Monarch butterfly on a tropical milkweed, Asclepias curassavica. |
Tropical milkweed is native to South America and has naturalized
into tropical areas worldwide, including southern California, southern Texas
and Florida. It is winter hardy in USDA
zone 9-11 and can be grown as an annual in colder parts of the U.S. It has striking red and orange flowers that
make it an attractive garden plant and its tender foliage seem to make it a
preferred host for monarch caterpillars.
That being said there are a number of reasons to plant more of the
native milkweed species rather than planting a lot of this
tropical species.
Some people have cited concern that this tropical species may
interfere with migration patterns, especially in the southern parts of the
US. There, the presence of large tropical
milkweed population may fool the butterflies into stopping their migration
before reaching their destinations in Mexico.
This has not yet been established by scientific studies. One negative factor that has been proven is
that tropical milkweed can host a parasite that can harm monarch butterfly
populations in overwintering butterflies.
I would refer you to this link on parasites on tropical milkweed for a more detailed discussion of this
issue.
The authors do not insist that all tropical milkweed be removed,
but recommend that all green foliage be removed from over-wintering plants
between October and February to prevent any parasites from surviving and
infecting any of the northward migrating butterflies along the way the following season. What would be best is to grow only native
species which die back to the ground naturally each year, thus ensuring that
there will be no parasites.
The remains of the tropical milkweed after 6 caterpillars fed on it. |
While we were thrilled with seeing the monarch caterpillars, we soon saw that there was a potential crisis developing. Within a couple of days the caterpillars had totally stripped the potted milkweed plants of all of their foliage. The caterpillers were approaching maturity (4th or 5th instar) but we still didn't want to risk starvation so close to maturity.
Fortunately there were the native milkweeds elsewhere on the
property. Since the tropical milkweed
was growing in pots, I was able to more the pots to where the native milkweeds
were growing and encourage the caterpillars to migrate onto the other plants. Before moving the caterpillars I tried to do
some research into what are the preferred host plants in the Mid-Atlantic
region. Much of the info on the internet
is anecdotal but I did find one scientific study that looked at nine North American milkweed species and
reported the survival rates for caterpillars reared on each. Of these nine I knew where I had three of
them growing on my property: Common milkweek (A. syriaca), butterfly weed (A.
tuberosa), and honeyvine (Cynanchum
laeve).
By August the caterpillars did not seem so interested in eating butterfly weed. |
While butterfly weed was among the better hosts based on that
study, late in the growing season its leaves are relatively tough compared to
my other two species (and much tougher than the tropical milkweed). By placing the tropical milkweed pots among
both common milkweed and honeyvine I was able to coax the caterpillars onto
those plants.
I was able to get the caterpillars to move onto the common milkweed plants. |
The honeyvine, with its thin, tender leaves seemed to be a big
hit. This was a bonus for me since the
honeyvine, which appeared last year as a weed, is an aggressive grower. The caterpillars, which are aggressive eaters
were a good match for the honeyvine and after a couple of days the vine had
been eaten back to an acceptable level.
So, based on this year's experience I will try to keep the potted
tropical milkweed in the sunroom over the winter so that I will have a head
start on growth next spring. To control potential
parasites I will cut the plants down to within 6" of the soil and remove all leafy sprouts between in late
winter.