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.
References:
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.