Tropical milkweed (Asclepias curassavica) at first glance seems to be a desirable garden plant given that it is a host plant for the monarch butterfly. Since we know the monarch population is in serious jeopardy, we might assume that tropical milkweed would help to slow the decline. However, several arguments have been raised both for and against the use of tropical milkweed.

Much has been written about the effects of Oe (Ophryocystis elektroscirrha), an obligate, protozoan parasite that infects monarch and queen butterflies with injurious effects. Well understood is the impact of Oe on non-migratory butterflies. Also substantiated by researchers is the notion that Oe is much more prevalent in areas where year round populations of monarchs reside due to the availability of tropical milkweed (Asclepias curassavica).

In regions where tropical milkweed has become naturalized, monarchs may establish year-round populations and fail to complete their migration. The incidence of Oe is much higher in these year-round populations of monarchs that do not migrate because the Oe is deposited on the leaves of tropical milkweed and passed on from generation to generation of butterflies.

In South Texas the situation is amplified because some researchers report that many monarchs stop before the migration is complete because of the availability of tropical milkweed. Scientists fear that Oe will proliferate among these overwintering butterflies and infect those that pass through on the journey to Mexico. Dr. Jeffrey Glassberg, president of the North American Butterfly Association, has seen little evidence that migrating monarchs stop in Texas. However he substantiates that the tropical milkweed that grows there year round supports a large population of Queens. Dr. Glassberg reports, "...when Monarchs migrate through the area in late October they continue to migrate southward and, so far as I can tell, few individuals pay any attention to the abundant A. curassavica." Dr. Glassberg would like to see data that supports the hypothesis that monarchs stop migrating when they encounter tropical milkweed.

ImageButterfly expert Dr. Lincoln Brower worries that resident Oe-infested monarchs will breed with migrating populations and seriously jeopardize the migration. He recommends limiting tropical milkweed to inside demonstration projects and keeping it inaccessible to wild monarchs. Dr. Jaret Daniels of the University of Florida says that planting tropical milkweed is better than no milkweed at all. No milkweed, no monarchs. Monarch Watch founder Dr. Chip Taylor neutralizes the argument by hypothesizing that which milkweed is planted is of little importance. More important is the fact that more habitat per day is being lost than can be replaced by gardeners everywhere planting tropical milkweed.

North America supports three main populations of monarchs. The California group migrates only as far as the California Coast each winter. The largest group breeds east of the Rocky Mountains and migrates to central Mexico. The third group is non-migratory and stays and breeds year round in southern Florida, Coastal Texas, Hawaii, and other tropical and subtropical areas. In these regions, monarchs have no need to migrate. Temperatures are warmer, so butterflies remain reproductive. Besides, plentiful tropical milkweed keeps them well nourished.

Researchers at the University of Georgia have found that Oe (Ophryocystis elektroscirrha) is much more widespread in the non-migratory monarchs. According to their findings, in the eastern migratory group less than 8% of the monarchs are infected. About 30% of the California monarchs are heavily infected, and a whopping 70% of the non-migratory monarchs of South Florida are infected.
Such statistics leave little doubt that the non-migratory populations are not as healthy as the migrating populations. Some monarchs heavily infected with Oe die before emerging from the chrysalis. Those that manage to emerge are often too weak to hang onto their larval cases, so they fall to the ground before their wings are expanded. These severely deformed butterflies live for a very short time.

In tropical regions, gardeners are encouraged to cut their scarlet milkweed twice each year; once in the fall and again in the spring. The rationalization is that if monarchs find no milkweed, they will not stop but will continue their migration. This would be helpful if gardens were the only places where tropical milkweed is found. The fallacy of this thinking is that since it has naturalized, it can be found everywhere. Most likely legions of people are not scouting the area and removing or cutting down tropical milkweed that has escaped. Consequently the monarch will always find a plentiful supply in waste places and fields where tropical milkweed has become well established.
Still, after all this, I have not answered the question about whether or not scarlet milkweed is good for monarch butterflies. It seems that the jury is still out, and there is no conclusive evidence to answer this question. We will all have to make up our own minds and decide for ourselves whether or not to plant tropical milkweed. It remains an important part of my own garden mainly because it is the only milkweed commonly available.
Future articles will explore different kinds of milkweed that might be chosen for gardens. Tropical milkweed may be the easiest to procure, but it is not our only choice.
Thanks to floridan for the photo of tropical milkweed and to htop for the image of the caterpillar on milkweed.