Target pest: Megastigmus spermotrophus (Hymenoptera: Torymidae), Douglas fir seed chalcid
Agent introduced: Mesopolobus spermotrophus (Hymenoptera: Pteromalidae)
Cameron et al. (1989) - in February and March 1955, releases were made at Kaingaroa (751 males, 378 females), Whakarewarewa (209 males, 132 females) (North Island) and Hanmer (251 males, 141 females) (South Island) forests. Some references say â€˜large numbersâ€™ of Mesopolobus spermotrophus were reared and released in 1956, but Forest Research Institute documents do not mention any emergence or release in 1956. Whatever did emerge would have been released in Kaingaroa and Whakarewarewa forests. In February and March 1957 releases were made in Hanmer and Dusky forests (South Island); numbers were not recorded but it is believed approximately 150 individuals were released in the former and 100 in the latter.
Cameron et al. (1989) - Mesopolobus spermotrophus has not been recovered since its release over 30 years ago and it is assumed it has not established. In 1963 and 1964 cones were collected from forests in which Mesopolobus spermotrophus had been released. Megastigmus spermotrophus-infested seed was present in all collections, but no Mesopolobus spermotrophus emerged. No special check of Douglas fir seed has been made since that time, and there is no record of the parastoid emerging from general seed collections made by the New Zealand Forest Service.
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 Mesopolobus spermotrophus, parasitising DFSC at a rate of 48.5%.
Landcare Research (2020i) - it is unlikely that a population of Mesopolobus spermotrophus has remained undetected for almost 70 years since its first release. More likely its recent discovery [see previous entry] is the result of an accidental incursion that occurred much later than the 1950s, with the importation of seed infested with DFSC and its parasitoid. Large amounts of Douglas fir seed were imported into New Zealand for genetic improvement in the 1980s so it may have established then.
Lee et al. (2021) - although Mesopolobus spermotrophus was released as a biocontrol agent in New Zealand in 1955, it seems unlikely that its establishment would have remained undetected until now. However, this species can be overlooked because Douglas-fir seed is usually collected before the seeds are parasitised. Thus, it is possible that Mesopolobus spermotrophus was missed in New Zealand for a substantial time, and perhaps even did establish after its release in 1955. However, it is also likely that it arrived in New Zealand as more recent accidental incursions in imported Douglas-fir seed, as large amounts were imported between the 1950s and 1980s, and Douglas-fir seed infested with DFSC and/or its parasitoids shows no external differences from healthy seeds.
Impacts on target:
Landcare Research (2020i), Lee et al. (2021) - with the high parasitism rate of the DFSC recorded in this study, it is plausible that this seed-attacking wasp is under successful biocontrol by Mesopolobus spermotrophus. However, while Mesopolobus spermotrophus was originally released to control DFSC due to concerns over its impact on Douglas fir seed production, the control of DFSC is now seen as unfortunate, as Douglas fir has escaped cultivation and has become a serious invasive species in New Zealand.
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.
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): 70â€“77 https://journal.nzpps.org/index.php/nzpp/article/view/11725