Colorado’s dry climate and diverse forests create an ideal environment for certain invasive insects to establish devastating populations. Since 2002, these non-native species have killed millions of trees across the state and caused extensive agricultural damage.
The Colorado Department of Agriculture’s CAPS program monitors these threats year-round, but homeowners and outdoor enthusiasts play a crucial role in early detection. Understanding which invasive insects pose the greatest risk helps you protect your property and report sightings that could prevent widespread infestations.
1. Emerald Ash Borer
The emerald ash borer (Agrilus planipennis) has earned its reputation as the single most destructive forest pest in United States history. This metallic green beetle, native to Asia, was first discovered in Michigan in 2002 and has since spread to multiple Colorado communities, including Loveland, Lakewood, and Wheat Ridge.
Adult beetles measure about half an inch long with distinctive emerald-green bodies and darker wing covers. They typically emerge in late spring and early summer, leaving D-shaped exit holes in ash tree bark. The real damage occurs beneath the surface, where larvae create serpentine galleries under the bark, disrupting the tree’s ability to transport water and nutrients.
Key Insight: The City of Loveland manages over 25,000 trees, and 4,000 of them are ash trees—all potentially vulnerable to this invasive species. Early detection is critical, as infested trees can die within two to four years without treatment.
Professional insecticide treatments offer the most effective control, though they must be applied by certified pest control specialists. Systemic insecticides work best when applied before infestation or during early stages. Severely infested trees often require complete removal to prevent the beetles from spreading to neighboring ash trees. You can report emerald ash borer sightings to the Colorado State Plant Health Director or through your local forestry department.
2. Mountain Pine Beetle
While technically a native species, the mountain pine beetle (Dendroctonus ponderosae) experiences epidemic outbreaks every 10 to 30 years that cause widespread forest devastation. Recent population explosions have affected Jefferson County and other Rocky Mountain regions, killing thousands of pine trees across Colorado’s forests.
These small, black beetles measure about the size of a grain of rice and target lodgepole, ponderosa, and Scotch pines. Adult beetles bore through bark to lay eggs, and their larvae feed on the tree’s phloem layer. A single tree can host thousands of beetles, and mass attacks overwhelm the tree’s natural defenses.
Early signs include pitch tubes—small, popcorn-like masses of resin on the bark where beetles attempted entry. As infestations progress, you’ll notice needles turning from green to yellow, then reddish-brown. Woodpecker activity often increases on infested trees as birds feed on beetle larvae.
Forest management strategies include removing infested trees before beetles emerge, typically during fall and winter months. Thinning dense forest stands improves remaining tree health and reduces susceptibility. The Jefferson County Invasive Species Management program provides technical assistance for property owners dealing with mountain pine beetle outbreaks and can help develop integrated management plans tailored to specific locations.
3. Gypsy Moth
The gypsy moth (Lymantria dispar), now often called the spongy moth, poses a significant threat to Colorado’s diverse tree species. This invasive insect feeds on over 300 plant species, with a particular preference for oak, aspen, willow, and cottonwood trees.
Adult males are brown and can fly, while larger females are white with black markings and remain flightless. The most destructive stage occurs during the larval period when caterpillars emerge in spring and begin defoliating trees. These hairy caterpillars grow up to two inches long and feature distinctive blue and red dots along their backs.
Pro Tip: Check firewood, outdoor equipment, and vehicles before traveling to prevent accidentally transporting gypsy moth egg masses. These tan, fuzzy masses can contain up to 1,000 eggs and often hide in wheel wells, under vehicles, or on camping gear.
Colorado has established quarantine zones where gypsy moth populations have been detected. The Colorado State Forest Service coordinates monitoring efforts using pheromone traps that can detect male moths from miles away. Heavy defoliation weakens trees and makes them vulnerable to secondary pests and diseases. Repeated defoliation over multiple years can kill even mature, healthy trees.
4. Russian Wheat Aphid
Agriculture faces serious economic threats from the Russian wheat aphid (Diuraphis noxia), a tiny pest that attacks wheat, barley, and other cereal crops throughout Colorado’s farming regions. These pale green or yellowish aphids measure only 2 millimeters long but reproduce rapidly, with females producing up to 80 offspring without mating.
Infested plants develop distinctive white or purple longitudinal streaks on leaves, and heavily damaged leaves roll lengthwise into tight tubes. Young plants may become stunted, and severe infestations can reduce crop yields by 80 percent or more. The aphids inject toxic saliva while feeding, causing damage far beyond simple nutrient removal.
Cold weather doesn’t eliminate Russian wheat aphid populations, as they can survive Colorado winters on volunteer wheat and winter grains. Natural predators include lady beetles, lacewings, and parasitic wasps, though these beneficial insects often can’t keep pace during population explosions.
Farmers typically employ integrated pest management approaches, combining resistant wheat varieties with carefully timed insecticide applications. Economic thresholds vary based on crop stage, with young plants requiring earlier intervention. The Colorado Department of Agriculture provides regular pest bulletins during growing season to help farmers make informed treatment decisions.
5. Asian Longhorned Beetle
The Asian longhorned beetle (Anoplophora glabripennis) remains one of Colorado’s most concerning potential threats, though active infestations have not yet been confirmed within the state. This large, distinctive beetle attacks hardwood trees including maples, elms, willows, and birches—species that comprise significant portions of Colorado’s urban forests.
Adults measure 1 to 1.5 inches long with a glossy black body covered in irregular white spots. Their most striking feature is their antennae, which extend 1.5 to 2.5 times their body length and feature alternating black and white bands. Exit holes are perfectly round, about the size of a dime, with smooth edges.
| Comparison Feature | Asian Longhorned Beetle | Native Longhorn Beetles |
|---|---|---|
| Body Size | 1-1.5 inches | Usually under 1 inch |
| Antennae Pattern | Distinct black/white bands | Variable, usually solid |
| Exit Hole Size | Dime-sized (10-12mm) | Usually smaller |
| White Spots | 10-20 irregular spots | Few or none |
| Host Trees | Healthy hardwoods | Typically stressed/dead trees |
The beetle’s life cycle takes one to two years to complete, and infested trees show no external symptoms during early stages. By the time you notice sawdust-like frass, oozing sap, or crown dieback, the infestation may already be severe. There is no cure for infested trees—complete removal and destruction of the wood is the only effective response.
Common Mistake: Never transport firewood from other states or regions. Asian longhorned beetles often spread through infested firewood, and moving even seemingly clean wood can introduce this devastating pest to new areas.
The USDA Animal and Plant Health Inspection Service coordinates detection and eradication efforts nationwide. If you suspect you’ve found an Asian longhorned beetle, immediately contact Colorado state officials and avoid moving any wood from the area.
6. European Paper Wasp
The European paper wasp (Polistes dominula) has become increasingly common across Colorado since its introduction to North America. While not as economically devastating as some invasive insects, this species aggressively outcompetes native wasps and can pose risks to people with allergies.
These wasps measure about three-quarters of an inch long with yellow and black markings similar to yellowjackets, but their bodies are longer and more slender. They have distinctive yellow markings on their faces and long, dangling legs visible during flight. Their nests are open-celled, umbrella-shaped structures typically built under eaves, deck railings, or in protected outdoor areas.
European paper wasps are more aggressive than native paper wasp species and build larger colonies, sometimes housing over 200 workers. They’re beneficial predators that feed on caterpillars, flies, and other garden pests, but their proximity to human activity and defensive behavior creates conflicts.
You’ll notice peak activity during late summer and early fall when colonies reach maximum size. Workers become increasingly aggressive when protecting nests, particularly during warm, dry weather. Unlike honeybees, paper wasps can sting multiple times and often attack in groups when their nest is threatened.
Safe removal requires professional pest control services, especially for large nests or those in high-traffic areas. Treating nests at night when wasps are less active reduces the risk of stings. The Colorado State University Extension offers guidance on distinguishing beneficial native wasps from invasive species to avoid unnecessary nest removal.
7. Brown Marmorated Stink Bug
The brown marmorated stink bug (Halyomorpha halys) has spread rapidly across Colorado since its introduction from Asia, becoming both an agricultural pest and a household nuisance. This shield-shaped insect measures about three-quarters of an inch long with a mottled brown coloring and distinctive white bands on its antennae and legs.
Agricultural damage occurs when stink bugs feed on fruits, vegetables, and field crops using their piercing-sucking mouthparts. They inject enzymes that break down plant tissue, creating dimpled, discolored areas on fruits and vegetables that render them unmarketable. Tree fruits, soybeans, sweet corn, and tomatoes suffer the most severe damage.
Important Note: The brown marmorated stink bug’s defensive odor glands release a pungent smell when crushed or threatened. This unpleasant odor can persist in homes and stain fabrics, so never squash these insects indoors.
Homeowners typically encounter stink bugs during fall when adults seek overwintering sites. Hundreds or even thousands may congregate on sunny sides of buildings before finding entry points through cracks, vents, or utility openings. They remain dormant through winter but emerge on warm days, often gathering around windows and light fixtures.
Prevention focuses on sealing entry points before fall migration begins. Inspect weather stripping, window screens, and foundation cracks. Vacuuming provides the most practical indoor removal method—immediately dispose of vacuum contents in sealed bags outside to prevent odor buildup. Outdoor populations require professional monitoring and treatment, particularly in commercial orchards and vegetable operations.
8. Japanese Beetle
The Japanese beetle (Popillia japonica) was discovered in Mesa County during summer 2022, prompting intensive eradication efforts by the Colorado Department of Agriculture. These distinctive metallic green and copper-colored beetles feed on over 300 plant species, including roses, grapes, linden trees, and various agricultural crops.
Adult beetles measure about half an inch long with metallic green heads and thoraxes, copper-colored wing covers, and small white tufts of hair along their sides and rear. They feed on plant leaves, creating a characteristic skeletonized appearance where only leaf veins remain. Mass feeding events can completely defoliate plants within days.
The beetles’ life cycle includes a damaging grub stage that feeds on grass roots, creating brown patches in lawns and golf courses. Adults typically emerge in late June and remain active through August. They congregate on plants in groups, making damage more severe and visible.
Colorado maintains quarantine zones around known infestations to prevent spread. These quarantines restrict the movement of nursery stock, soil, grass sod, and other materials that might harbor beetles or grubs. Homeowners in quarantined areas should check with local extension offices before moving plants or soil off their property.
Pro Tip: Japanese beetles release aggregation pheromones that attract more beetles to feeding sites. While pheromone traps are widely marketed, research shows they often attract more beetles than they capture, potentially increasing damage to your yard. Hand-picking beetles into soapy water early in the morning when they’re less active provides more effective control for small populations.
The Colorado Department of Agriculture works with local stakeholders to implement integrated pest management strategies, combining insecticide treatments, pheromone monitoring, and public education to prevent Japanese beetle establishment in additional Colorado counties.
9. Hemlock Woolly Adelgid
The hemlock woolly adelgid (Adelges tsugae) presents a serious threat to Colorado’s hemlock stands, though populations remain limited compared to eastern United States infestations. These tiny insects, related to aphids, feed on hemlock trees by inserting their piercing mouthparts into twig bases and extracting sap.
You’ll recognize hemlock woolly adelgid by the white, woolly egg sacs that appear at the base of needles, resembling small cotton balls. These protective coverings shield the insects and their eggs from predators and harsh weather. Heavy infestations turn branches gray and give trees a snowy appearance even during summer months.
Infested hemlocks show gradual decline over several years. Needles turn grayish-green and drop prematurely, branches die back from tips, and overall growth slows dramatically. Young trees may die within four years, while mature trees can linger for a decade before succumbing. The adelgids produce two generations per year, with populations building rapidly when conditions favor their survival.
Colorado’s typically dry climate provides some natural resistance to hemlock woolly adelgid, as they prefer humid conditions. However, irrigated landscapes and riparian areas offer suitable habitats. Monitoring involves inspecting hemlock branches during late fall through early spring when white egg sacs are most visible.
Treatment options include horticultural oil sprays for small trees and systemic insecticides for larger specimens. Biological control using predatory beetles shows promise in some regions, though cold tolerance limits effectiveness in Colorado’s climate. Early detection remains crucial—report suspected infestations to forestry officials immediately to enable rapid response before populations explode.
10. Sawfly Species
Various invasive sawfly species threaten Colorado’s forests and ornamental plantings, with the European pine sawfly and introduced pine sawfly causing the most widespread damage. Despite their name, sawflies are actually wasps that feed on pine, spruce, and other coniferous trees during their larval stage.
Sawfly larvae resemble caterpillars but can be distinguished by having more than five pairs of prolegs and lacking the tiny hooks found on true caterpillar prolegs. They feed in groups, often arranged in distinctive patterns along needles. Mature larvae measure one to two inches long with varied coloration depending on species—European pine sawfly larvae display gray-green bodies with darker stripes and a shiny black head.
Feeding damage appears dramatic as larvae consume entire needles, often stripping branches completely. Young trees may be defoliated in severe infestations, while mature trees typically survive one season of defoliation but suffer stress that makes them vulnerable to other pests and diseases. Multiple years of defoliation can kill even established trees.
Life cycles vary by species, but most invasive sawflies produce one or two generations annually. Adults emerge in spring, and females use their saw-like ovipositors to insert eggs into pine needles. Larvae hatch within weeks and feed voraciously through late spring and early summer before dropping to the soil to pupate.
Control strategies depend on infestation severity and tree size. Hand-picking larvae provides effective control for small trees and light infestations. For larger trees or heavy infestations, insecticide applications targeting young larvae offer the best results. Natural predators including birds, parasitic wasps, and small mammals help suppress populations, though they rarely prevent outbreak conditions.
Conclusion
Colorado’s diverse ecosystems face ongoing threats from these ten invasive insect species, but early detection and rapid response can prevent localized problems from becoming widespread disasters. Property owners should regularly inspect trees and plants for signs of infestation, particularly during spring and early summer when many invasive insects are most active.
If you discover any of these invasive insects on your property, document the location with photographs and contact the Colorado Department of Agriculture or your local CSU Extension office.
The CAPS monitoring program relies on public reports to track invasive species distribution and coordinate control efforts. You can file reports directly with Colorado Parks and Wildlife or contact the State Plant Health Director at 303-371-3355.
Prevention remains your most powerful tool against invasive insects. Inspect firewood before transportation, clean outdoor equipment when moving between regions, and purchase plants only from reputable nurseries that follow quarantine protocols. Your vigilance protects not only your property but also Colorado’s forests, agricultural lands, and natural ecosystems for future generations.
