Importance of long‐term research in classical biological control: an analytical review of a release against the cabbage seedpod weevil in North America

Cabbage seedpod weevil,Ceutorhynchus obstrictus(Marsham) (Col., Curculionidae), is an invasive alien pest that is spreading in North America. To aid with planning for introductions of European parasitoids in North America, we examined the status of the only classical biological control release against this pest in North America, which in 1949 introducedMesopolobus morys,Stenomalina gracilisandTrichomalus perfectus(Hym., Pteromalidae). Weevils and parasitoids were reared in 2005 from mass collections of seedpods ofBrassica napus,Brassica rapaandRaphanus raphanistrum(Brassicaceae) from 18 sites in the Fraser Valley, near Vancouver, British Columbia, Canada. Of the three European parasitoid species that were originally released, onlyS. graciliswas found. The predominant hymenopterous parasitoid species wereTrichomalus lucidus,S. gracilis,Mesopolobus moryoides(Pteromalidae),Necremnus tidius(Eulophidae) andEupelmus vesicularis(Eupelmidae). These constituted over 97% of the parasitoids reared, although overall parasitism was low. OnlyM. moryoidesis clearly North American in distribution; other thanS. gracilis, the remaining species were either accidentally introduced or are Holarctic in distribution. Based on these results, re‐releases ofM. morysandT. perfectusin North America should be considered as part of a classical biological control programme. However, redistribution ofS. gracilisis not recommended at present because of potential conflicts with biological control programmes against weeds. Ongoing re‐examination of classical biological control programmes can further our understanding of failure of release programmes, particularly when re‐examination can be made in the light of improved taxonomy and systematics of the target and agent species.