The plant genus Asclepias, or milkweed, has become synonymous with efforts to support declining pollinator populations. While milkweed attracts butterflies and other insects, one non-native variety, tropical milkweed, has sparked significant debate. Readily available in garden centers, it is a popular choice for those looking to help Monarch butterflies. However, its unique growth habit presents a complication that can potentially harm the very insects it is meant to support. The controversy centers on how this plant interacts with Monarch migration and a debilitating microscopic parasite.
Identifying Tropical Milkweed
Tropical milkweed (Asclepias curassavica) is also known as bloodflower or scarlet milkweed. This perennial subshrub is native to the American tropics, including Mexico, Central and South America. In its native range or mild climates, the plant remains evergreen and can grow up to three or four feet tall. Gardeners in colder regions often grow it as an annual because it cannot survive a hard frost.
The plant is easily distinguished from most native North American milkweeds by its striking, vibrantly colored flowers. Clusters of blooms typically feature bright red or scarlet corollas contrasting with yellow or orange hoods. Tropical milkweed also has narrow, lance-shaped leaves. Like other members of the genus, it produces a milky sap when the stem or leaves are broken.
Essential Role as a Larval Host Plant
The entire Asclepias genus is ecologically important because it is the exclusive host plant for Monarch butterfly larvae. Monarch caterpillars feed only on milkweed leaves, making the plant necessary for the butterfly’s life cycle. Female Monarchs instinctively lay their eggs on milkweed, ensuring the newly hatched caterpillars have an immediate food source. This specialization is a highly evolved symbiotic relationship.
As the caterpillars consume the leaves, they ingest toxic cardenolides present in the plant’s sap. The larvae sequester these steroids in their tissues without being harmed. This process makes the caterpillars and adult butterflies distasteful and toxic to many vertebrate predators, which is advertised by their bright warning coloration.
Year-Round Growth and the OE Parasite Risk
The primary ecological concern with tropical milkweed arises in regions where it does not naturally die back during the winter, such as the Southern United States and California. Unlike native milkweeds, which go dormant and decompose in the fall, A. curassavica continues to grow year-round in mild climates. This lack of natural dormancy prevents the seasonal cleansing of the plant, allowing a debilitating pathogen to accumulate.
The pathogen is Ophryocystis elektroscirrha (OE), a microscopic protozoan parasite that infects Monarchs. Infected adult Monarchs scatter OE spores onto the milkweed leaves and eggs. When a caterpillar hatches and begins feeding, it ingests these spores, leading to an infection that replicates inside the insect’s body. The OE spores build up on the persistent tropical milkweed over successive generations.
Severe OE infection can result in adult butterflies unable to fully emerge from their chrysalis or emerging with deformities, such as crumpled wings. Even mildly infected butterflies suffer reduced body mass, shorter lifespans, and decreased flight endurance, which hinders migration. The constant availability of tropical milkweed also encourages Monarchs to continue breeding in the winter instead of entering the necessary reproductive diapause for migration. This disruption can lead to non-migratory populations with a much higher rate of OE infection.
Responsible Gardening Practices and Native Options
Gardeners who wish to support Monarchs can mitigate the risk posed by tropical milkweed through management. In areas where the plant remains active year-round, it should be cut back completely to the ground in the late fall or early winter. This action forces a temporary dormancy, removing the parasite load from the leaves and stems before the next generation of Monarchs arrives.
The most effective solution is to switch to milkweed species native to the specific region. Native milkweeds, such as Asclepias tuberosa (Butterfly Weed) or Asclepias syriaca (Common Milkweed), are naturally adapted to the local climate. They will die back on their own, preventing parasite accumulation. These native options co-evolved with local Monarch populations and naturally support the butterfly’s migratory patterns. By choosing a native milkweed, gardeners ensure their habitat contribution supports local ecosystems without increasing the risk of parasitic disease.