Monarchs: the basic research

Hi all, I've had this topic in the works for awhile. It's a complicated one and seems to be increasingly relevant.
Monarch butterflies (Danaus plexippus) are well-known to pretty much any American. They are brightly-colored and come with interesting qualities that are key to their survival. In their larval stage they feed almost exclusively on butterfly milkweed plants (Asclepias tuberosa) OR a climbing member of the milkweed family--sandvine/swallowvine/smooth swallow wort (Cynanchum laeve)  (USDA info here). This diet is what allows the monarch to be toxic to predators. Another important factor in the monarch species is their great migration which, being under threat for a number of reasons, means that the species itself is under threat.

Basics of monarch life cycle

Scientists began studying the monarch migration in the 1870s. With new technological developments in the 20th century they were able to get a good understanding of the nature of this phenomenon. Urquhart's work in the first half of the 20th century achieved the participation of thousands of people tagging monarchs for his study. In 1971, K. and C. Brugger discovered overwintering populations in the mountains of Mexico, specifically on Oyamel fir trees in the Sierra Pelon. During metamorphosis, monarchs synthesize magnetic particles into their thorax and it is presumed that this helps them with navigation. In the mountains in Mexico where they overwinter, magnetic readings are 100x higher than normal.

There is also an overwintering location in southern California, where the monarchs rest on eucalyptus and Monterey pine. The Monterey and Sierra Pelon sites seem to be preferable to the monarch because they protect them from extreme temperatures. When severe storms hit some of the overwintering peaks Mexico, the local monarch populations suffered (Brower, 1995).

A couple of technical methods for analyzing the monarch migration were found in the literature. Isotopic studies seemed to be the most current. Cardenolide is the toxin that larvae sequester from milkweed. Scientists were able to do cardenolide 'fingerprinting' on monarchs and determine the geographic region where the larvae had fed (Brower, 1995). The map below shows a rough sketch of the migration as described in the articles.

Monarch migrations. Trees show reserves where monarchs overwinter.

The eastern migration takes place in stages. The first generation migrates up to Louisiana, Texas, and sometimes Florida. Some make it further. The monarchs may breed for one or two more generations during the summer as they move overall in a clockwise pattern and return to the Oyamel fir trees in Mexico.

References:

Brower, L. (1996). Monarch butterfly orientation: missing pieces of a magnificent puzzle. The Journal of Experimental Biology, 199:93-103. http://jeb.biologists.org/content/199/1/93.full.pdf .

USDA (2014). Invasive Species Alert. Monarch Joint Venture. http://www.fs.fed.us/wildflowers/pollinators/Monarch_Butterfly/documents/Swallow-wort_flyer_MJV.pdf .

Wassenaar, L. I. & Hobson, K. A. (1998). Natal origins of migratory monarch butterflies at overwintering colonies in Mexico: New isotopic evidence. PNAS, 95(26): 15436-15439. http://www.pnas.org/content/95/26/15436.long .

CORRECTION NOTE: Previously I wrote that the caterpillars feed on Common Milkweed, or Asclepias syriaca. I was mistaken. They prefer Butterfly Milkweed, or Asclepias tuberosa.