Asclepias curassavica has many common names including tropical milkweed, bloodflower, Mexican butterfly weed and wild ipecacuanha. The genus name, Asclepias, is derived from the name of the Greek god of healing, Asklepios, as tropical milkweed has many traditional medicinal uses despite its mild toxicity to vertebrates.
Tropical milkweed is a tender perennial subshrub, typically 1m (3 ft.) tall. The leaves are lanceolate, oppositely arranged, and 6 to 15 cm (2-6 in.) long. Like other milkweeds, Asclepias curassavica produces a milky white latex sap that exudes from damaged parts of the plant. According to Plant Physiological Ecology by Hans Lambers, F. Stuart Chapin III and Thijs L. Pons (Springer Science+Business Media, LLC, 2008), this sap contains the heart-affecting cardiac glycosides calotropine and calactine.
The bright scarlet to orange flowers are borne in groups of up to twelve in umbels from terminal or axillary peduncles. Each individual flower is approximately 12.5 mm (0.5 in.) across, with five sepals and five linear, reflexed petals that are fused at the base. The yellow corona is composed of five hood-shaped structures, each containing a single horn. The blossoms are pollinated by wasps and butterflies; however, some Australian and Costa Rican populations have been reported to be self-pollinated. Tropical milkweed blooms throughout the warm season wherever it grows. The paired seed pods are elongate and split open along a vertical line of dehiscence. When dry, 70 to 80 dark brown ovate seeds are released. Each seed is well-equipped for wind dispersal, possessing a silky coma at one end and a thin papery wing around its entire edge.
Originally from somewhere in the Americas (see previous BPotD entry for discussion: Asclepias curassavica, Asclepias curassavica has been introduced to Asia, North America, Australia, Africa, and Europe. It has naturalized and become invasive in many tropical or subtropical regions of the world. The species is successful in a wide variety of environments, including wetlands, grasslands and disturbed areas. In parts of Cambodia and Papua-New Guinea, tropical milkweed has been known to invade coconut (Cocos nucifera) plantations.
Home gardeners in the United States have sometimes been encouraged to plant tropical milkweed as a food source for monarch butterflies (Danaus plexippus). Unfortunately, new research from the University of Georgia (see: Satterfield, D. A. et al. (2015). Loss of migratory behaviour increases infection risk for a butterfly host. Proceedings of the Royal Society B-Biological Sciences. doi: 10.1098/rspb.2014.1734 ) suggests that the widespread cultivation of tropical milkweed in the southern United States may be doing more harm than good. Like milkweed species that are endemic to the United States, tropical milkweed can play host to a protozoan parasite, Ophryocystis elektroscirrha (What is OE?). This parasite is ingested by monarch larvae, internally damaging the caterpillar. Upon emergence from the chrysalis, the adult butterfly is covered with dormant spores–and may be so deformed or weak that it cannot fly (in some cases, even too weak to emerge from the chrysalis).
Under normal circumstances, the long migration to Mexico would curb widespread transmission and infestation, as only healthy monarchs would survive the journey. However, unlike native milkweeds, tropical milkweed does not senesce in the winters in the southern United States. This means that monarch populations can continue to breed on tropical milkweed throughout the winter instead of migrating to Mexico. Infected butterflies can then pass the spores on to their offspring, and potentially to healthy butterflies returning from Mexico. As a result of this, gardeners in the southern USA are encouraged to plant the winter-dormant native milkweeds instead of tropical milkweed.