12 Weird (but COMMON) Caterpillars in Colorado (2023)
What kinds of caterpillars can you find in Colorado?
Caterpillars are some of the MOST fascinating insects in the world! It always amazes me that caterpillars eventually turn into butterflies or moths.
There are hundreds of different caterpillar species found in Colorado!
Since it would be impossible to list them all in one article, I chose the 12 most interesting and common ones to share with you. 🙂
#1. Monarch Caterpillar
- Danaus plexippus
- This famous caterpillar is plump with black, white, and yellow bands.
- Its legs and pro-legs are pronounced, and each end of its body has spindly black tentacles.
- The Monarch’s preferred host plant is milkweed.
Like the adult butterfly, the Monarch is one of the most well-recognized caterpillars in Colorado!
Their distinctive stripes and tentacles make them look cartoonish. But this highly visible coloring sends a message to predators: Back Off!
Monarch Caterpillars are toxic to most animals, and at the very least, taste bad! This poison comes from their diet, which is almost entirely made up of milkweed. Toxins from the milkweed plant stay in the caterpillar, producing a bitter taste and poisonous effects.
If you have milkweed in your yard or nearby, your chance of finding Monarch Caterpillars is excellent! Honestly, there is nothing more fun than finding these colorful insects on our milkweed plants and getting to watch them transform into adults!
- Coloring is light green with small yellow dots along the sides.
- This species is small and relatively thin and appears velvety.
- Cabbageworms’ host plants are Brassicas, including cabbage, kale, broccoli, and chard.
In Colorado, this species is often called the Imported Cabbageworm because it isn’t native to North America. It was introduced in shipments of cabbage and other brassica plants and soon became an invasive species.
Cabbageworms are considered agricultural pests and can do severe damage to crops to their host plants. Cabbage, brussels sprouts, broccoli, and cauliflower are all susceptible to damage. For a home gardener, the best way to deal with Cabbageworms is to prevent a large infestation. Plant covers, regular weeding, and varied plantings can all help with preventing this hungry invader!
One reason Cabbageworms are so damaging is that they are voracious eaters! They can easily skeletonize entire plants, eating everything but the toughest stems and midveins. Boring through heads of cabbage and making huge dents in broccoli are no problem for this Very Hungry Caterpillar!
Cabbageworms grow into Cabbage White Butterflies, which are one of the most abundant butterflies in Colorado! If you see a white butterfly in the spring, chances are it’s a Cabbage White!
#3. Woolly Bear
- Pyrrharctia isabella
- Coloring is black with a wide rusty-red band in the middle.
- As its name suggests, the Woolly Bear caterpillar is covered in dense, coarse hairs.
- Transform into Isabella Tiger Moths.
- Woolly Bears are generalist feeders, meaning they will live on and eat nearly any plant!
As a kid, I can remember hearing tons of stories about Woolly Bear caterpillars – and the coolest part about them is that most of the stories are true! One myth you might have heard is that Woolly Bears can predict the type of winter we’ll have. Unfortunately, this one isn’t true. But there’s plenty of other interesting facts about this cute little caterpillar!
The most fascinating thing about Woolly Bears is the way they hibernate.
That’s to say, they don’t hibernate at all! Instead of burrowing or pupating to escape the cold, Woolly Bears allow themselves to freeze solid. They have a unique chemical in their blood that allows them to thaw out and continue in the spring as if nothing happened!
You may have heard that Woolly Bear Caterpillars are venomous, but this isn’t entirely true. Their hairs don’t contain any toxins or irritants, but some people are sensitive to the hairs and may get a slight rash if they touch one. It’s best to observe the species without touching them, just in case.
Woolly Bear Caterpillars and their adult-form, Isabella Tiger Moths, are found in incredibly varied climates, even the Arctic! Because they will eat almost anything, including herbs, tree leaves, and grasses, they can be found pretty much anywhere plants are growing. Look for them in groups near the base of plants.
Just don’t expect them to take over for your meteorologist! 🙂
#4. Viceroy Caterpillar
- Limenitis archippus
- The coloring is mottled brown or green and white to resemble bird droppings.
- Two dark-colored horns on the head and small spines on the body.
- The chrysalis also resembles bird droppings hanging from a tree branch.
- The preferred host plants of Viceroy Caterpillars are willow, poplar, and cottonwood trees.
Viceroy Caterpillars are one of the ugliest caterpillars in Colorado!
This is by design; their lumpy, mottled appearance makes them look like bird droppings, warding off predators!
Interestingly, this isn’t the Viceroy’s only protection against predators. Viceroy Caterpillars eat plants that are rich in salicylic acid, which they store in their bodies. When predators try to eat them, they are rewarded with a strong, bitter flavor and an upset stomach. One taste and they learn to stay away!
Viceroy Caterpillars primarily live in open forests or fields, and they’re found across many different climates. Look for them during spring and summer, which is when the adults typically mate.
Interestingly, Viceroy and Monarch Caterpillars look almost identical.
#5. Large Maple Spanworm
- Prochoerodes lineola
- Gray-brown coloring with small black spots scattered on the body. Often this species has a bark-like pattern.
- Thin and stick-like with a knob on each end.
- Large Maple Spanworms use a huge variety of plants and trees as hosts: birch, maple, cherry, apple, oak, poplar, walnut, and willow trees; geranium, soybean, blueberry, and currant plants; and grass.
This species has one of the best camouflages of any caterpillar in Colorado!
Large Maple Spanworms look exactly like a bit of twig on a tree, even from up close! So, it’s hard to imagine any predator observant enough to try and eat one, which is precisely its goal.
Their camouflage is the only defense Maple Spanworms have because they aren’t poisonous. They’re a favorite snack for determined birds!
When they’re not imitating sticks, Large Maple Spanworms have an interesting way of getting around! They plant their front legs, arch their back in the air, and bring their back legs forward to meet their front. Then, they throw their front forward and repeat the whole process. They’re one of many caterpillars that move like this, earning them nicknames like inchworm, looper, or spanworm.
Large Maple Spanworm Moths are just as adept at camouflage as their larva – they look exactly like dead leaves clinging to a branch!
#6. Variegated Fritillary Caterpillar
- Euptoieta claudia
- Stripes of black, red, and white run the length of the body.
- Black branched spines stick out from each body segment in even rows.
- Variegated Fritillaries will use any plant in the violet or alder family as a host plant. These include common blue violets, yellow alder, and pansies.
Variegated Fritillary Caterpillars share the same name as their adult-form butterflies. They eat ornamental plants like violets, pansies, and passionflower.
Look for these caterpillars in Colorado in meadows, open lots, and fields.
The Variegated Fritillary’s chrysalis is the most beautiful of all the caterpillars in Colorado. This protective shell is where the caterpillar transforms into the adult butterfly. Its pearly white color and shiny gold spikes make it look like an expensive jeweled pendant!
#7. Curve-Lined Owlet Moth Caterpillar
- Phyprosopus callitrichoides
- The coloring is shades of brown and cream, occasionally near black.
- The distinctive body shape is spiky, contorted, and asymmetrical, like a dry leaf.
- Greenbriers are the host plant of choice for this species.
There’s a good chance you’ve seen a Curve-Lined Owlet Caterpillar in northeastern Colorado!
However, you may not have even realized it because this species is an expert at camouflage. Its body is meant to look like a dry, curled leaf clinging to a branch. I think it succeeded!
Interestingly, it isn’t only the larva of this species that hide in plain sight. Curve-Lined Owlet Moths also have coloring and texture that resembles a dry leaf. So, it seems like even though the insect goes through a complete metamorphosis, it keeps some of the survival traits into adulthood!
The host plant of the Curve-Lined Owlet Caterpillar is greenbrier, a vine plant common in many habitats. This species prefers woodland and nearby clearings, but it’s sometimes spotted in more developed areas like office parks or lush backyard gardens.
#8. Milkweed Tussock Caterpillar
- Euchaetes egle
- Tufts of black and white hairs cover the body, with one line of black-centered orange tufts along the back.
- The body is relatively thin and up to 1.5 inches long.
- As its name suggests, this species’ preferred host is Milkweed.
Despite looking very different, Milkweed Tussock Caterpillars in Colorado have a lot in common with Monarch caterpillars!
Milkweeds are the preferred host plant for both species. However, the really interesting thing about them isn’t what they eat; it’s why! Milkweed Tussock Caterpillars store a poisonous chemical from milkweed called cardiac glycoside in their body.
It doesn’t harm the caterpillar, but it does an effective job of making the caterpillar both disgusting and dangerous for many predators! In fact, Blue Jays have been known to vomit after eating just one of these furry little caterpillars.
The fascinating thing about cardiac glycoside is it stays in the caterpillar’s body through its transformation into a moth. So, even though Milkweed Tussock Moths don’t eat milkweed, they still have the benefits that the caterpillar’s diet created!
#9. Parsley Caterpillar (Black Swallowtail)
- Papilio polyxenes
- The coloring is bright green, with rings of black and yellow dots down the length of the body.
- The head and thorax are slightly larger than the back, and the legs and pro-legs are pronounced and visible even from a distance.
- Parsley plants are the preferred host of this species.
Parsley Caterpillars, sometimes called parsley worms, are the larva of the Black Swallowtail butterfly. They get their name from their preferred host and favorite snack, the parsley plant.
At first glance, it’s easy to mistake a Parsley Caterpillar for a Monarch. But, the coloring is slightly different, and the stripes on the Monarch Caterpillar are a bit thinner. It would also be unusual for a Monarch Caterpillar to eat parsley or other garden herbs since they eat milkweed almost exclusively!
If you’re a gardener, you might consider Parsley Caterpillars a bit of a nuisance since they can decimate a parsley plant quickly. But, if you plant some extra, you’ll be rewarded with sightings of the beautiful Black Swallowtail butterfly in a few weeks!
#10. Eastern Tiger Swallowtail Caterpillar
- The coloring is bright green with two large eyespots in white, black, and blue.
- The thorax is much larger than the middle and tail, giving the caterpillar the look of an enlarged head.
- The favorite host plants of this species are the tulip tree and wild black cherry.
This species is one of the strangest-looking caterpillars in northeastern Colorado!
The appearance of Eastern Tiger Swallowtail Caterpillars is so unique they inspired a Pokemon! Caterpie (the Pokemon) has the same features, including its horn-like Osmeterium, bulbous thorax, and large round eyespots. I consider this to be quite an honor! 🙂
This caterpillar’s primary defense is mimicry, using its unique shape and coloring to imitate a snake’s head. Additionally, as a young caterpillar, its color is brown and white to mimic bird droppings!
Interestingly, the adult Eastern Tiger Swallowtail Butterfly stands out in a crowd with its unique coloring and pattern.
Another defense of the Eastern Tiger Swallowtail is its Osmeterium, a horn-like organ that can be projected from the caterpillar’s head. Its dual functions are to mimic a snake’s forked tongue and to smell horrible to predators! Interestingly, to humans, the odor is strong but pleasant, like grass and pineapple!
#11. Cecropia Moth Caterpillar
- The coloring of this species changes with each instar growth. Most commonly seen in late instar; green with yellow, blue, and red bumps topped with black spikes.
- It has a large, fleshy body and very obvious leg appendages.
- Cecropia Caterpillars prefer birch, cherry, and maple trees for host plants.
Cecropia Caterpillars look more like aliens than anything!
Their bulbous bodies and multicolored, spiked nodules truly look like something from a sci-fi movie.
Look for Cecropia Caterpillars on maple, birch, and apple trees during late spring. They remain in their caterpillar stage for about two weeks before encasing themselves in their chrysalis for winter.
This species is one of many caterpillars in Colorado called a “silkworm”. The name refers to the silk cocoon they spin around their chrysalis in preparation to become a moth. The cocoons are brown and cling to the side of host plants, and look like dead leaves.
As strange as Cecropia Caterpillars look, it’s nothing compared to the adult Cecropia Moth. Not only is the pattern and coloring beautiful, but this moth is also ENORMOUS! In fact, it’s the largest moth in North America!
#12. Io Caterpillar
- Automeris io
- The coloring is green with two stripes along each side, one red and one white.
- Tufts of short green spines cover the body, with patches of light green showing through.
- Io Caterpillars use hackberry and willow trees as host plants.
Io Caterpillars are highly venomous, and their sting is excruciating!
Fortunately, the sting is rarely severe enough to seek medical attention. Instead, most experts recommend removing the spines with scotch tape, then applying ice to the sting. Over-the-counter antihistamines and pain relievers can also help.
Of course, the best way to stay safe is to avoid touching Io Caterpillars. Since they’re so recognizable, this should be pretty easy!
Their tufts of green spines are distinctive and hard to miss against darker green leaves or brown bark. The red and white stripes on the sides are also helpful – they clearly say, “Stay back, I’m dangerous!”
If you know you’re going to be gardening or doing yard work in an area with Io Moths or their caterpillars, it’s a good idea to wear a hat and gloves. That way, if you happen to brush against one, you won’t have to worry about exposed skin!
Adult Io Moths are just as distinctive as their larva – if not more so! They have a beautiful pattern with large, prominent eyespots. There are two color morphs:
Do you need more help identifying caterpillars in Colorado?
Here are some recommended books on Amazon!
Which of these caterpillars have you seen in Colorado?
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Trying to find out about large dark brown caterpillar similar shape to Eastern tiger swallowtail.