Establishment of Urophora cardui (Diptera: Tephritidae) on Canada Thistle, Cirsium arvense (Asteraceae), and Colony Development in Relation to Habitat and Parasitoids in Canada

Investigations of the European Urophora cardui (L.) (Diptera: Tephritidae) as a potential biological control agent were initiated three decades ago by Agriculture Canada and the Commonwealth Institute of Biological Control within the Canadian biological control program against introduced thistles (Zwolfer 1965; Peschken 1971). Sparked by this program, the biology of this monophagous stem-gall former on the weed Canada thistle Cirsium arvense (L.) Scop., Asteraceae) was studied extensively. Zwolfer (1967, 1979, 1982, 1988), Zwolfer and Arnold-Rinehart (1993) Zwolfer et al. (1970) and Schlumprecht (1989) reported on its biology, phenology, field host range, parasitism, patchy occurrence within its large general range, and its habitat preferences. Zwolfer et al. (1970) showed that habitat characteristics greatly influence pupation and emergence of adults. Mortality of U. cardui larvae from one of its main parasitoids, Eurytoma robusta Mayr. (Hymenoptera: Eurytomidae), decreases with an increase in gall size and the number of cells per gall while mortality due to E. serratulae, another main parasitoid, is only slightly affected by gall size (Zwolfer and Arnold-Rinehart 1993). The gall is a weak physiological sink when young and rapidly ceases to function as a sink as it ages (Forsyth 1983). In the laboratory, increasing gall size correlated with an increase in the number of main shoots, and with a decrease in plant vigour as measured by several parameters, such as height, and root and shoot fresh weight.