Monarch Caterpillars And Milkweed

September 13, 2023

The excitement of finding monarch butterfly caterpillars has been keeping my attention for the last couple of weeks. While looking closely at the showy milkweed, Asclepias speciosa, and the narrowleaf milkweed, Asclepias fascicularis, we discovered that there were caterpillars on the narrowleaf milkweed. Last year, the majority of caterpillars we observed were on the showy milkweed. The small flowers and narrow leaves of A. fascicularis make it easier to find these beautiful striped caterpillars. Yesterday, we found 12 monarch caterpillars ranging in size from tiny light-colored caterpillars to robust ones with brilliant green, black, white, and yellow stripes.

I found out from my research that this is because monarch caterpillars, or larvae, go through five stages of development (besides the egg, pupa, and adult butterfly).

Each of these larval stages is called an instar, and the first instar occurs when the caterpillar hatches out of the tiny white egg. The first instar is pale green color and is shiny and translucent. It eats the egg that it came out of and then begins to eat the milkweed leaf it was hatched on. In about two to three days, it undergoes a molt and loses its transparency and the black, yellow, and white bands become more prominent. It also gets what are called tentacles, which are noticeable on the front of the body. Very small back tentacles also begin to develop. This second instar monarch larva has a greater number of bristle-like setae all over its body. This stage lasts about the same number of days (2-3) as the first stage. Molting is the shedding of the skin so sometimes you can find the shed skin on the milkweed plants. The 2nd instar caterpillars are between 6 to 9 mm in length. Both the first and second instar when disturbed will drop off the leaf and hang by a fine thread produced by silk-producing organs called spinnerets which are in the caterpillar’s head below its mouthparts.

During the third instar, larvae grow to between 10 to 14 mm long and the front and back tentacles (or what looks like antennae) are longer and can clearly be seen. Another morphological difference is that the front pair of legs are smaller than the other two pairs and are beginning to move or migrate closer to the caterpillar’s head. If disturbed, the third instar will drop off the leaf to the ground and curl into a ball rather than hang by a thread. This third stage lasts about the same as the others, about 1 to three days depending on temperature.

By the fourth instar, the monarch caterpillar can be between 13 and 25 mm with long front and back tentacles. There are noticeable white spots on the five pairs of prolegs or ‘false’ legs at the rear of the caterpillar’s body. This stage also lasts 1 to 3 days.

The fifth instar is the final larval stage and lasts 3 to 5 days. The caterpillar is as large as it’s going get, between 25 and 55 mm in length. It can be found away from milkweed plants as it seeks the right place to pupate. When it finds that place, a tree branch, plant stem, leaves, fence posts, or whatever protected surface it can create a silk pad on, it hangs head down from the pad and then molts for the final time becoming a chrysalis.

The chrysalis stage lasts 10 to 14 days and then the adult monarch butterfly finally emerges! Learning all of this allows me to carefully look at the caterpillars and make an educated guess as to what instar I am looking at and how old it is. We are hoping to find pupa on the surrounding plants and watch the monarchs emerge.

Unlike the larvae that only eat milkweed, adult Monarchs use many nectar plants for food. Planting milkweed and other nectar sources benefits monarch butterfly populations and is an easy way to help increase their numbers.