Dr. Lincoln Brower, the world's foremost monarch butterfly expert, answered the question in his January 2014 essay, "On the wisdom or lack of wisdom in planting Asclepias curassavica outside of its normal range."
Asclepias curassavica is a tropical American milkweed and its natural distribution is Mexico, Central America, South America and the Caribbean (see Woodson 1954 reference below). It is very likely that curassavica has been intimately involved with the long term evolutionary history of the monarch butterfly in the Neotropics but NOT in the temperate zone. In that sense, allowing curassavica to reproduce in North America is encouraging an exotic weed.
I have cultivated curassavica in a greenhouse at Amherst College in Massachusetts, in a large milkweed garden at the University of Florida in Gainesville, and at my former home near Gainesville. It is an ideal food plant of the monarch butterfly and is cultivated extensively to maintain commercial and experimental monarch cultures. However:
The first problem with planting curassavica is that monarchs are highly attracted to it, lay their eggs and usually overwhelm the plants with caterpillars.
The second problem, and a more serious one, is that when monarchs are in their non-reproductive phase (gonads repressed in the fall), they will be almost irresistibly attracted to curassavica, remain near the plant, and come into reproductive condition. When this happens, as far as we know, monarchs lose their migratory urge.......and probably, as individuals, never get it back.
Evidence of this was dramatically demonstrated in Gainesville. I had about 50 well developed potted curassavica plants that I set out in my yard and another 100 or so that we planted in a garden at the University. Fall migrants were highly attracted to the plants, laid eggs and there were so many caterpillars that I had to cull them. At home where I kept a closer eye on them, the migratory monarchs produced an early fall generation and their offspring then produced a second generation. The caterpillars formed their chrysalids by the dozens under the eaves of my house. By then it was mid- to late November, and the temperature cooled down, slowing development. Then a frost occurred and 100 percent were killed in the chrysalid stage. (Gainesville generally has two or three killing frosts each year.) I had this happen over several years.
I also visited an enormous county milkweed garden near Tampa, Florida, in the fall, and the monarchs had completely stripped hundreds of the curassavica plants. Since all native Asclepias die back in Florida, what could the butterflies then do?
Another problem with establishing what becomes a continuously breeding population of monarchs is that the incidence of the protozoan parasitic disease (Ophryocystis elektroscirrha) increases with time. If the diseased individuals then somehow end up breeding with or joining the overwintering clusters of monarchs, then the incidence of the disease will very likely rise with detrimental effects on the migratory monarch populations.
Professor Sonia Altizer at the University of Georgia, who is the world expert on OE, agrees with me.
I would limit curassavica to be used in inside demonstration projects, growing the plants in an enclosed area totally inaccessible to the wild monarchs.
Altizer, S.M., Oberhauser, K.O., and Geurts, K.A. 2004. Transmission of the protozoan parasite, Ophryocystis elektroscirrha, in monarch butterfly populations: implications for prevalence and population-level impacts. In: Oberhauser, K.S. and Solensky, M. (eds). The Monarch Butterfly: Biology and Conservation. Cornell University Press.
Woodson, R. E., Jr. 1954. The North American species of/ Asclepias/ L. Annals of the Missouri Botanical Garden 41: 1-211.