cardui and thistles

[Erik Runquist]

Has anyone, or does anybody know of any theories about how the spread of 
exotic thistles has affected cardui immigrations? I suspect that ladies are 
generally able to reach higher population densities (and reach farther 
north) than pre-colonization times because of these pervasive weeds.  I 
realize that this is a largely untestable hypothesis, but here I almost 
exclusively find caterpillars on bull and Canada thistles.
Just curious…
Erik Runquist
Ashland, OR


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