A look at Monarch caterpillars

On a recent trip to Vinton County to photograph the old Moonville Railroad Tunnel, we made a quick stop at the Wayne National Forest Welcome Center. The display of native plants was impressive as was the informative signage. As we walked around observing butterflies we started carefully checking for Monarch eggs and caterpillars. We were excited to find the tiny caterpillar in my photograph.

Monarchs lay eggs one at a time on milkweed plants, most frequently on the underside of leaves. When milkweed is scarce, they may load a single plant with eggs, but they usually lay only one egg on a plant. The eggs have ridges and taper to a point on top. The black head capsule of the caterpillar can be seen inside eggs about to hatch.

When temperatures are sufficiently warm – between 20 – 27 degrees C (70s and upper 60s F) – the eggs hatch three to five days after they are laid. In cooler temperatures they can take as many as 20 days to hatch. A newly-hatched caterpillar often eats its eggshell first. It will then eat the milkweed leaf, frequently leaving a characteristic arched hole in the leaf. Remember, Monarch caterpillars feed exclusively on plants in the milkweed family.

The caterpillars eat, grow and molt their outer skins four times, going through five instars (a period between larval molts). The fifth instar caterpillars are about two inches long and have yellow, black and white stripes and four fleshy black tentacles-two in front and two in the rear. In warm conditions, the caterpillars are ready to pupate 14 to 18 days after the eggs are laid, according to the authors of “Milkweed, Monarchs and More – A Field Guide to the Invertebrate Community in the Milkweed Patch.”

About 10 days after it is formed, the chrysalis begins to darken and the familiar patterns of the monarch butterfly’s bright, orange and black wings become visible under a transparent cuticle. The butterfly ecloses (emerge from pupal stage), pumps up and dries its wings and is ready to fly in a matter of hours.

Three or more generations emerge each summer. Those eclosing in June and July have an adult life span of four to five weeks. In the north, the final generation of Monarchs become adults in mid-august through mid-September and will migrate to overwintering sites in Mexico and along the Pacific coast of California, where some survive up to eight months. This journey might entail traveling nearly 2,000 miles one way. Once in Mexico, the butterflies congregate in massive numbers in a very few favored locales. Northward migration reaches the US in early March. Females lay eggs on emerging milkweeds. The offspring of this first brood then colonize the remainder of the breeding range in eastern North America, according to the ODNR publication, “Milkweeds and Monarchs.”

There is great concern over the decline in the Monarch populations. What can we do? Try including some native species of milkweed to your landscape. Monarch Watch is an excellent source for milkweeds, visit the website at monarchwatch.org/milkweed/market.

Picking tomatoes and cucumbers? Making salsa and pickles? Digging potatoes? This is certainly a busy time in the vegetable garden.

Don’t forget to take some time to walk around your flower beds and check for butterflies and caterpillars. Don’t focus on the weeds.

Remember to email your gardening questions to Brown County Master Gardener Volunteer Mike Hannah at mhannah2@msn.com.


By Faye Mahaffey

Faye Mahaffey is an OSU Master Gardner volunteer who writes on local gardening stories and techniques.