Biocontrol introduction
Target pest: Pseudotsuga menziesii (Pinales: Pinaceae), Douglas fir
Agent introduced: Megastigmus spermotrophus (Hymenoptera: Torymidae), Douglas fir seed chalcid
Released:
Landcare Research (2020i) - 1920s
Release details:
Landcare Research (2020i) - accidentally introduced into New Zealand in imported Douglas fir seed.
Establishment:
Landcare Research (2020i) - clearly well established by 1950s as it was impacting Douglas fir seed production.
Impacts on target:
Landcare Research (2020i), Lee et al. (2021) - investigating the prevalence of the Douglas fir seed chalcid (DFSC) (Megastigmus spermotrophus) as part of research on biocontrol options for wilding conifers, and as part of a study aimed at documenting the success and failure of all weed biocontrol agents that have been released in New Zealand, in the 2019/20 summer Douglas fir cones were collected at 13 sites throughout New Zealand (three in the North Island, 10 in the South Island). From over 21,000 seeds removed from the cones only 17 DFSC adults and larvae were recovered, which equates to an extremely low rate of attack on the seeds (0 to 0.85% per site). This low rate of seed attack, much lower than the average of 20% seed destruction reported in the 1970s, can be explained by the recovery of the parasitic wasp Mesopolobus spermotrophus, parasitising DFSC at an overall rate of 48.5%, though levels approached 90% at some sites.
Lee et al. (2021) - suppression of DFSC populations by Mesopolobus spermotrophus will benefit commercial Douglas-fir seed production in New Zealand, but almost certainly negates any further interest in DFSC as a potential biocontrol agent for wilding Douglas-fir seed in this country.
General comments:
Landcare Research (2020i) - due to concerns over the Douglas fir seed chalcid (DFSC)’s impact on Douglas fir seed production, an ectoparasitic wasp (Mesopolobus spermotrophus) was intentionally released as a biocontrol agent for the DFSC in 1955. However, because Douglas fir has escaped cultivation and has become a serious invasive species in New Zealand, it is now seen as unfortunate that as a result of parasitism of DFSC by Mesopolobus spermotrophus it has evaded a potential biocontrol agent that held promise for reducing its invasiveness and spread.
References
Landcare Research (2020i). Douglas fir evades biocontrol due to parasitism. Weed Biocontrol: What's New? 94, Nov 2020. https://www.landcareresearch.co.nz/publications/weed-biocontrol/weed-biocontrol-articles/douglas-fir-evades-biocontrol-due-to-parasitism
Lee S, Fowler SV, Lange C, Smith LA, Evans AM (2021). Unexpected parasitism of Douglas-fir seed chalcid limits biocontrol options for invasive Douglas-fir in New Zealand. New Zealand Plant Protection 74(1): 70–77 https://journal.nzpps.org/index.php/nzpp/article/view/11725
