The Monarch Butterfly, Danaus plexippus, is one of the world’s most beloved insects. Sadly, their very existence is under serious threat. There are a number of reasons why. We can help reverse one of their threats by planting milkweed plants in our yards and avoiding the use of insecticides.
Monarchs lay their eggs only on milkweed plants, and that’s what their caterpillars eat. But, the number of milkweeds has decreased 21 percent since 1995. That’s due to genetically engineered crops, principally corn and soybeans, that can withstand Round-up, a powerful herbicide. Farmers used to kill weeds on their croplands by tilling between rows to chop them up and turn the soil, but some milkweeds still survived. Now, however, an entire field can be sprayed with Round-up, including the food crops. Pretty much nothing can escape the spray; the crops live, the weeds die. Croplands used to be significant habitats for milkweeds.
Monarchs face other threats, too, including illegal deforestation in their winter sanctuaries, bad weather and possibly climate change. All this combined reduced the Monarch’s fall migration from 550 million individuals in 2004 to only 33 million in 2013. Some recent good news indicates their population was up in 2014.
We may not be able to individually control other harmful factors, but we can definitely play a role in helping these beautiful butterflies, simply by planting milkweeds, as many as you have space for.
Here the female is laying an egg. Notice she’s curling her abdomen upward to deposit it on the underside of a milkweed leaf, where it will be better hidden from the sharp eyes of predators. Eggs are sticky and stay attached. They’re very small — not much larger than the period at the end of this sentence. They hatch in five to 10 days.
The caterpillars in the above photo are about 2 inches long. After hatching, their first meal was probably their eggshell, rich in protein. They periodically rest, but, otherwise, it’s all about eating, eating, eating to store up fat and proteins for their later metamorphosis into adult butterflies. They will soon be ready to pupate.
As caterpillars eat and grow larger they outgrow their skin and shed it several times in a process called molting. They usually go through five or six molts. There’s a term for the stages between each molt — instar. Between hatching and the first molt a caterpillar is called a first instar. Between the first and second molt, it’s a second instar, and so on.
When a caterpillar is ready to pupate, it moves a few feet off the plant and attaches itself to something solid to hang from. This might be the stem of a neighboring plant, a twig, the lip of a flowerpot or even the underside of a deck chair! With secretions from its mouth, it deposits a sticky silk pad to hang from, attaches its hindmost pair of prolegs and takes on a characteristic J shape.
Prolegs are the caterpillar’s stubby, fleshy legs with black feet that you can easily see above. They help caterpillars cling to plants and other surfaces. Prolegs are temporary and disappear altogether during pupation.
Now shedding its skin for the last time, it isn’t recognizable as a caterpillar nor a butterfly. The skin on its back has split open, revealing the underlying skin which will become its chrysalis (often called, inaccurately, a cocoon.) The old skin (the black-striped part) is being crushed upward as the caterpillar maneuvers its body free of it. Eventually the old skin will be just a small dark, dry wad with the chrysalis hanging below, and will fall off.
Here’s the chrysalis (KRIS-uh-lis). The caterpillar within is undergoing a process that converts its body into a head, eyes, internal organs, wings and all the other body parts that constitute an adult butterfly. The chrysalis is extremely delicate. If it falls, the butterfly will die. Even if no obvious harm comes to them, Monarch pupae have a high mortality rate, for unknown reasons. What goes on inside the chrysalis
The metamorphosis takes about 10 to 14 days. As the caterpillar develops into a butterfly, little by little the shape of its body and its tightly folded wings can be seen. Here’s a composite showing its emergence. Notice the wings, all wadded up. They’re floppy and wet. Slowly, they’ll take shape as the butterfly inflates them with liquid drawn from its abdomen. This gives them the form and, once dry, rigidity that’s necessary for flight. If anything interferes with this process or causes the wings to become misshapen before they dry, it’ll be partially or wholly crippled. If it can’t fly, it’ll starve to death.
Here’s how to tell if a Monarch is female or male. Note the heavier veins in the wings of the female and they lack the two small, black spots you see on the male’s wings.
Monarchs live two to six weeks, most of the time mating and laying eggs. Summer generations live two to six weeks. The final generation emerges in late summer and early fall and migrates south to overwinter in Mexico. Monarchs in the West migrate to California. They will live eight or nine months, long enough to begin their migration northward and produce the first spring generation.
Although Monarch caterpillars feed on milkweed leaves, the adults eat nectar from various flowers. Pop quiz: What sex is this one?**
**Compare it to the composite photo above it and you’ll find that it’s a male.
Beautiful National Geographic video of Monarchs
Visit Monarchwatch.org for in-depth information about the Monarch