Summary: Unlike many damaging caterpillars, the Monarch caterpillar's diet consists almost entirely of milkweed leaves, so they will not pose a threat to your garden or yard.
Hooray for small favors! At last a caterpillar that won't destroy my landscape plants and trees. The Monarch caterpillar eats milkweed leaves and nothing else. Farmers love having the Monarch as a guest to rid fields of the persistent milkweed plant.
The sap of milkweed leaves contains a chemical called cardiac glycoside. This chemical is stored in the caterpillar's body. If a predator tries to attack the monarch caterpillar, it will taste this repulsive, poisonous chemical. Ingestion of the Monarch caterpillar usually results in illness.
The day before the monarch caterpillar emerges the egg turns black. After the caterpillar emerges from its shell, it eats it! It grows and grows during its time in the larval stage of its life, which is about 14 days. In fact, a monarch caterpillar can grow over 2,000 times its original mass! Its main responsibility
during this time is to eat plenty of food. This weight gain is important in determining how big the monarch will be as a full-grown butterfly. The large amount of milkweed leaves allows the insect to store a lot of lipids that are useful when it is an adult trying to survive the winter.
A monarch caterpillar goes through five different stages called “instars.” The caterpillar grows bigger and molts its skin throughout each stage. The official term for a caterpillar's skin is “cuticle.” The caterpillar also has an exocuticle, which is a waxy substance that protects the insect from losing water.
The body of a monarch caterpillar is fascinating. It has three pairs of true legs connected to its thorax, and it also has a few pairs of prolegs connected to its abdomen. The prolegs have tacky pads called crochets attached at the bottom.
Monarch caterpillar predators have a special technique for finding their prey. They observe milkweed leaves for damage giving a good indication that the Monarch caterpillar is present. However, the caterpillars also know how predators can find them, so they do a lot of moving from leaf to leaf, leaving no forwarding address.
Another escape tactic for the Monarch caterpillar is to use silk as a type of rope. If it senses a predator nearby, the caterpillar can crawl down to the ground by dispensing a string of silk. Like many other types of caterpillars, it will curl up into a ball and do the “play dead” routine when touched.
A Monarch caterpillar will give off certain clues when it is ready to pupate. First, it will stop eating. Second, it will move from the milkweed leaf it was living on, moving to a safer place. Finally, it will hang off a twig or leaf in the shape of a “J”. Within a day, the monarch caterpillar will form into a chrysalis. (Interesting fact of the day: A butterfly develops from a chrysalis, but a moth develops from a cocoon.)
Because Monarch caterpillars have a diet that entirely consists of milkweed leaves, they will not pose a threat to your garden or yard. They are also relatively harmless to humans. These unique caterpillars grow into beautiful butterflies, so if you see them near your home, they will be fun to observe as they go through the stages of metamorphosis.