One Variety of Milkweed May Harm Monarch Butterflies

Planting a bright orange milkweed in your garden? Make sure it’s not tropical milkweed.

Asclepias curassavica has many common names including tropical milkweed, bloodflower, Mexican butterfly weed and wild ipecacuanha. It does not winter over in Ontario but has been grown as an annual.  In southern U.S., including Florida, it is widely planted.

Research from the University of Georgia suggests that tropical milkweed may be doing more harm than good. Tropical milkweed can play host to a protozoan parasite, Ophryocystis elektroscirrha. 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 is not dormant in the winters in the southern U.S. 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.

To encourage monarch butterflies in Ontario, you can plant common milkweed (Asclepias syriaca), which has a pale mauve flower, but tends to spread aggressively by rhizomes. Swamp milkweed Asclepias incarnata), which has pale pink flowers; the white-flowered whorled milkweed (Asclepias verticillate); and the butterfly weed (Asclepias tuberosa), noted for its brilliant orange flowers, are not rampant spreaders; and they are good hosts for monarchs.