Why Monarch Butterflies Need Milkweed

monarch butterfly on asclepias plant
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Why Monarch Butterflies Need Milkweed

Why Are People Planting Flowers for Monarch Butterflies?

Monarch butterflies, Danaus plexippus, are big black-and-orange butterflies, found all across North America. They are easily recognized, familiar butterflies. Ecologists were shocked to discover monarch butterfly numbers have dropped over 90% since 1990. In July 2022, the International Union for Conservation of Nature added monarchs to their list of Endangered Species, animals at serious risk of going extinct.

North America has two populations of monarch butterfly. The eastern monarch population is found from the Atlantic to the Rocky Mountains. These butterflies spend the winter in Mexico and migrate north in the spring. Coming out of Mexico, they reproduce in the southern U.S. and their offspring migrate farther north. These monarchs mate, lay eggs, and their larvae develop in the middle of the continent and, when mature, migrate still further north to produce a third generation, within the same year, in northmost U.S. and southern Canada. As fall approaches, this third generation of eastern population monarchs fly south to Mexico. (Science does not fully understand how monarchs know to make this journey, but every year they do it.)

West of the Rocky Mountains, the western monarch population, which almost never contacts the eastern population, overwinters in chosen spots here and there along the California coast. In spring, these butterflies leave their winter homes and spread northward and eastward, the third generation reaching the western slopes of the Rocky Mountains and Washington State by the end of the summer, when the adults migrate back to the coast.

Both populations are drastically down in number, despite being independent of each other. When numbers drop dramatically, likely something important for life has changed for the worse. To prosper, every organism requires food, water, favorable weather, and to avoid predators and diseases. In 2010, insect ecologists started looking for what might have changed for monarchs. Shared by both eastern and western monarch populations were land use changes that reduced monarch food. Both populations probably had some loss of winter habitat and perhaps a couple of years when the weather wasn’t favorable as well. The focus on monarch food is because we do not just want to know why there are so few monarch butterflies, we want to have them be abundant again. Planting flowers for monarch food is seen as the best way to help monarch numbers grow again.

Looking back, you can see how development over the last 30 years reduced monarch food. Farming has gotten more intensive, growing crops up to the edge of the road with no ditch for weeds, and spraying with better herbicides to eliminate weeds within the fields. Milkweeds grew in both places. Highway departments promoted cleaner roadsides, which are often cut too often for wildflowers to flower, or herbicides leave only grasses, which rarely feed butterflies. Elsewhere we paved the ground for better parking lots and around our new housing developments and strip malls, chose plants for easy maintenance, with no messy flowers…all these changes, added together over many states, meant that a monarch butterfly could fly for miles and miles without finding food or a milkweed plant to lay its eggs on.

This problem is something any American wanting to save the monarchs from declining any further can help with: grow flowers for adult butterflies to feed on and grow milkweeds for monarch caterpillars to eat.

The Food Needs of Monarch Butterflies

Monarch butterflies, like most butterflies, start life as an egg which hatches into a caterpillar. The caterpillar eats and eats, growing. Then it forms a chrysalis (cocoon) and after ten days, emerges as a butterfly. Adult monarch butterflies mate, and lay eggs, and in August start their migration south for the winter. Monarch caterpillars can only eat milkweeds, plants of the genus Asclepias (pictured). On other plants they starve. As their broad distribution suggests, pretty much everywhere in North America, a monarch will be looking for a milkweed upon which to lay its eggs.


Adult monarch butterflies are much less picky. They get their nutrition—enough to fly from Maine to Mexico—from drinking the nectar from flowers. Many flowers have good, sugar-rich nectars—beebalms, sedums, sunflowers—and feed monarch butterflies well. Since they migrate across the continent, they need patches of flowers all along the route.

To stabilize and increase monarch numbers back toward the abundance of the 1990s, people are urged to plant milkweeds for the caterpillars and grow flowers for the butterflies.

What Plants to Grow to Feed Monarchs?

Adult monarch butterflies are pretty good general feeders: they feed on nectar from agastaches, asters, bee balms, coreopsis, cosmos, goldenrods, mints, marigolds, sedums, sunflowers, verbenas, yarrows, zinnias, and many more. Lots of plants appeal to monarchs. We brighten our own environment when we provide food for monarch butterflies.

A few flower shapes are unusable by butterflies: closed flowers, for example sweetpeas, and very deep tubular flowers, such as trumpet vine. They can’t reach the nectar in those flowers. Flat flowers or ones with short tubes are much easier for them.

Migrating monarchs need stations with flowers all along their route. But if you want monarchs hanging out in your yard, plant so that nectar-filled flowers are available all summer, and that will tempt them to stay.

The caterpillars are more of a problem to feed. You must have milkweeds.

Why Do Monarch Caterpillars Require Milkweeds?

Plants, rooted and unable to run away from animals that eat them, long ago evolved poisons to discourage animals. These chemicals are generally specific to one group of plants, so that if animals adapt to eating them, they can’t switch to the neighboring plants. Milkweeds developed cardiac glycosides, also called cardenolides, toxins that block nerve function and so will stop the heart. In a large enough dose, they are fatal to birds, mammals, and people. At lower concentrations they cause nausea and vomiting.

Monarch ancestors evolved methods to eat the cardiac glycosides without being poisoned, and, indeed, to store the glycosides in their bodies. With cardiac glycosides in them, monarch butterflies are poisonous to their predators. Researchers investigated this. They found that young blue jays, after swallowing a monarch caterpillar and then painfully vomiting it up, wouldn’t touch another. Milkweed poisons protect monarch butterflies from predators. Their dramatic orange and black coloration makes it easy for birds and other predators to learn which butterflies to avoid.

That’s the monarch point of view. From the view of milkweeds, monarch caterpillars are nasty animals that eat their leaves, and they defend themselves with cardiac glycosides. Different milkweed species have different amounts of cardiac glycosides in their tissues. (Don’t believe statements that “milkweeds are delicious.” Some are, yes. But some are poisonous. You need to know which milkweed species is delicious.) Monarch butterflies fly over the many species of milkweed in North America and, like Goldilocks, want ones with not too much toxic (larvae don’t grow well) and not too little (larvae don’t get cardiac glycoside protection) but ones that are “just right.” For example, a properly protected monarch has enough cardiac glycosides in its wing that a bird can take a single bite of the wing and find it nauseating and leave the battered butterfly to fly onward.

What Milkweed Should You Grow?

If you live where monarchs are found, there are milkweeds native to your area that are excellent host plants. Native means it will grow well for you. This is important, because caterpillars eat plants; if you successfully attract monarchs to your milkweed, they will consume its leaves. A healthy milkweed will grow fast enough that it survives the damage from monarch caterpillars.

The complication is that there isn’t one milkweed that everyone should grow. Different milkweeds are native to different parts of North America, so the recommended species change with region. (North America has about 130 different species of native milkweeds.) The Xerces Society, dedicated to insect conservation, has excellent information on their website about monarch butterflies and growing milkweeds. I will summarize that list to emphasize that milkweeds differ across North America.

The Xerces Society divides the U.S. up into six regions: Northeast, Southeast, South Central, West, California, and Arizona. The Northeast, Southeast, South Central, and the part of the West Region that is east of the Rocky Mountains all are within the eastern monarch population. The West, California, and Arizona are the western monarch population.

For the Northeast Region, from the middle of the Dakotas south to Kansas, east to the Atlantic Ocean and south to Missouri, Kentucky, and Virginia, five milkweed species are recommended. These are: the common milkweed (Asclepias syriaca), once common on roadsides, the beautiful orange or yellow butterfly milkweed (also called butterfly weed and pleurisy root, Asclepias tuberosa), swamp milkweed (A. incarnata) which has gorgeous red-purple flowers, whorled milkweed (A. verticillate) with small and delicate white flowers, and poke milkweed, (A. exaltata) a white-flowered, large-leafed species for wooded areas. If you live in the East Region, one of these will grow well for you, maybe all of them.

Along eastern side of the Rocky Mountains, common milkweed is replaced by a very similar species, the showy milkweed (Asclepias speciosa). There, this more drought-tolerant species is a better choice than the common milkweed.

The Southeast Region goes from Arkansas and Louisiana to the Atlantic coast in North Carolina south to Florida. In this more humid area butterfly milkweed and whorled milkweed are recommended, but also white milkweed (A. variegate), which has a tight cluster of white flowers, and the sandhills or pinewood milkweed (A. humistrata) a pink flowered species of sandy soils. On very wet soils, aquatic milkweed (A. perennis) with pretty pinkish flowers will not just grow but thrive.

The South Central Region is Oklahoma and Texas, warm but dry. The recommended milkweeds are the green flowered green antelopehorn milkweed (Asclepias viridis), the antelopehorns milkweed (A. Asperula) which has flowers in a striking green and white ball, and zizotes milkweed (A. oenotheroides) with clusters of very big flowers. These handle the heat well and are known favorites of monarchs coming out of their winter retreat in Mexico.

For the West Region, you can grow showy milkweed, which is native across all the west except the California coast and Arizona. Also recommended for the West Region is the lovely pink-and-white flowered Mexican whorled milkweed (A. fascicularis).

In California, in addition to Mexican whorled milkweed and showy milkweed, the pale desert milkweed (Asclepias erosa), the dramatic California milkweed (A. californica) with its big, deep red flowers, the smaller heartleaf milkweed (A. cordifolia), the white and drought-tolerant woolly milkweed, (A. vestita) and the delicate woolly pod milkweed (A. eriocarpa) are monarch favorites The list is long because there are so many climates and soils within California; follow recommendations for your particular climate zone.

Finally, in Arizona’s extremely dry conditions, butterfly milkweed, antelopehorn milkweed, the lovely narrow-leaved rush milkweed (A. subulate) and the striking white Arizona milkweed (A. angustifolia) grow well and support healthy monarch butterflies.

These recommendations are based 1) growing locally-native milkweeds, 2) whether monarchs thrive on the milkweed (not too toxic, not too edible) and 3) whether the milkweed is available from nurseries (only over the last few years have people asked for milkweeds, so there’s more demand than supply) .

Monarch butterfly numbers are down, but monarchs are not yet extinct. Butterfly populations are capable of rapid increases in good conditions, and in fact, some of the last five years have shown encouraging increases in monarch numbers. Monarch experts believe that by providing food, monarchs populations have an excellent chance of recovering substantially and they ask everyone to help.

Important Note: If you are planting for monarchs, be sure NOT to put insecticide anywhere near the plants. Wild insects are much more sensitive to poisons than the pests we buy insecticides to kill, so just a trace of insecticide can be fatal to a monarch.