Tropical Milkweed (Asclepias curassavica) is a vibrant, non-native species favored by gardeners for its bright red and yellow flowers and ease of cultivation. Monarch butterflies (Danaus plexippus) rely exclusively on milkweed plants as the host for their eggs and the sole food source for their caterpillars. A concern has emerged regarding whether this popular plant may actually be detrimental to monarch health and migratory success.
Why Tropical Milkweed Poses a Risk
The primary concern with tropical milkweed stems from its perennial nature in warm climates, where it does not naturally die back during the winter months. Native milkweed species naturally senesce in the fall, removing the monarch’s food source. This dormancy provides an environmental cue that triggers the butterflies to enter diapause and begin their migration south to Mexico.
When Asclepias curassavica remains green year-round, particularly in the southern United States and California, it disrupts this natural cycle. The continuous availability of host plant tissue encourages monarchs to breed continuously. This year-round breeding creates resident populations that do not complete the necessary long-distance flight, increasing their susceptibility to disease and risk of mortality from cold snaps.
Understanding the OE Parasite
The continuous presence of tropical milkweed fosters the buildup of a debilitating pathogen called Ophryocystis elektroscirrha (OE). This single-celled protozoan parasite infects monarchs when caterpillars ingest microscopic spores found on the milkweed leaves or eggs. The parasite multiplies inside the monarch during the larval and pupal stages, coating the outside of the newly emerged adult butterfly’s body with millions of spores.
High OE spore loads severely reduce the monarch’s fitness, leading to a shorter lifespan, reduced mating success, and physical deformities. Infected adults may fail to emerge from the chrysalis or emerge with crumpled wings that prevent flight. The spores are easily transferred by infected butterflies laying eggs on the milkweed, contaminating the host plant. The evergreen nature of tropical milkweed allows OE spores to accumulate to dangerously high concentrations, maximizing transmission rates.
Planting Guidelines by Region
For gardeners in the northern and central parts of the monarch’s migratory range, where freezing temperatures occur, tropical milkweed often dies back naturally. However, it is recommended to avoid planting it entirely in favor of native species. If A. curassavica is present, gardeners should actively cut the plant back to the ground, typically to six inches or less, no later than early fall, around September 1st.
This aggressive pruning mimics the natural senescence of native milkweeds, ensuring late-season monarchs enter diapause and continue their journey south. The removal of all leaves and stems also eliminates the surface where OE spores accumulate, reducing the infection risk.
Non-Migratory Zones
In mild, non-migratory zones, such as the Southern United States and the California coast, the risk is persistent year-round. Cutting back tropical milkweed must be done repeatedly throughout the fall and winter months. Diligent removal of all foliage every few weeks prevents the continuous buildup of OE spores and discourages perpetual breeding. This practice is essential to reduce the high infection rates seen in resident monarch populations.
Recommended Native Alternatives
The most effective way to support healthy monarch populations is by planting milkweed species native to the local region. Native milkweeds naturally align with the monarch’s lifecycle, providing the host plant during the appropriate breeding season before dying back in the fall to cue migration. Selecting native species ensures the plant’s growth cycle supports migratory behavior and naturally removes the OE parasite load each winter.
There are over 60 species of milkweed native to North America, offering suitable options for nearly every climate and soil condition. Widely adaptable native alternatives include Common Milkweed (Asclepias syriaca), which thrives in dry, sunny areas; Swamp Milkweed (Asclepias incarnata), which adapts well to moist soils; and Butterfly Weed (Asclepias tuberosa), valued for its bright orange blooms and suitability for drier soils.