14 Types of Caterpillars in Colorado (and How to Identify Each One)
September 18, 2026
Colorado’s caterpillars range from harmless woollybears wandering across hiking trails to spiny stingers best admired from a distance. The CaterpillarIdentification.org database lists 81 documented caterpillar species across the state, and knowing the difference between a friendly monarch larva and a painful slug caterpillar matters for anyone gardening, hiking, or letting kids explore the backyard.
This rundown covers 14 of the most recognizable types of caterpillars in Colorado, organized by where you’ll actually run into them: garden beds, cottonwood groves, alfalfa fields, and foothill shrublands. Each entry includes host plants, identifying features, and whether it’s safe to pick up.
1. Saltmarsh Caterpillar
The saltmarsh caterpillar (Estigmene acrea) is one of two “woollybear” species Coloradans are most likely to spot wandering across sidewalks and trails in late summer and fall. According to PlantTalk Colorado, the most common species in Colorado are the yellow woollybear and saltmarsh caterpillar, and both earn the “woollybear” nickname from their hairy appearance, wandering habits and the fact that they feed on a variety of plants.
Despite the dense, bristly hairs, saltmarsh caterpillars don’t sting and rarely cause garden damage worth worrying about, since the caterpillars rarely cause concern because the damage they do occurs so late in the growing season. If you’ve heard that a woollybear’s stripe width predicts how harsh winter will be, CSU’s plant pathology team notes there’s little evidence behind that folklore, though it remains a favorite piece of Colorado lore passed down every autumn.
Pro Tip: Saltmarsh caterpillars are generalist feeders that will happily crawl across a driveway looking for a new host plant. Relocating one to a nearby garden bed is safer than leaving it on pavement, and it won’t bite or sting during the move.
2. Leafy Spurge Hawk Moth Caterpillar
Few types of caterpillars in Colorado have a backstory quite like the leafy spurge hawk moth (Hyles euphorbiae). Colorado State University Extension notes this species was deliberately introduced, explaining that this species has been purposefully introduced into Colorado for the suppression of leafy spurge, an invasive rangeland weed.
The caterpillars are hard to miss once they reach their final instars. Late stage caterpillars develop vivid markings that include yellow or red striping, light colored spots, red and black prolegs, and full grown caterpillars may be about 4 inches long. The species is currently established in Larimer and Morgan Counties, but probably has a considerably wider range within the state. Adults are true hawk moths, and adults emerge in late May or June and may sometimes be seen visiting flowers at dusk… hover when feeding, much in the manner of a hummingbird.
This introduced species also shows up in neighboring states and provinces as its range expands north; gardeners tracking similar biocontrol insects across the border can compare notes with resources on insects found in Canada.
3. Eastern Tiger Swallowtail Caterpillar
The eastern tiger swallowtail (Papilio glaucus) is one of the showier butterfly caterpillars in Colorado’s garden and woodland edges. Young larvae protect themselves through disguise, since the young Eastern Tiger Swallowtail caterpillar will mimic the appearance of bird droppings to ward off predators, a strategy that fools most hungry birds long enough for the caterpillar to keep growing.
Older caterpillars trade camouflage for intimidation. Their bright green with black and white like eyes pattern creates a convincing snake-face illusion, and this caterpillar can be seen in northeastern Colorado, eating various plants, tulip trees, and wildflowers before it can grow up to 2 inches at full size. Gardeners in other tiger swallowtail territory, including those researching caterpillar species in Georgia, will recognize the same false-eyespot defense.
4. Silver-Spotted Skipper Caterpillar
The silver-spotted skipper (Epargyreus clarus) is small but distinctive once you know what to look for. Per Animal of Things’ Colorado caterpillar research, the Silver-spotted Skipper Caterpillar is a type of caterpillar found in Colorado… small, with its length ranging from 1.5 to 2.5 cm and width from 0.6 to 1 cm.
Its coloring makes identification straightforward: this caterpillar has a light grey body and several white spots across its surface… it has a brownish-orange head with two large orange-yellow eyespots, an eyespot pattern that likely startles predators. The larvae aren’t picky eaters, and the larvae feed on various herbaceous plants, including legumes and Asteraceae, which explains why they show up in Colorado gardens planted with clover, locust, or wild sunflowers. Gardeners comparing notes with caterpillar identification guides from Maryland will find the same species feeding on similar legume hosts.
5. Slug Caterpillar
Slug caterpillars earn their name from a slow, gliding motion across leaves rather than a typical caterpillar crawl. In Colorado, this group is represented by Acharia horrida, and A-Z Animals’ Colorado field notes describe it as one of the most common type of caterpillars found in ash and deciduous trees throughout Colorado.
Handle these with caution. If disturbed, if these tiny creatures are touched or frightened, it will raise its head and spread its tail… then spray a yellow foul liquid that causes irritation of the skin, but is not poisonous. Despite the defensive display, most sources agree it poses little real danger; separate reporting on the species confirms the Slug Caterpillar is harmless to humans and poses no threat beyond mild, temporary skin irritation.
Important Note: Slug caterpillars in the family Limacodidae vary widely in sting intensity across North America. Colorado’s species is mild compared to relatives found in warmer climates, such as those covered in guides to caterpillar species in Florida, where several stinging slug caterpillars cause much sharper reactions.
6. Spiny Elm Caterpillar
The spiny elm caterpillar is the larval stage of the mourning cloak butterfly (Nymphalis antiopa), and Colorado naturalists consider it one of the state’s most frequently spotted species. Regional caterpillar surveys describe the Spiny Elm Caterpillar as one of Colorado’s most common caterpillar species… usually found on elm trees, though it also feeds readily on willow, hackberry, and cottonwood.
Mature larvae are dark, almost black, with a scattering of fine white speckles and a row of brick-red spots down the back, all bristling with branched, blue-black spines. The spines look intimidating but aren’t venomous; they simply deter birds looking for an easy meal. Because mourning cloaks overwinter as adults rather than pupae, Colorado residents sometimes spot the butterflies flying on unseasonably warm winter days, well before most other species emerge. Similar elm-feeding populations turn up in colder climates too, including in guides covering caterpillars found in Maine.
7. Monarch Caterpillar
No list of butterfly caterpillars in Colorado is complete without the monarch (Danaus plexippus), instantly recognizable by its bold banding. Field guides describe it as a plump insect with black, white, and yellow stripes, a pattern that signals danger to predators rather than hiding from them.
That warning is backed by chemistry. This caterpillar is toxic because of the number of milkweed plants they eat, and toxins absorbed from milkweed sap stay in the caterpillar’s tissue long after it becomes a butterfly. Distribution follows the food source closely: if you are looking for this beautiful creature, they hang out wherever milkweed is found… the Monarch can be found in mountains and plains of Colorado.
Pro Tip: Planting native milkweed species like showy milkweed (Asclepias speciosa) is one of the most effective ways to attract monarchs to a Colorado yard, and it supports pollinators tracked in broader regional surveys such as guides to bee species in Colorado.
Monarchs also pass through Iowa and other Midwestern flyway states during their multi-generational migration, a route covered in more detail in resources on caterpillar species in Iowa.
8. Pine Webworm
Pine webworms build communal silk shelters among pine needles, and their feeding damage is often more obvious than the caterpillars themselves. While the classic pine webworm (Pococera robustella) is best documented in the eastern and central United States, Colorado’s ponderosa and lodgepole pine forests host closely related needle-webbing pests that Colorado State University catalogs alongside it, including entries for Pine Tip Moths and the Zimmerman Pine Moth in its statewide arthropod reference.
Whichever species is responsible, the damage pattern looks similar: clusters of needles bound together with silk, brown and skeletonized where larvae have been feeding, usually on the lower and middle branches of young or stressed pines. Homeowners who spot webbed pine needles in Colorado’s foothill communities are more likely dealing with a pine tip moth than the true pine webworm, but management for both follows the same basic playbook of pruning infested tips and avoiding unnecessary pine stress. Comparable pine-feeding insects are covered in broader surveys of insect species in New York, where true pine webworms are far more established.
9. Ugly Nest Caterpillar
The ugly nest caterpillar (Archips cerasivorana) lives up to its unglamorous name. CSU Extension’s tent-caterpillar research explains that ugly nest caterpillars can be found on chokecherry, where they produce a messy nest of silk mixed with bits of leaves and insect frass, a shelter that looks far scruffier than the neat tents built by true tent caterpillars.
Despite the unappealing nest, this species rarely causes lasting harm to chokecherry stands, and the webbing mostly matters as a visual nuisance rather than a serious pest problem. Gardeners who prune out active nests before larvae disperse can reduce the following year’s population without resorting to chemical treatment. Similar chokecherry-feeding insects appear in comparable habitats further east, including populations discussed in guides to caterpillars in Tennessee.
10. Eastern Tent Caterpillar
Colorado sits at the edge of the eastern tent caterpillar’s (Malacosoma americanum) range, and CSU Extension confirms it shows up mainly in one region: in northeastern Colorado, the eastern tent caterpillar (M. americanum) can occasionally be found on fruit trees. Unlike its more destructive cousin, this species tends to appear in scattered, low numbers rather than large outbreaks.
Colorado is actually home to four species of tent caterpillars, and telling them apart comes down to host plant and timing as much as appearance. The table below breaks down the two most commonly confused species.
| Feature | Eastern Tent Caterpillar | Western Tent Caterpillar |
|---|---|---|
| Primary host | Fruit trees (cherry, apple) | Aspen, mountain-mahogany |
| Region in Colorado | Northeastern Colorado | Statewide, especially foothills and mountains |
| Peak activity | Spring, after bud break | May to early June |
| Population impact | Occasional, localized | Can cause widespread aspen die-off |
Growers managing fruit trees may find useful comparisons in guides covering caterpillar species in Oklahoma, another state where eastern tent caterpillars target orchard trees each spring.
11. Western Tent Caterpillar
The western tent caterpillar (Malacosoma californicum) is, by a wide margin, the more consequential of Colorado’s tent-making species. CSU Extension notes it most often is seen infesting aspen and mountain-mahogany during May and early June, and describes it plainly as the most common and damaging tent caterpillar, sometimes producing widespread outbreaks that have killed large areas of aspen.
Outbreak years can strip entire hillsides of aspen canopy, though healthy trees typically refoliate and recover within a season or two. Two additional regional relatives round out the state’s tent caterpillar lineup: in stands of Gambel oak, the Sonoran tent caterpillar (M. tigris) occurs and the M. incurvatum discoloratum can be found feeding on cottonwoods in western Colorado’s Tri-River area. Anyone comparing native caterpillars across regions can browse the broader reference on common types of caterpillars for context on how tent-building behavior evolved across multiple moth families.
12. Beet Armyworm
The beet armyworm (Spodoptera exigua) is a crop pest rather than a garden curiosity, and Colorado growers watch for it closely each season. CSU Extension’s field crop guide describes it as a greenish striped caterpillar closely resembling the clover cutworm but lacking the black markings around the abdominal spiracles, a subtle distinction that matters for accurate identification.
Its appetite is broad but has a clear favorite: this insect will attack a variety of broadleaf and grass crops, but sugarbeet is a preferred host, and populations build quickly because the beet armyworm has two to three generations per year in Colorado. Home gardeners growing beets, chard, or spinach in eastern Colorado occasionally see damage from this species even without commercial-scale sugarbeet nearby. Growers in other agricultural states, including those referencing guides to insect species in Texas, deal with the same pest on cotton and vegetable crops.
13. Colias Eurytheme Caterpillar
Also called the alfalfa caterpillar, Colias eurytheme is the larval form of the orange sulphur butterfly and a familiar sight in Colorado’s hay fields. CSU Extension’s identification notes describe it as dark green with a narrow white stripe on each side of the body and a fine red line running through the stripe, a combination that separates it from other green crop-feeding caterpillars.
Texture is the fastest way to confirm an ID in the field: the alfalfa caterpillar is distinguished from other Colorado crop caterpillars by short, dense, velvet-like hairs covering the head and body. True to its common name, it’s primarily an alfalfa pest, though it occasionally turns up in other legume crops as well. Colorado’s alfalfa-growing regions share this pest with similar Midwestern hay ground, including areas profiled in guides to caterpillar species in Wisconsin.
14. Spotted Tussock Moth Caterpillar
The spotted tussock moth caterpillar (Lophocampa maculata) rounds out this list as one of the more strikingly patterned species in Colorado’s foothills. Regional identification resources confirm the spotted tussock moth caterpillar (Lophocampa maculata) is a type of caterpillar found in Colorado, and it’s most active during a specific window each year.
Size and timing help separate it from lookalikes: this species can grow up to 2.5 inches long and are seen between late summer and late fall in the foothills of Colorado, feeding on a range of deciduous trees and shrubs along the way. Like several other tussock moth relatives cataloged by CSU Extension, dense outbreaks occasionally draw attention in spring, though the individually wandering caterpillars most people encounter show up later in the year. Readers curious how this species compares to caterpillars found in other ecosystems can browse broader entomology coverage in guides to different types of insects.
Colorado’s caterpillar diversity reflects the state’s dramatic range of elevation and habitat, from milkweed-lined plains where monarchs breed to aspen groves where western tent caterpillars occasionally defoliate entire hillsides. Most species on this list are harmless to handle, but a few, particularly the slug caterpillar, are worth observing rather than touching. For readers who want to keep exploring, resources on how many caterpillar species exist worldwide and guides to raising insects as pets offer good next steps, as does the fuller extended list of Colorado caterpillar species for anyone who wants to go beyond these 14.