Monarch butterfly survival is closely tied to milkweed, the sole host plant for their caterpillars. Female Monarchs deposit their eggs exclusively on Asclepias species, providing larvae with their only source of food. Given the decline in Monarch populations, conservation efforts have focused on planting milkweed to support their life cycle. This effort, however, requires careful selection, as not all milkweed species support Monarch health equally or align with their natural history.
The Species to Avoid
The species that poses a risk to Monarch health is Tropical Milkweed, identified scientifically as Asclepias curassavica. This plant is native to Mexico, Central America, and South America, and is not naturally found in most of the United States or Canada. It is widely available at garden centers because it is easy to grow, produces bright, attractive flowers, and readily hosts Monarch caterpillars.
Unlike native milkweed varieties, A. curassavica is a perennial that does not die back when cold weather arrives in warmer regions. Its continuous growth in areas like the southern United States allows Monarchs to breed throughout the winter, disrupting their natural migratory cycle. This year-round availability is the root cause of the problems, as the presence of this evergreen host plant can inadvertently lead to a buildup of disease.
How Tropical Milkweed Harms Monarchs
The danger associated with year-round tropical milkweed is its role in the life cycle of the protozoan parasite, Ophryocystis elektroscirrha, or OE. This parasite accumulates on the milkweed foliage and is then ingested by Monarch caterpillars when they feed. Since the milkweed does not die back in warm climates, the OE spores are not killed off by a winter freeze, allowing the spore load to reach dangerous concentrations on the plant’s leaves.
When an infected female butterfly lays eggs, she dusts the eggs and the surrounding milkweed with millions of microscopic OE spores. The resulting caterpillars consume these spores, which germinate inside their gut and reproduce asexually, causing damage during the pupal stage. Infected adults often emerge with deformed wings, are unable to fly, or have significantly shortened lifespans. Studies have shown that non-migratory Monarch populations in areas with year-round milkweed exhibit a much higher rate of heavy OE infection, sometimes exceeding 70%.
The year-round presence of A. curassavica also interferes with the Monarch’s innate migratory cues. Monarchs that emerge in late summer are genetically programmed to enter a non-reproductive state and fly south, triggered by cooling temperatures and dwindling host plants. When tropical milkweed remains available in the fall, it encourages Monarchs to continue breeding instead of migrating, leading to “migration disruption.” These non-migratory populations are left vulnerable to sudden cold snaps, which can kill them and their offspring.
Prioritizing Native Milkweed Species
The most responsible action for supporting Monarch butterflies is to plant milkweed species native to your specific region. Native milkweeds naturally senesce, or die back, in the fall, which eliminates the OE spore load on the plant and encourages the Monarchs to begin their migration south. This seasonal growth cycle ensures a fresh, low-parasite environment for returning Monarchs in the spring.
Gardeners in the eastern and central regions of North America should prioritize species like Common Milkweed (Asclepias syriaca), Swamp Milkweed (Asclepias incarnata), or Butterfly Weed (Asclepias tuberosa). Western gardeners have options such as Narrowleaf Milkweed (Asclepias fascicularis) or Showy Milkweed (Asclepias speciosa). Consulting local native plant societies or regional milkweed maps can help ensure the selection of the most appropriate variety for your location.
For those who already have tropical milkweed growing, it is possible to manage it to minimize harm if replacement is not immediately feasible. The recommended practice is to cut the plant back aggressively, often to the ground or within six inches of the soil, during the late fall or early winter months. This forced dormancy simulates the natural die-back of native species, removing the parasite-laden foliage and encouraging the Monarchs to resume their migratory journey.