(A.leucophylla, A.l. var.obtusa, A.erosa var. obtusa,A.obtusata, A.Rothrockii) Milkweeds are amongst the most important insect plants. Butterflies and some beetles use milkweeds as nectar sources and the foliage as food. This means the plant will look great and unusual in spring half eaten and mangy by fall, then it goes deciduous.
The alkaloids associated with this milkweed and other milkweeds give the monarch
and other butterflies that feed on it protection from predators. Alkaloids from the wrong milkweed (South American, Mexican, etc.) can expose the butterflies to predation. If the monarch or other butterfly has not evolved with the milkweed they may have limited tolerance for the particular alkaloid or latex of the plant species. The California flyway runs from Baja to Canada, it does not include Mexico proper nor Central America. If you live in Chicago you can plant Mexican species (Asclepias mexicana) or Asclepias tuberosa, don't plant our species. You might look at the Monarch entry in James Scott's Butterflies of North America and articles by Fink, L and L.P. Brower.
I would guess the symptoms to be similar to the problem of intolerance to legumes that some people have.
Communities for Asclepias erosa:Great Basin Sage, Creosote Bush Scrub, Valley Grassland and Central Oak Woodland.