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Biocontrol introduction

Target pest: Hypericum perforatum (Malpighiales: Hypericaceae), St John's wort

Agent introduced: Zeuxidiplosis giardi (Diptera: Cecidomyiidae), St John's wort gall midge



Import source:


Import notes:

Hancox et al. (1986), Cameron et al. (1989) - Zeuxidiplosis giardi was imported into New Zealand from Australia in 1960-61.

Release details:

Hancox et al. (1986), Cameron et al. (1989) - released at 18 sites throughout the South Island, as far south as Wanaka (Otago).


Hancox et al. (1986) - has established well and reaches high densities in the stony riverbeds of the northern South Island. It is also present, although only in low numbers, to an altitude of 830 m in northwest Nelson where there is considerable winter snowfall. It has not established south of the Nelson region.

Landcare Research (2014c) - established in northern South Island.

Impacts on target:

Hancox et al. (1986) - it has been observed that where pockets of maximum galling occur there is no flowering and that seedlings are killed by midge activity. Recently, however, Dimeromicrus sp. [Cameron et al. (1989) reports this as Torymoides sp.], a hymenopteran parasitoid, has been recorded from Z. giardi in the Nelson area. It is not known whether mortality from this parasitoid is significant.

Cameron et al. (1989) - no attempt has been made to measure the impact of Z. giardi on H. perforatum in New Zealand.

Landcare Research (2014c) - often causes severe stunting.

Paynter et al. (2018) - Zeuxidiplosis giardi is localised and ineffective. Heavily parasitised (41% parasitism) to an extent that is likely to significantly influence its efficacy.

Fowler et al. (2023) - investment in H. perforatum biocontrol totalled NZ$0.28 million (2022 rates) associated with the 1943-1992 releases of the chrysomelid beetles, Chrysolina hyperici and C. quadrigemina, and Z. giardi. Modelling shows biocontrol effectiveness increased linearly from 1943 to reach 99% control nationwide by 1993 (when H. perforatum was no longer considered a significant agricultural weed). It is estimated that H. perforatum biocontrol in New Zealand provided a national benefit of NZ$15.5 million in 2022, with a historical benefit-cost ratio of 6,254:1. Uncertainties remain concerning whether biocontrol caused all reductions in H. perforatum, or whether infestations of this weed were partially replaced by other weeds. Despite such caveats, benefits of H. perforatum biocontrol to New Zealand appear huge and sustainable. [Note that the control of H. perforatum is largely due to the chrysomelid agents - see the Paynter et al. (2018) entry above regarding the effectiveness of Z. giardi.]

Impacts on non-targets:

Paynter et al. (2004) - field surveys of two native plant species related to H. perforatum, Hypericum gramineum and H. japonicum, were undertaken. Pre-release laboratory testing predicted severe non-target damage, but potentially underestimated non-target impacts in that H. gramineum and H. japonicum weren't tested. Surveys on these species are on-going and it is too early to assess non-target impacts, but to-date no non-target feeding by Z. giardi has been observed.


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 SV, Barringer J, Groenteman R, Humphries G (2023). Biocontrol of St John’s wort (Hypericum perforatum) provides huge ongoing benefits to New Zealand agriculture. New Zealand Journal of Agricultural Research. Published online 20 Jul 2023. https://doi.org/10.1080/00288233.2023.2232762

Hancox NG, Syrett P, Scott RR (1986). Biological control of St John’s wort (Hypericum perforatum) in New Zealand: a review. Plant Protection Quarterly 1(4): 152-155 https://caws.org.nz/PPQ12/PPQ%2001-4%20pp152-155%20Hancox.pdf

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

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