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144 D. P. Peschken
Table 33 continued
Evaluation of Control Attempts
Year of
Species and Province Origin Year Number Recovery
Quebec Germany 1975 96
France, Austria 1976 451 1977-79
plus lab. reared
from French stock
Lab. reared from stock 1977 99
collected in Canada
Field collected 1979 290 1981
at Compton, Quebec galls
New Brunswick France, Austria 1976 191 1977-82
plus lab. reared
from French stock
Lab. reared from stock 1977 207 1978-82
collected in Canada
TOTAL 4932 flies
plus 290
galls
Lema cyanella (L.)
Switzerland· 1978 28 1979
1979 68
1980 35 1981
1982 31 1982
TOTAL 162
• These were laboratory reared beetles from stock collected in Switzerland .
.. Not reported in Peschken 1971.
The cool climate combined with predation prevented establishment of A. carduorum.
The weevil, C. litura, is established in four provinces and probably can be colonized in
all agricultural areas of Canada. However, damage to Canada thistle by larval mining is
small. Furthermore, Canada thistle is a major weed in the grain belt of the three Prairie
Provinces, where destruction of eggs and larvae by cultivation of thistles along field
margins prevents the buildup of dense populations. U. cardu; is not established in
western Canada (except for a small colony at Cam rose , Alberta), but it can probably be
established anywhere in the agricultural areas of eastern Canada. The galls do cause
noticeable stress to thistles growing in competition with other vegetation (Pesehken el
al. 1982). Overdispersal prevented the buildup of dense populations and reduced
effectiveness so far. It is not known why U. cardu; did not become established in western
Canada. The 35% rate of mor.tality alone does not explain the failure of the release
colonies, many of which bred well in the year of their release. Possibly severe spring
frosts during the pupation period were a problem. Most release stock originated from the
upper Rhine Valley which is considerably warmer and moister than the Prairie Provinces.
Lalonde & Shortbouse (1982) suggest that inadequate moisture on the release sites in
western Canada prevented the breakdown of the callus plug in the exit channel out of the