> His theory is that the agricultural practices of early settlers, which cleared trees, created a population explosion of the milkweed that is the host plant of monarchs. More milkweeds resulted in more monarchs and monarchs began to migrate in response to population pressure, he believes.

So it sounds like it was all pine in that area, deforestation created an environment for monarchs, and they moved in. Monarchs are not native to the area, so I'm not sure that they can be used as a defence to keep all the eucalyptus.