Milkweed is Both Necessary and Dangerous for Monarchs

Milkweed is both necessary and dangerous for Monarch butterflies, Danaus plexippus, and here’s why.

Milkweed is a perennial in the genus Asclepius, comprised of approximately 140 species and the only one Monarchs lay their eggs and the only food source for their caterpillars. The Monarchs prefer some species of milkweed to others.

The Monarch’s eastern migratory population will have three or four generations born in North America each Spring and Summer before migrating back to the oyamel fir forests of Central Mexico. However, this population has multiple threats, including that only two percent of the forest remains due to deforestation. At the same time, with the growing presence and popularity of native and tropical milkweed by those seeking to help them, more Monarchs overwinter here, putting them at significant risk of infection from a protozoan disease spread by infected Monarchs and spores on the milkweed.

Reasons Milkweed is Necessary for Monarchs:

1. Food source: Monarch caterpillars exclusively feed on milkweed leaves. The toxic compounds found in milkweed plants, called cardiac glycosides, make the caterpillars toxic to predators. By feeding on milkweed, monarchs acquire this toxicity, which helps protect them throughout their lifespan. Without milkweed, monarch caterpillars would not have a food source, hindering their growth and survival.
2. Reproduction: Monarch butterflies rely on milkweed as a host plant for egg-laying. Adult female monarchs lay their eggs on milkweed leaves. When the eggs hatch, the emerging caterpillars have immediate access to their food source. The availability of milkweed determines the success of monarch breeding and the continuation of their population.
3. Habitat and migration: Milkweed is a critical habitat for monarch butterflies. It provides shelter, nectar sources, and an ideal environment for pupation and overwintering. In the fall, the Eastern migratory monarch population in Canada and the United States migrate thousands of miles to reach overwintering sites, primarily in the oyamel fir forests of Central Mexico and increasingly warmer southern states from California to Florida. Milkweed patches are vital fueling stations during this journey, providing nectar for the butterflies to rest and refuel.
4. Biodiversity: Milkweed is essential for overall biodiversity as it supports numerous other pollinator species, such as bees, wasps, and butterflies. By planting milkweed, we help monarch butterflies and contribute to maintaining a healthy ecosystem with diverse flora and fauna. Due to various factors, including habitat loss, agricultural practices, and herbicides, milkweed populations have declined significantly in recent years.

Reasons Milkweed is Dangerous for Monarchs:

1. Mismanagement of milkweed: “Tropical milkweed (Asclepias curassavica) is a non-native milkweed that has exploded in popularity in response to the demand for milkweed,” according to the Xerces Society.
“It is simple to propagate, allowing growers to rapidly produce the plant for quick sale. The plant is also attractive, both to humans and monarchs, providing flowers and lush green foliage throughout the growing season – and that’s a problem.”
A monarch-specific vector, Ophryocystis elektroscirrha (OE), is a protozoan disease that infects and kills Monarchs. OE is spread via infected female Monarchs and spores that emerge with caterpillars onto the milkweed that others pick up. The Monarchs’ eastern migratory population is the least infected compared to the Western and the non-migratory South Florida populations. The availability of milkweed year-round in Florida’s warm climate results in the highest OE infection levels. “This “resident” population has the highest level of infection, with over 70% of the population heavily infected with OE,” according to www.MonarchParasites.org
 . “Other non-migratory monarchs live in Hawaii, the Caribbean Islands, and Central and South America. More recently, resident populations have been noted in coastal Texas, Louisiana, and Georgia due to the presence of non-native tropical milkweed, which flowers throughout the winter in these mild climates, reducing the need for these monarchs to migrate. Nearly 100% of these residents are heavily infected with OE.”

Monarch reliance on milkweed provides us important reasons to cultivate preferably native milkweed in the warmer months and prune them all back during cooler months in warmer climates to mitigate the dangers of milkweed that, when mismanaged, can encourage overwintering rather than their healthy migration.

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