Milkweed and the Monarch

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Photo by Getty Images/Jeff Stefan.

The North American monarch (Danaus plexippus) population has decreased substantially since the 1980s. Though monarchs are not an endangered species, their declining numbers are a cause for concern among conservationists. The primary reason for their decline is thought to be habitat destruction, particularly the destruction of habitat for milkweed (Asclepias spp.) plants. Monarchs depend on milkweed for several vital reasons; adult monarchs feed on the nectar from the flowers, and female monarchs only lay their eggs on milkweeds, because monarch larvae won’t eat any other plant.

Monarchs are large, showy butterflies that attract a lot of interest from the general public for their famous international migration from Michoacán, Mexico, to southern Canada and back. This yearlong cycle, spanning several generations of monarchs, means that these creatures can be found throughout the country, from the northern states bordering Canada, to ever-sunny Florida, and even all the way to the Golden Coast. This wide range means that it’s easy for any gardener to make a difference; growing milkweeds is easy, and hand-raising monarch larvae is likewise straightforward.

The Ins and Outs of Milkweed

Milkweeds are flowering perennials in the genus Asclepias, many of which are native to North and South America. The “milk” in milkweed refers to the white latex inside the plant that oozes out when the stem breaks. This “milk” is the reason that monarchs will only lay their eggs on milkweed plants. Milkweed latex contains cardenolide alkaloids that are harmful to most insects — as well as small birds — but not to monarchs. These alkaloids, which are a subdivision of cardiac glycosides, are molecules that arrest an animal’s heart. Monarchs use milkweed alkaloids as a chemical defense; they eat the milkweed leaves and sequester the alkaloids in their abdomen and wings. The presence of alkaloids make the insect taste bitter and deters predators, who will often spit them out. For example, a bird that swallows a monarch will either regurgitate its prey, or suffer the effects of the cardiac glycosides and die.

Asclepias syriaca – Common Milkweed. Photo by Adobe Stock/annavolotkovska.

Growing milkweed isn’t difficult; they don’t need rich soil, nor do they require extensive tending. When growing milkweeds to attract and raise monarchs, be sure to source seeds and plants from species native to your local area. Some milkweed species, such as common milkweed (Asclepias syriaca), whorled milkweed (A. verticillata), swamp milkweed (A. incarnata), and butterfly weed (A. tuberosa) grow throughout the migration range for monarch butterflies. While butterfly weed has spectacular orange flowers attractive to hungry adult monarchs, it’s common milkweed that attracts female monarchs in particular. You should plant common milkweed to capture eggs and hand-raise larvae, as it’s their favorite to eat.

Seed Stratification

Milkweed seeds usually benefit from cold stratification, which is the storing of seeds in a cold environment to increase germination. In the wild, winter provides this service. If you buy seed, however, you’ll have to do it yourself.

To do this, take some sterile potting mix and wet it lightly. Squeeze the mix to expel any excess liquid, and then embed seeds within the mix. You don’t need very much potting mix for this — just barely enough to surround each seed. Place the mixture in a plastic zip-close bag, write the date and type of seed on the bag, and place the bag in the fridge. For most types of milkweed seed, 30 days of stratification is sufficient. If you forget this step — or don’t have 30 days to wait — don’t worry: some of your seed will still sprout, although it will likely take longer and not be as uniform. Just plant twice as many seeds as you anticipate for mature plants.

Getting Started in the Garden

You can plant milkweed seed in flats and transfer the seedlings to a garden or a container, but do so when the first 2 to 4 true leaves emerge and the plants are still small. At that point the roots won’t have overgrown a typical well in a nursery flat.

Milkweeds don’t need large doses of fertilizer, if any. In fact, too much nitrogen can inhibit flowering. After the seeds sprout, simply keep competing plants weeded until the seedlings grow tall enough to tower over their competitors.

First-year milkweeds generally won’t flower. The most important thing happening in the first year is the formation of rhizomes — thick, underground stems capable of sending down roots and sending up shoots. In their second spring, the plants will sprout again, sending new shoots from the rhizomes. Under most circumstances, you won’t need to do anything to your milkweed rhizomes over the winter. However, if you live in a location where the ground doesn’t freeze in winter, experience a severe drought, or grow your plants in containers, watering your plants occasionally will help the rhizomes survive until spring.

Asclepias tuberosa – Butterfly weed. Photo by Chris Colby.

In their second year, initial growth will be quick, fueled by the starches stored in the rhizomes. The plants may flower in the second year, but this isn’t guaranteed. Most milkweeds won’t flower until the third year, but it’s worth the wait. One of the more complex flowers among plants, these flowers attract a wide variety of insects. The flower will mature into a seedpod that will eventually rupture, releasing seeds to be dispersed in the wind.

To attract monarchs, plant your milkweeds among other native, flowering plants. Established milkweeds won’t require maintenance; however, continue to check for pests and infestations.

Plague of Pests

Managing milkweed pests is complicated by the fact that you’re raising the plants so that a “pest” insect will lay eggs on them. Ironically, the resulting caterpillars will consume the very plants you are growing. For this reason, insecticides are out of the question. Fortunately, most common milkweed pests are easy to deal with.

The two most common milkweed pests are milkweed aphids and milkweed bugs. Milkweed aphids (Aphis nerii) are small, yellow aphids that mass on the stems of the plants. A healthy ladybug population will keep these infestations to a manageable level. However, if a plant is heavily infested, spray the plant with a mixture of water and a bit of dish soap to physically dislodge the aphids. You can also simply run your fingers up and down an infested stem and crush them.

Oncopeltus fasciatus – Milkweed bugs. Photo by Chris Colby.

Milkweed bugs (Oncopeltus fasciatus) are hemipterans, or true bugs, that feed on milkweed seeds. Of the many different species, the large milkweed bug is the most common in North America. Milkweed bugs cluster on the seedpods. The nymphs are orange and the adults are black and orange. Small infestations don’t harm the plants greatly, and insect predators, especially mantises, can keep their numbers low. If control is needed, they can be hand-picked in the morning, when cool temperatures slow their movement.

The Makings of a Monarch

Like all butterflies, monarchs undergo a complete transformation from larvae to adult. The full monarch life cycle includes an egg, five larval instars, a chrysalis, and finally, an adult butterfly. Female monarchs favor new, tender milkweed leaves and lay their eggs near the growing tip of the plant. Within 3 to 5 days, the eggs hatch into first-instar larvae — tiny, mostly white caterpillars that eat their egg casings before eating the leaf surrounding them. As the caterpillars grow, they go through four more instar stages — each larger than the last. Each stage has its own pattern, although they’re all highly similar. In the fifth and final instar, the caterpillars chew the underside of a leaf stalk until the leaf droops. They then hang upside down on the inside of the leaf and consume it. It takes 10 to 14 days for the larvae to grow from the first instar to a hanging fifth instar.

When monarch caterpillars first emerge from their cocoons, they’re mostly white. As they enter the second instar, they begin to show the trademark yellow and black bands of a monarch caterpillar. Photo by Cris Colby.

Through the second and third instar, the caterpillar is predominately yellow, with thinner black bands. Photo by Chris Colby.

As it moves into the fourth instar, the black bands become more pronounced. Photo by Chris Colby.

Finally, when the caterpillar reaches the fifth instar, the final color pattern is fully established.Photo by Chris Colby.

The transformation into a butterfly begins when a fifth-instar larvae finds a site to form a chrysalis. It uses silk to attach itself to the underside of a leaf stalk, and then hangs upside down for several hours with its head raised, resembling the letter “J.” After hours of inaction, the monarch writhes about and sheds its skin, revealing the green surface of a chrysalis below. The skin frequently stays attached to the silk suspending the chrysalis. The transformation from caterpillar to chrysalis takes less than 2 minutes, and begins the moment the caterpillar’s skin starts to tear.

The chrysalis hangs for about 10 days, or until the butterfly is ready to emerge. At the beginning of this stage, the inside of the chrysalis looks like undifferentiated “goo” — sometimes jokingly referred to as caterpillar soup — because there’s very little internal structure. In fact, there’s neither a caterpillar nor a butterfly inside, only tiny circles of tissue, called imaginal discs. These will mature into wings and other anatomies on the adult butterfly.

Photo by Chris Colby.

The night before the butterfly emerges, the chrysalis turns from green to transparent, and the wings of the butterfly are clearly visible. The following morning, the casing of the chrysalis cracks and reddish-black fluid — waste trapped in the chrysalis — drips out. After it emerges, the butterfly hangs upside down for a period of time to allow its wings to dry. Temperature and humidity affect the drying time; on a warm, dry day, the butterfly may be ready to fly in about 5 minutes, and on a cool, rainy day, the butterfly may hang onto its chrysalis for hours. Eventually, the butterfly will flap its wings, warm up its flight muscles, shake off the last of the moisture, and fly away.

Lend Nature a Helping Hand

Watching the step-by-step process of a caterpillar become a butterfly is something that everyone should have the opportunity to experience. If you grow milkweed and you’d like to see this transformation up close, you have two options: you can watch them grow in your garden, or you can capture monarch larvae and raise them in captivity. If you choose to watch from afar, monarchs will find your milkweed plants and do everything they need to transform and reproduce on their own. However, capturing caterpillars or eggs and raising them in captivity has one huge benefit — it protects the monarchs from predation and parasitism.

While monarchs are chemically protected against many predators and parasites, that protection isn’t always enough. Caterpillars are still vulnerable to predation by mantids, ants, some wasps, and mice, as well as select birds, lizards, and toads. But their biggest threat is tachinid flies, which look similar to house flies. Tachinid flies specifically target caterpillars when looking for a place to lay their eggs. The parasite eggs rest inside the caterpillar until it has pupated. After the monarch caterpillar has turned to goo, the tachinid eggs hatch and mature inside the chrysalis. One year, these flies killed 70 percent of the viable chrysalises in my garden.

Photo by Chris Colby.

To protect your hand-raised monarchs from tachinid parasites, be sure to capture caterpillars early. I’ve never had a tachinid develop in any monarch that I’ve captured as a second-instar larvae or younger.

You can hand-raise a monarch in any simple enclosure. You can purchase specialty insect habitats, but I just use quart-sized canning jars covered with perforated plastic cling wrap to allow airflow. No matter what habitat you use, be sure to add a dried milkweed stalk to each jar. This gives your caterpillar something to attach itself to when it’s time to pupate. Feed your caterpillar a fresh milkweed leaf every morning. Each day, gently remove the caterpillar, rinse the frass (insect feces) out of the jar, and add a few drops of water — enough to form a small pool at the bottom of the jar. Keep in mind that as the caterpillar gets bigger, it may require more than one leaf a day, or may need to have its jar cleaned more often.

The morning after the chrysalis turns clear, take it out of the jar and prop the milkweed stick up in a safe, outdoor location so the monarch can emerge from the chrysalis and have room to spread its wings. From there, wait and let nature take over once again.

North American monarchs are one of the few butterfly populations that migrate. While it takes four generations of monarchs to make the journey from Mexico to Canada, it only takes one generation to make the returning flight. Photo by Getty Images/cicloco.

Seeing a monarch butterfly spread its wings and take flight is the culmination of months of gardening and weeks of “larval wrangling,” but all of the effort is well worth it. The color on just-emerged monarchs is vivid, and watching a monarch emerge and fly off is a wonderful experience for those who raise them.

Chris Colby is a writer and editor with a background in biology. He has an in-ground garden by the side of his house and a container garden on his driveway. He mostly grows vegetables, but is becoming increasingly interested in native plants that feed bees, butterflies, and other beneficial garden insects. He lives with his wife and an undisclosed number of cats in Bastrop, Texas.

Mother Earth Gardener
Mother Earth Gardener
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