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Urophora cardui
(L.) (Diptera:
Tephritidae)
Cirsium arvense (L.) Scop.. 141
assumed that larval mining does not prevent the early emerged rosettes from growing
into taller shoots. This also means that damage to the thistles is light.
The development of the weevils and thistles was monitored in detail at one release
site in Ontario. The weevil was released on a dense patch of Canada thistle, and
eventually it spread to other more or less contiguous thistle patches on the same
permanent pasture covering about 1 ha. On the original release patch, the level of
mined stems peaked at 72% in the fifth growing season with an average of 2.75 larvae
per stem, and thistle density subsequently diminished to zero (Peschken & Beecher
1973, Peschken & Wilkinson 1981). However, on another patch in the same pasture,
thistle density did not diminish although 77-91 % of the stems were mined for four
years, while on a third thistle density declined to zero per m 1 with only up to 2% of
the stems being mined. Thus no consistent correlation between an increase in the
weevil population and a decline of the thistles was observed. The rust Puccinia
punctiformis (Str.) Roh!. and the thistle feeding beetles Cleonus piger Scop. (Coleoptera:
Curculionidae) and Cassida rubiginosa Muell. (Coleoptera: Chrysomelidae) were also
present on this release pasture, and dense thistles tend to decrease to scattered shoots
behind an advancing front (Amor & Harris 1975). It is assumed that the decline of
thistles on some of the patchcs was caused by one or a combination of these factors.
Similarly, the fluctuating thistle densities at release sites in western Canada could not
be associated with levels of weevil infestation.
(a) Ecology
This gall fly occurs from France in the west (ZwOlfer 1967) to near the Crimea
(Oirlbeck & Oirlbeck 1964) and Siberia in (he east (Leclercq 1967), and from Sweden
in the north (Zetterstedt 1847) to the Mediterranean in the south (Zwolfer 1967).
Areas in half shade seem to be preferred over those in full sun (ZwOlfer 1967).
The fly is monophagous on Canada thistle and oviposits into the vegetative buds of
the main or side shoots. In Saskatchewan, the oviposition period extends from the end
of May to the third week in July. The larvae overwinter and pupate in spring inside a
multi-locular lignified gall. There is a strong correlation in the number of larvae and the
size of field collected or laboratory produced galls (Pesch ken et al. 1982).
(b) Releases
A total of 4932 flies and 290 galls was released in 14 locations in the four western
provinces of Canada and in 10 locations in Ontario, Quebec, and New Brunswick (Table
33). In western Canada, galls were usually produced in the year of the release, and on
two sites it survived for two and three years respectively. However, the fly died out in
western Canada except for a very small colony at Camrose, Alberta.
In contrast, U. cardu; is established at five locations in Ontario, Quebec, and New
Brunswick. Near Sussex, New Brunswick, the fly had been released at two locations,
4.5 km apart. The population of one release had spread over 1000 ha by 1980, but only
6% of the shoots were galled in anyone thistle patch. By 1981, populations of the two
release sites had merged and spread over 3000 ha. On some thistle patches up to seven
galls were found on one thistle shoot and there was an average of three on the attacked
shoots (O.B. Finnamore. 1981. personal communication). In 1979. 13% of a sample of
larvae was diagnosed to contain Nosema sp. (Microsporidia) spores and the percentage
was higher in dead than in live larvae. Three percent of the larvae were parasitized by
Habrocyrus elevatus Walker (Hymenoptera: Pteromalidae). In 1980, only 3% of the