Responses of Chalcidoidea (Hymenoptera) parasitoids to invasion of the cabbage seedpod weevil (Coleoptera: Curculionidae) in western Canada.
Invasion of the European weevil, Ceutorhynchus obstrictus (Marsham), was investigated through surveys of its range and population densities in Alberta and Saskatchewan from 2001 to 2005. After it was first reported in southern Alberta, C. obstrictus rapidly expanded its range and abundance. Our more recent surveys indicate that its northward expansion has slowed, but that it has continued to extend its range eastward to southcentral Saskatchewan. The distribution and abundance of parasitoids of C. obstrictus in Alberta and Saskatchewan were investigated from 2003 to 2005 by mass rearing canola pods infested with C. obstrictus larvae. Although weevil populations were not parasitized for several years immediately following its introduction to southern Alberta, a surprisingly diverse assemblage of Chalcidoidea parasitoids, comprising 12 species from four families, were recently reared from weevil-infested canola siliques in Alberta and Saskatchewan. The Chalcidoidea fauna of C. obstrictus include species with both Nearctic and Holarctic distributions, with some species having restricted host ranges and others that are more niche than taxon-specific. These Chalcidoidea species appear to have expanded their host ranges to parasitize C. obstrictus in the region. Most parasitism is attributable to Trichomalus lucidus (Walker), Chlorocytus sp., and Pteromalus sp. (Pteromalidae), and Necremnus tidius (Walker) (Eulophidae). Parasitism levels varied considerably over the three years of this study. From 2003 to 2005 increases in parasitism occurred among all four of the species dominating the parasitoid fauna of C. obstrictus, but greater increases were observed for Chlorocytus sp. and Pteromalus sp. than for T. lucidus. Parasitoid species have sometimes caused substantial levels of host mortality, although current levels are usually less than 15% for all species combined and so are not sufficient to control weevil populations. Implementing a classical biological control program for C. obstrictus by reconstructing its European natural enemy complex is being considered, but it is still uncertain whether parasitism levels by native Chalcidoidea will increase over time since considerable year-to-year variation has been found. Parasitism levels of C. obstrictus should therefore continue to be monitored to assess whether a classical biological control program should be implemented.