at Palmerston North, 60% at Christchurch; resulting in a seed fall of 5,250 seeds/m2 at Palmerston North, 429 seeds/m2 at Christchurch; culminating in a seed bank of 23,695 seeds/m2 at Palmerston North, 4,312 seeds/m2 at Christchurch. Cydia ulicetana accounted for most of the seed predated at the Palmerston North site (17%) while E. ulicis was dominant at the Christchurch site, accounting for 42%. Modelling by Rees & Hill (2001) [see Rees & Hill (2001) entry above] (which found the primary drivers of gorse invasion and cover to be seedling recruitment and survival, and environmental disturbance) indicates that at the Christchurch site, with low seed fall and probable low seedling survival due to grass cover, the population of gorse should reduce over time. The Palmerston North site would need a very high level of disturbance and low seedling survival to reduce the gorse population. Sites throughout the country vary hugely, but managing for low seedling recruitment will be key for obtaining the greatest impact from seed-feeding biocontrol agents. Management practices that kill plants, prevent or substantially reduce subsequent recruitment, and reduce seedling survival will be required to reduce gorse cover, regardless of seed predation levels.

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 E. ulicis, 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) - host range tests prior to release carried out in both England and New Zealand showed E.ulicis could not attack any other plant.

Paynter et al. (2004) - surveys record no non-target feeding, as predicted by laboratory tests.


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.

Gourlay AH, Partridge TR, Hill RL. (2004). Interactions between the gorse seed weevil (Exapion ulicis) and the gorse pod moth (Cydia succedana) explored by insecticide exclusion in Canterbury, New Zealand. In: Cullen, J.M., Briese, D.T., Kriticos, D.J., Lonsdale, W.M., Morin L. and Scott, J.K. (eds) Proceedings of the XI International Symposium on Biological Control of Weeds. CSIRO Entomology, Canberra, Australia, pp. 520–522

Gourlay H (2007a). Gorse seed weevil: Exapion ulicis. The Biological Control of Weeds Book - Te Whakapau Taru: A New Zealand Guide (Landcare Research) [Updated 2020] https://www.landcareresearch.co.nz/discover-our-research/biodiversity-biosecurity/weed-biocontrol/projects-agents/biocontrol-agents/gorse-seed-weevil/

Gourlay H (2007b). Gorse pod moth: Cydia succedana. 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/gorse-pod-moth/

Gourlay H (2020a). Gorse seed weevil: Exapion ulicis. The Biological Control of Weeds Book - Te Whakapau Taru: A New Zealand Guide (Landcare Research) [Update of Gourlay (2007a)] https://www.landcareresearch.co.nz/discover-our-research/biodiversity-biosecurity/weed-biocontrol/projects-agents/biocontrol-agents/gorse-seed-weevil/

Hill RL, Gourlay AH, Fowler SV (2000). The biological control program against gorse in New Zealand. In Proceedings of the X international Symposium on Biological Control of Weeds 2000 Jul (Vol. 917, pp. 909-917). Montana State University Bozeman, Montana, USA. https://www.landcareresearch.co.nz/assets/researchpubs/biologial_control_gorse_Hill_2000.pdf

Hill RL, Ireson J, Sheppard AW, Gourlay AH, Norambuena H, Markin GP, Kwong R, and Coombs E. (2007). A global view of the future for biological control of gorse, Ulex europaeus L. In Proceedings of the XII International Symposium on the Biological control of Weeds (eds Julien MH, Sforza R, Bon MC, Evans HC, Hatcher PE, Hinz HL, Rector BG), pp. 680-686. CAB International , Wallingford, UK. http://citeseerx.ist.psu.edu/viewdoc/download?doi=

Landcare Research (2023d). Are gorse seed feeders hungry enough for a change? Weed Biocontrol: What's New? 105, August 2023 https://www.landcareresearch.co.nz/publications/weed-biocontrol/weed-biocontrol-articles/are-gorse-seed-feeders-hungry-enough-for-change/

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 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

Rees M, Hill RL (2001). Large-scale disturbances, biological control and the dynamics of gorse populations. Journal of Applied Ecology 38(2): 364-377 https://doi.org/10.1046/j.1365-2664.2001.00598.x