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Monarchs Need Native Milkweed

Arkansas Natural Heritage Commission - Thursday, February 26, 2015

Often called the “king” of butterflies, iconic black and orange monarchs are probably one of the most recognized and well-known North American butterflies. Early settlers dubbed them “King Billies” after the black and orange colors associated with William of Orange, which is also believed to be the source of the common name “monarch.”

Every spring and summer, monarchs breed throughout the United States and southern Canada. In the fall, adults of an eastern population migrate up to 3,000 miles to Mexico, where they overwinter; in the western United States, monarchs migrate to eucalyptus groves in coastal California to overwinter. When spring comes around again, monarchs leave their overwintering location and return northward to lay their eggs on milkweed plants.

In the past 20 years, the North American population of monarchs has decreased by 90 percent, down from a population of approximately 1 billion in 1996-1997, to a population of approximately 33 million in 2013-2014. Loss of overwintering habitat and loss of milkweed habitat are thought to be the greatest contributors to the population decline.

When loss of milkweed habitat was discovered as a possible cause, many people became involved, planting milkweed plants in gardens, along highways, and on public lands. A new study has found that this approach has caused as many problems as it was hoped it would solve. The reason is that the species of milkweedmost commonly available to plant in the United States is tropical milkweed (Asclepias curassavica), which is not native to North America.

Asclepias curassavicaIt turns out that tropical milkweed doesn’t die back in the winter like native milkweed does, creating new winter breeding sites, keeping monarchs from migrating farther south. Although that might not sound like such a bad thing, it poses a great threat to monarchs – it hosts a parasite called Ophryocystis elektroscirrha (OE). Monarch caterpillars ingest the parasite while feeding on milkweed and then emerge from their chrysalises covered in OE spores. Infected monarchs are weakened by OE and don’t live as long, and an OE-infected monarch that tries to migrate will probably die before it reaches the overwintering sites in Mexico.

The newest study, reported in January by the Proceedings of the Royal Society B. concluded that monarchs who stayed in the southern United States for the winter were five to nine times more likely to be infected with OE than migrating butterflies. In some of the winter sites monitored, 100 percent of the monarchs sampled were infected. And, the study also predicted that the problem could harm butterflies returning from Mexico who would pass through the winter breeding sites and could lay eggs on infected milkweed or mate with infected butterflies.

What can be done? Plant only native varieties of milkweed. In Arkansas, that includes curly milkweed (Asclepias amplexicaulis), tall green milkweed (Asclepias hirtella), purple milkweed (Asclepias purpurascens), butterfly milkweed (Asclepias tuberosa), white milkweed (Asclepias variegata), horsetail milkweed (Asclepias verticillata), and spider milkweed (Asclepias viridis).


What if you’ve already planted tropical milkweed? All is not lost. Experts advise that you cut the tropical milkweed back every few weeks during the winter. And, of course, plant more native milkweed.

Where can you buy native milkweed plants or seeds? The milkweed plants listed above (and other native plants from our Native Plant Guide) can be purchased from most regional and local nurseries. Other resources are listed on our Native Gardening web page

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