Colorado is home to dozens of caterpillar species, from tiny cutworms that chew through garden seedlings to three-inch hornworms that strip tomato plants overnight. Some build conspicuous silk tents in your trees, others are welcome visitors that become native butterflies. Knowing which caterpillars you’re looking at helps you decide whether to leave them alone or take action.
Tent-Making Caterpillars on Trees
The most visible caterpillars in Colorado are the tent makers, species that spin conspicuous silk structures on tree branches in spring and early summer. The forest tent caterpillar is the most common and damaging species in urban areas, feeding on aspen, ash, and various fruit trees. Western tent caterpillars, recognizable by their blue-white stripe with black bands, show up on aspen and mountain mahogany during May and early June but also infest fruit trees, bitterbrush, and ornamental shrubs. In northeastern Colorado, you may occasionally spot eastern tent caterpillars on fruit trees as well.
Sonoran tent caterpillars stick to stands of Gambel oak, while a related species targets cottonwoods and poplars along river corridors. During outbreaks, tent caterpillars can completely strip a tree’s leaves and then migrate to neighboring hosts. Repeated defoliation over multiple years causes branch dieback, slowed growth, and occasionally kills the tree outright. Most years, though, healthy trees recover after a single defoliation.
After midsummer, fall webworms take over as the most common tent-making caterpillar. They feed on many plants but favor chokecherry and cottonwood. Their webs look messier and tend to enclose the tips of branches rather than forming the neat fork-shaped tents of spring species. The adult form is a nearly pure white moth.
Hornworms in the Garden
Hornworms are among the largest caterpillars in Colorado, with some reaching three inches or more. The two species gardeners encounter most are the tomato hornworm and the tobacco hornworm, both of which chew leaves and fruit on tomatoes, peppers, and potatoes. Despite its name, the tobacco hornworm is actually the more common of the two in Colorado vegetable gardens, though both can show up on the same plant.
These caterpillars are green with diagonal white stripes and a distinctive horn on the rear end. They’re surprisingly hard to spot despite their size because their coloring blends with tomato foliage. A single hornworm can rapidly defoliate a plant, so checking the undersides of leaves regularly through summer is worthwhile. If you find a hornworm covered in small white cocoons, leave it alone. Those cocoons belong to parasitic wasps that are doing your pest control for you.
The whitelined sphinx is Colorado’s most common hornworm overall, but it feeds on a wide variety of wild plants and rarely damages crops. Its adult form is a large, fast-flying moth often mistaken for a hummingbird at dusk.
Forest Pests and Outbreak Cycles
The Douglas-fir tussock moth is a native Colorado insect whose caterpillars attack Douglas-fir, white fir, and spruce trees, primarily along the Front Range. Outbreaks follow a fairly predictable cycle of every 7 to 10 years and can last up to three years. During peak outbreaks, tussock moth caterpillars cause severe defoliation that kills fir trees, particularly those already stressed by drought.
Other forest caterpillars include tiger moth larvae, which spin dense silk mats on ponderosa pine, lodgepole pine, pinyon, Douglas-fir, and juniper. Pine webworms tie together ponderosa pine needles along the Front Range. Ugly nest caterpillars target chokecherry, leaving behind messy nests of silk mixed with leaf fragments and droppings. None of these typically reach the destructive scale of tussock moth outbreaks, but they can make landscape trees look rough.
Caterpillars That Sting
Most Colorado caterpillars are harmless to touch, but a few species carry venomous spines or irritating hairs connected to toxin-producing glands. Contact causes pain, itching, burning, swelling, and sometimes blistering. Saddleback caterpillars, io moth caterpillars, flannel moth caterpillars, and buck moth caterpillars are among the species capable of stinging. Some caterpillars shed tiny hairs that drift on the breeze and land on exposed skin, eyes, or clothing, causing irritation without direct contact.
If you brush against a stinging caterpillar, pressing adhesive tape against the skin can help pull out embedded spines. Washing the area with soap and water, then applying ice, reduces most symptoms. Reactions that spread beyond the contact site or involve difficulty breathing warrant immediate medical attention.
Butterfly Caterpillars Worth Protecting
Not every caterpillar in your Colorado yard is a pest. Several native butterfly species depend on common garden and landscape plants during their larval stage, and removing them means losing the butterflies they become.
Black swallowtail caterpillars, green with black bands and yellow spots, feed on dill, parsley, fennel, caraway, and other herbs in the carrot family. Anise swallowtails use many of the same host plants. If you find caterpillars munching your parsley, consider planting extra rather than removing them. Oregon swallowtail caterpillars feed on French tarragon, while giant swallowtail larvae use rue and gas plant.
The Colorado hairstreak, the official state insect, has a caterpillar that feeds exclusively on Gambel’s oak leaves. Eggs are laid on oak leaves or bark in late summer, overwinter in place, and hatch in early spring when new leaves emerge. Adults fly from June through August, producing just one generation per year. Preserving Gambel’s oak in your landscape directly supports this species.
When Caterpillars Are Active
Colorado’s caterpillar season follows a rough timeline tied to elevation and temperature. Tent caterpillars are the earliest arrivals, hatching and feeding from early April through late June at lower elevations. At higher elevations, the same species may not appear until May or early June. Fall webworms pick up after midsummer, and their webs remain visible into early autumn. Garden pests like hornworms and cutworms are active through the warm months, peaking in midsummer when vegetable gardens are in full production.
Most caterpillar species in Colorado produce one generation per year. They hatch, feed for several weeks, pupate, and emerge as moths or butterflies that lay the next year’s eggs. This single-generation pattern means that caterpillar damage, while sometimes dramatic, is usually concentrated in a narrow window.
Managing Caterpillar Problems
For most tree-infesting caterpillars, natural controls do the heavy lifting. Parasitic wasps, birds, and diseases keep populations in check without any intervention most years. When tent caterpillars or webworms are concentrated on a few branches, physically removing the silk nests while caterpillars are still inside is the simplest approach. A stick or gloved hand works fine for small tents; a pole pruner handles higher branches.
When populations are heavy enough to threaten tree health, several spray options are effective. A naturally occurring soil bacterium sold as Bt (a biological insecticide) kills caterpillars that eat treated leaves without harming bees, birds, or pets. Spinosad and pyrethrins are other options with relatively low environmental impact. Timing matters more than product choice: sprays only work while caterpillars are still actively feeding on the tree, not after they’ve left to pupate.
In the vegetable garden, handpicking hornworms is the most practical control. Check plants in the early morning or evening when caterpillars are most active. For cutworms that sever seedlings at the soil line, wrapping a small cardboard collar around the base of transplants creates an effective physical barrier.