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Biological control of Canada thistle
One insect, the weevil Ceuthorhynchus litura, has been established on
Canada thistle at one site in Alberta. The larvae bore into the stems of
the thistle and weaken the plants, sometimes killing them. However, the
thistle plants seems to be able to compensate for this damage. C. litura
increases and spreads very slowly from its release point and does not seem
promising as a biological control agent.
A beetle, Lema cyanella, which feeds on the leaves of Canada thistle, has
been approved for release in Canada. It has not yet been released in
Alberta because of problems in rearing enough beetles for release, but
further attempts will be made. Other insects are being studied for possible
biological control of Canada thistle.