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148 P. Harris. and A. T. S. Wilkinson
Table 34
thistle. The eggs are laid in small groups through the bracts at the top of the bud. A
second-instar larva hatches in 5-8 days and bores down a corolla tube into the ovariole.
The ovariole and the adjacent receptacle become enlarged and the larva feeds inside this
gall. Lignified tissues which develop on the outside of the gall gradually coalesce with
those of adjacent galls so that the larvae are eventually enclosed in a hard multilocular
gall.
Zwolfer (1972) and Redfern (1968) reported that approximately one quarter of the C.
vulgare heads collected in their studies in western and central Europe were attacked by U.
stylala. This is much lower than the population reached at the best site in British
Columbia. On the European continent, 37% of the larvae were parasitized (Zw6lfer
1972) and 24% in Britain (Redfern 1968). No larval or pupal parasitoids were encountered in
Canada. In certain regions of Europe, strains of U. stylala are specialized on certain other
thistles, but no attack of other thistles by the strain released has been found in Canada.
(b) Releases
The releases of U. stylala are summarized in Table 34. The fly established itself readily
at sites with a dense stand of C. vulgare. It has subsequently declined or disappeared
where the thistle became sparse following the reestablishment of grasses and other
herbaceous perennials as has happened at Danville and lie Jesus, Quebec, and Cranbrook,
British Columbia. At aka, Quebec, the thistle was mowed one year after release
and the colony disappeared. The release sites at Nanaimo and at Cloverdale, British
Columbia, were also destroyed but the fly can be found in the surrounding area. The
current status of the other release sites is unknown. The results of establishment suggest
that fly dispersal is slow and they require a relatively stable and dense stand of C.
vulgare for survival.
Open releases and recoveries
(Savi) Ten. in Canada.
of Urophora stylala F. on Cirsium vulgare
Year Location Origin No. released Year of recovery
1973 Nanaimo, Germany - 174 adults 1974 (site cleared
British Columbia Switzerland and mowed in 1975)
1973 Westham Isle. Germany - 462 adults 1974
British Columbia Switzerland
1973 Cloverdale, Germany - 132 adults 1974
British Columbia Switzerland
1973 Cranbrook, Germany - 459 adults 1974-1976 (no
British Columbia Switzerland thistles or U.
stylala in 1981)
1976 aka, Quebec France - 132 adults Site mowed - no
Austria establishment
1977 Danville, Quebec British Columbia 157 adults 1978
1977 lIe Jesus, Quebec British Columbia 248 adults 1978
1978 New Denver, British Columbia 957 pupae 1979
British Columbia
1978 Williams Lake, British Columbia 2000 pupae 1979
British Columbia
(lots 6069, 7017, 69)