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To Help Monarch Butterflies, The Milkweed You Plant Matters

To Help Monarch Butterflies, The Milkweed You Plant Matters

By Therese Ciesinski, GardenSMART

You may have heard recently that the migratory monarch butterfly has been declared endangered by the International Union for Conservation of Nature. Western monarch populations are down by 99.9% since the 1980s, and in the east, populations have gone down 84% since 1996.

Habitat destruction, climate change, and the use of the herbicide glyphosate to kill weeds, among other threats, have created a perfect storm of circumstances leading to a decrease in the only food monarch butterfly caterpillars have evolved to eat: milkweed.

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Monarch butterfly sipping nectar from swamp milkweed, Asclepias incarnata. Photo by R. A. Nonenmacher, CC BY-SA 4.0, via Wikimedia Commons.

The situation looks bad for monarchs (Danaus plexippus plexippus), but it’s not hopeless. There is still lots that can be done to help stop the decline. Unlike so many big environmental problems where it seems one person’s contribution won’t make a difference, there is something tangible that home gardeners can do. And it’s not even particularly difficult.

How do we make a difference? By planting monarch caterpillar food. Planting milkweed in our home gardens can help keep the species afloat. In fact, as bad as the decline in monarch numbers are, getting the word out about the necessity of planting milkweed is one reason why populations haven’t completely collapsed.

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Tropical milkweed, Asclepias curassavica, hosts a parasite that harms monarchs. Photo by Ricard Busquets, CC BY 4.0, via Wikimedia Commons.

But it needs to be the right milkweed, because the wrong milkweed can cause more harm. It’s important to plant only milkweed that’s native to your area. It turns out that tropical milkweed (Asclepias curassavica), as pretty as it is, hosts a protozoan parasite, Ophryocystis elektroscirrha, that infects monarchs during metamorphosis, preventing them from completing their development.

Most infected monarchs either can’t emerge from the chrysalis or have deformed wings and are unable to fly. The few that do survive are infected with the pathogen, and go on to spread it to more tropical milkweed. Most milkweeds in North America are annuals, but invasive tropical milkweed survives through the winter in southern regions, and it is freezing temperatures that kill the pathogen.

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Swamp milkweed, Asclepias incarnata. Photo by peganum from Small Dole, England, CC BY-SA 2.0, via Wikimedia Commons.

The milkweed that’s safe for most to plant is swamp milkweed, A. incarnata. It is native throughout most of the U.S., with the exception of the West Coast and Arizona. This plant has soft pink flowers, rather than the brilliant yellow and orange flowers of tropical milkweed. It dies back each winter. Monarchs lay their eggs on it, the caterpillars eat the leaves, and numerous butterflies and insects drink the nectar from its flowers.

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Showy milkweed, Asclepias speciosa. USFWS Mountain-Prairie, Public domain, via Wikimedia Commons.

In the West, narrow-leaved and showy milkweed (A. fascicularis and A. speciosa) are the two natives recommended by the Xerces Society. Planting milkweed in western states is more involved than in the east – there are areas where it should not be planted, so as not to disrupt natural migratory patterns – so some research before buying plants or seeds is suggested.

You can find regional milkweed guides on the Xerces Society website at this link. More information on selecting and growing the right milkweed for your area is also available at Monarch Joint Venture.


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