Biocontrol introduction
Target pest: Plutella xylostella (Lepidoptera: Yponomeutidae), diamondback moth
Agent introduced: Diadegma semiclausum (Hymenoptera: Ichneumonidae) = Diadegma eucerophaga, Angitia cerophaga
Imported:
1936-37, 1940-41
Import source:
England (1936-37), Canada (1940-41)
Import notes:
Cameron et al. (1989) - a European strain of D. semiclausum was imported from England in 1936-37. Between September 1936 and January 1937, 11 consignments of this species were received, totalling 20,830 parasitoid cocoons (some reared from the 25,410 host pupae sent containing either D. semiclausum or D. fenestrale), from which at least 8,425 adult D. semiclausum emerged. Six subsequent importations between mid-January and late-February 1937 contained a mixture of D. semiclausum and D. fenestrale, from which at least 4,894 adults (both species combined) emerged. Diadegma semiclausum was mass-reared in field cages at Palmerston North from September 1936 and from October 1936 until early 1937 imported consignments of both D. fenestrale and D. semiclausum were immediately sent from Palmerston North to Hawke’s Bay for mass-rearing in field cages. Each species was kept separate until early January 1937, when the colonies were combined and transferred from Palmerston North to Nelson [Cameron et al. (1989) expressed doubt that these are distinct species, but see the taxonomic note in ‘General comments’ section]. A Canadian strain of D. semiclausum was imported from Canada in 1940-41. Four consignments in 1940-41 of at least 609 individuals (mostly cocoons but some adults) were used for laboratory rearing but the colony soon died out. Three subsequent shipments in 1941 of 4,921 cocoons yielded 603 adults, approximately 300 of each sex. The Canadian strain was not released.
Released:
1936
Release details:
Cameron et al. (1989) - only the European strain was released [see Cameron et al. (1989) entry in ‘Import notes’ section]. Unknown numbers were released at Hawke’s Bay, North Island, in 1936-37. Combined releases of D. fenestrale and D. semiclausum were made in 1937 in Hawke’s Bay (more than 6,600) and in 1937-38 at Nelson, Waimea West and Blenheim in the South Island (31,229). In 1937, two releases of D. semiclausum, each of 400 adults, were made at Lincoln, Canterbury, South Island and there were releases of 31,229 at Nelson and 10,000 at Marlborough in the South Island. A further 500 were released in Canterbury in 1938-39 and again in 1939-40. In 1940, 8,100 were liberated in Alexandra, Central Otago, South Island and 2,400 in Auckland, North Island. Unspecified numbers were released in South Canterbury in 1941-42 and releases were planned for South Otago and Southland but it is not recorded if these eventuated.
Establishment:
Cameron et al. (1989) - several adults were recovered in the season of release from Hawke’s Bay and Palmerston North in the North Island and Lincoln (Canterbury), Nelson and Marlborough in the South Island. The following season, 1938-39, many adults were recovered from the three South Island regions where over 70% larval parasitism was recorded. It was considered established in Nelson, Marlborough, Canterbury and North Otago in 1939-40. No recoveries were made in Palmerston North from 1937 until 1940-41, when it was found to have established and spread rapidly, at which time it was also established and widely dispersed in Hawke’s Bay, North Island and the northern half of the South Island. It was established throughout New Zealand by 1945.
Impacts on target:
Cameron et al. (1989) - in conjunction with the introduced biocontrol agent Diadromus collaris and the naturally occurring fungus Zoophthora radicans, D. semiclausum has provided a high level of control of the diamondback moth complex (impact on the two most common Plutella spp. is unknown as studies have not distinguished P. xylostella from P. antiphona) in North Island cruciferous fodder crops. However, insecticide applications are still sometimes necessary on these crops in the South Island, and are essential on brassicas grown for human consumption.
Cameron et al. (1993) - Diadegma semiclausum, in conjunction with Diadromus collaris [see the D. collaris introduction entry], is categorised as exerting “substantial” control (defined as “other control measures are only occasionally required”) over P. xylostella.
Impacts on non-targets:
Cameron et al. (1989) - Diadegma semiclausum also attacks potato tuber moth (Phthorimaea operculella) [an introduced pest in New Zealand]. In New Zealand it has been bred from this host at Oamaru (North Otago), Lincoln (Canterbury) and Nelson in the South Island, and Auckland in the North Island.
General comments:
Taxonomic note (10 January 2023) - Cameron et al. (1989), referencing a 1938 publication, noted there was doubt that D. semiclausum and D. fenestrale were distinct species because, while the diagnostic differences were clear, under certain conditions the two species could interbreed, with the morphological characters used varying widely and a range of intermediate forms found in the laboratory and field. Based on this information, Wagener et al. (2006) recognised that additional studies were needed to confirm the existence of separate species; their molecular study showed both species clearly separated on distinct clades in the phylogenetic trees. Juric et al. (2017) concluded that since the two species are morphologically very similar it is conceivable that D. fenestrale has inadvertently been introduced to some countries together with, or even instead of, D. semiclausum. The New Zealand specimen (from Pukekohe, Auckland) used by Wagener et al. (2006) as part of their study was identified as D. semiclausum.
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
Juric I, Salzburger W, Balmer O (2017). Spread and global population structure of the diamondback moth Plutella xylostella (Lepidoptera: Plutellidae) and its larval parasitoids Diadegma semiclausum and Diadegma fenestrale (Hymenoptera: Ichneumonidae) based on mtDNA. Bulletin of Entomological Research 107(2): 155-164
Wagener B, Reineke A, Löhr B, Zebitz CPW (2006). Phylogenetic study of Diadegma species (Hymenoptera: Ichneumonidae) inferred from analysis of mitochondrial and nuclear DNA sequences. Biological Control 37(2): 131-140 https://doi.org/10.1016/j.biocontrol.2006.01.004
