How To Identify Monarch Butterflies

I’m as excited as anyone about supporting Monarch butterfly conservation efforts. We plan to buy several swamp milkweed plants to add to the existing trees, shrubs and flowering plants around our property.

A monarch rests on asters. Image courtesy of the National Park Service.
A monarch rests on asters. Image courtesy of the National Park Service.

I got to see the Monarchs many years ago while on a visit to Pacific Grove, Calif. I’m not sure, however, that I’ve ever spotted a Monarch in New Mexico, and a map I checked had a giant question mark for much of New Mexico and Arizona, even though our states border Mexico. Although singularly striking, Monarchs look similar to a few other butterflies. What’s more, the critical aspect of saving them requires providing habitats for the butterflies’ larvae. Enter milkweed.

A flowering milkweed plant in the pollination gardens at the Arizona-Sonora Desert Museum outside Tucson, Ariz.
A flowering milkweed plant in the pollination gardens at the Arizona-Sonora Desert Museum outside Tucson, Ariz.

Monarch life cycle

Monarch butterflies lay their eggs on milkweed plants, which also attract other pollinators. The Monarchs find milkweed even when it has finished flowering for the season. The eggs are tiny (about the size of the head of a pin), but as they take on adult form, the larvae, or caterpillars, emerge and grow rapidly.

Monarch butterfly lifescyle. Courtesy of the U.S.D.A. http://www.fs.fed.us/wildflowers/pollinators/Monarch_Butterfly/biology/index.shtml
Monarch butterfly life cycle. Courtesy of the U.S.D.A. http://www.fs.fed.us/wildflowers/pollinators/Monarch_Butterfly/ biology/index.shtml

Like many gardeners, I freak out a little when I see a big caterpillar on a plant. I can’t abide tomato hornworms and the damage they inflict, and we also have had branches of several small trees stripped by the io moth caterpillar. This bad boy stings and releases a poisonous substance that can irritate the skin or cause severe reactions in some people. Last summer, we found a similar greenish-yellow caterpillar on fennel plants, or what was left of them. I think I’ve identified them now with some research.

The io caterpillar is similar in color, but has a fuzzy, fluorescent look. And it stings!
The io moth caterpillar is similar in color, but has a fuzzy, fluorescent look. And it stings! Check out the stripped branches in every direction.

So, I wanted to be sure that I can easily recognize the Monarch caterpillar before trying to attract them to my yard. In fact, all gardeners who want to help the Monarchs should be able to recognize the insect in all forms. I would hate to hand-pick a Monarch caterpillar and end its life cycle! Strangely, finding quality resources to help me identify the caterpillars, and especially to share photos, did not come easy. I finally tracked down some information.

Monarch or Queen?

Although I don’t want to hurt the chances of any butterfly, I was relieved to discover that the caterpillars on the fennel were most likely Queen butterflies, not Monarchs. Queen butterfly caterpillars also feed on milkweed, and can share plants with Monarchs. The butterflies look really similar, but have a few distinctive differences.

Here's a Queen butterfly enjoying milkweed in fall at the Arizona-Sonora Desert Museum.
Here’s a Queen butterfly enjoying milkweed in fall at the Arizona-Sonora Desert Museum. Notice the white dots that extend toward the body.
Monarch wings look like stained glass designs. Image courtesy of National Park Service.
Monarch wings look like stained glass designs, and the color varies slightly throughout. Image courtesy of National Park Service.

Queen, Monarch and Viceroy butterflies have similar colors and patterns. Monarchs are a deeper orange, and the hue changes slightly throughout the wings. The best way to tell the adults apart is the location of white spots on the wings. The Queen butterfly has dots that extend into the orange areas of the wings. But Monarchs have a distinctive stained glass appearance and no dots outside the black edges of the wings. Viceroys are smaller than Monarchs and have triangular white dots under their wings.

Just to confuse matters: Here's a Viceroy adult. Courtesy of the NPS.
Just to confuse matters: Here’s a Viceroy adult. Courtesy of the NPS.

You can distinguish the Monarch caterpillar from other caterpillars by the antennae. The Queen butterfly has three sets of antennae and filaments, and the Monarch has only two – one antenna pair on the front end and a filament pair on the back end. The Queen caterpillar spots an extra set about one-third of the way down from the front. Viceroy caterpillars do not look like Monarch larvae; the Viceroy caterpillar has a brown, rough appearance.

Close-up of a Monarch caterpillar. Note two sets of protrusions, while the Queen has three.
Close-up of a Monarch caterpillar. Note two sets of protrusions, while the Queen has three. Image courtesy of NPS Photo/Alicia Lafever.

The Monarch young larva is nearly clear until it begins to eat milkweed and grow. The larval portion of the life cycle lasts about 10 to 16 days. They can become plump just before pupating, having eaten plenty of milkweed. The caterpillars move away from milkweed as they prepare to evolve to pupae.

Queen butterflies are pretty as well. This one enjoys a butterfly weed flower in Northeastern New Mexico.
Queen butterflies are pretty as well. This one enjoys a butterfly weed flower in Northeastern New Mexico.

Check the Resources page for excellent pages and videos on identifying and protecting Monarchs. Happy hunting and pollinating!