Doug Taylor
Hi, two things
In mid June this summer, we photoed some Monarch caterpillars feeding on Asclepias ovalifolia in a public garden in Red Deer, Alberta. The 4th and 5th stage caterpillars preferred the seed pods to the leaves.
There is a noticeable difference in air density and oxygen partial pressure between the butterfly reserves in Mexico (8,600 feet asl) and the central plains and lower elevations where the butterflies migrate to. Perhaps the butterflies don't fly past Mexico because they can't fly over it.
Doug