Once the temperatures warm up across the Colorado Front Range, insect populations explode. And while most aren’t harmful or even a nuisance to residents, there are a handful of bugs that can do serious damage to trees throughout the state.
If you want to keep your trees safe, beautiful, and strong this summer, then it pays to know which Colorado tree bugs to look out for so you can prevent them from doing any lasting damage to your precious trees.
Here are some of the most common and destructive tree bugs and insect pests to keep your eyes peeled for in the Denver foothills this summer, and how you can tell if they are wreaking havoc on your trees.
NOTE: For information about other pests and insects that attack trees on the Front Range, see our Colorado Tree Pests page.
EAB adult in a tunnel, photo by Eric R. Day, Virginia Polytechnic Institute and State University, Bugwood.org
1. Emerald Ash Borer
Emerald ash borer (EAB) has become the most famous tree pest in the US in the last two decades. This invasive wood boring beetle from Asia was first discovered in the Midwest in 2002. Since then, it has spread to 36 states, primarily in the Midwest and the Eastern US. The only two western states with confirmed populations of EAB are Oregon and Colorado.
This incredibly harmful pest is responsible for the death of tens of millions of ash trees in the US every single year. Luckily for us, only a few areas in Colorado have fallen victim to EAB. It was first confirmed in the city of Boulder in 2013 and has been slowly spreading southward. In 2020, it was found in Arvada, and the most recent confirmed infestation was in Littleton in 2023, putting this destructive pest practically on our doorstep.
The dark green adult beetles, half an inch long, are only active on the trees from May to July, laying eggs on ash trees. When the larvae hatch, they crawl underneath the bark and start feeding. They feed on the sensitive inner back, damaging the tree’s ability to move water and nutrients throughout the tree, ultimately killing the tree in just a few years. This is bad news for a state where 15% of all trees are ash trees.
Signs of Emerald Ash Borer Infestation
- Crown dieback, starting near the top of the tree
- Yellow foliage
- Winding, S-shaped markings in the sapwood under the outer bark
- Cracking bark
- Excessive woodpecker damage
- D-shaped exit holes
If you suspect EAB is attacking your ash trees, act immediately if you hope to save your tree. Properly administered insect control can fight off infestations and allow the tree to recover if applied before too much damage is done.
Jeffco Invasive Species Management and Jeffco CSU Extension are partnering to promote EAB awareness within our county. Stay up-to-date here.
2. Bronze Birch Borer
Also a wood-boring bug, the larvae of bronze birch borer aggressively destroy birch trees. They act similarly to EAB, with adults laying eggs on tree bark and larvae hatching and making their way underneath the bark to feed on the interior wood.
Signs of Bronze Birch Borer Infestation
- Crown dieback, starting near the top of the tree
- Ridges or bumps on branches
- D-shaped exit holes
Although they look similar to aspen trees, birches are a distinct species. Here in Jefferson County, CO, you may find the Western river birch (Betula occidentalis) and dwarf birch (Betula glandulosa), both of which are native to Colorado. Birches prefer cooler, shadier planting sites. If you have birch trees on your property, keep them watered and mulched, which helps to regulate temperatures at the base of the tree, reducing stress and the likelihood of infestation. The river birch is somewhat more borer-resistant than the dwarf birch.
An adult poplar borer, photo by Jean Pinon, Institut National de la Recherche Agronomique, Bugwood.org
3. Poplar Borer
These wood-boring insects attack poplars, cottonwoods, and aspen trees, Colorado’s most famous fall color tree and the most widespread tree in North America.
Poplar borers are most active throughout the summer months. Though they attack and weaken trees, they don’t normally kill host trees. Weaker trees are more susceptible to pathogens, however, so further decay is likely.
Signs of Poplar Borer Infestation
- Holes with blackish stains on the bark
- Oozing sap
- Frass or wood shavings at the base of the tree
Weak and stressed trees are most susceptible to attack from poplar borers. Proper pruning, watering, mulching, and fertilizing is your best defense against this harmful pest. Poplar borers are also attracted to trees in full sun.
Adult at the entrance of gallery of the elm tree in Colorado, photo by Whitney Cranshaw, Colorado State University, Bugwood.org
4. Elm Bark Beetles
Elm bark beetles, which are native to North America, primarily attack the American elm but can also be seen on English elms. The small, black adult beetles emerge in May and June. They will fly from dying elm trees where they overwintered in search of new host trees, where they feed on small branches in the canopy. The female beetles tunnel beneath the bark to lay their eggs.
Elm bark beetles are notorious for carrying the fungus that causes Dutch elm disease (DED) and are the primary means of this disease spreading and killing elm trees. Once infected with DED, elm trees can die in as little as one year. There is no cure for DED so infected trees should be removed to prevent spreading the disease.
We’ve recently seen a return of Dutch elm disease in Denver after a decade without any notable cases.
Signs of Elm Bark Beetle Infestation
- Boring dust at the base of the tree
- Wilting, yellow, or browning foliage in random parts of the canopy
- Dark streaking in newly formed branches
The beetles prefer older elm trees and ones that are stressed by drought or other issues. Watering mature elm trees regularly can help keep them healthy and minimize the likelihood of attack and infection. However, when the Front Range experiences a cool, wet spring and early summer, the fungus is more likely to spread as elm bark beetles fly in search of new food and egg laying sites.
An adult Lilac ash borer, photo by Eugene E. Nelson, Bugwood.org
5. Lilac Borer / Ash Borer
Not to be confused with the emerald ash borer, which is a wood boring beetle, the lilac borer or ash borer is a clearwing moth that closely resembles the paper wasp.
Adults are active in late spring. This is another pest that seriously harms Colorado’s ash tree population. Damage is done in a similar manner to EAB, with larvae feeding on the sapwood underneath the bark.
Signs of Lilac/Ash Borer Infestation
- Tunneling in the sapwood underneath the bark
- Limb dieback
- Swelling on the stems
Lilac/ash borers typically target highly stressed trees suffering from drought or heat problems. Proper watering is the most important factor when trying to prevent attacks from this pest. Fresh pruning wounds are also attractive to adult moths. Avoid pruning ash trees when adults are active in the summer.
Ips engraver beetle, photo by Edward H. Holsten, USDA Forest Service, Bugwood.org
6. Ips Beetles
Also known as engraver beetles, ips beetles attack pine and spruce trees, including lodgepole and ponderosa pines. There are 11 species of ips beetles native to Colorado and all are capable of killing spruce and pine trees.
These pests’ tunnel under the bark, leading to dieback in the canopy and possibly death to trees. Ips beetles are generally attracted to stressed trees caused by drought, construction damage to roots, or overwatering. They rarely attack healthy trees. However, they’re especially problematic during periods of prolonged drought, and are also attracted to freshly cut or broken wood, such as from pruning or logging, and wind or snow damage. In the Denver foothills, they can kill whole forests in a single season after a particularly bad winter for snow damage and during especially dry summers.
Signs of Ips Beetle Infestation
- Winding galleries underneath the bark
- Yellowish or reddish-brown boring dust in crevices or at the base of the tree
- Excessive woodpecker activity or damage, as they seek to feed on ips beetles
Keeping pine and spruce trees healthy will limit or prevent attacks from ips beetles. The best way to do this is with effective watering and deep root fertilization.
For more details about recognizing signs of ips beetle infestation, treatment options for infested trees, and preventive measures, see our article about ips beetles in the Front Range.
Honeylocust and black locust trees are susceptible to infestation by the honeylocust plant bug, photo by John A. Weidhass, Virginia Polytechnic Institute and State University, Bugwood.org
7. Honeylocust Plant Bug
These damaging bugs are often in full force at the end of May. They target the honeylocust tree. These translucent green bugs can jump far distances, often jumping onto people.
Honeylocust plant bugs generally target new foliage. Populations typically diminish in June, eventually allowing trees to push out new growth undamaged.
Signs of Honeylocust Plant Bug Infestation
- Yellow or brown spotting foliage
- Twisted leaves
- Twig dieback
Chemical applications often aren’t necessary unless spring attacks are especially heavy.
8. Mealybugs
These white, soft-bodied insects can cause a lot of stress to a variety of different trees. These ones are easy to identify, as the females are covered with a white, waxy, cottony substance. The white “fluff” helps protect them from excessive heat and moisture loss.
They are often found on plant stems and leaves, where they pierce the surface to suck out moisture and sugar, causing damage and premature leaf drop due to moisture loss.
Where you find mealy bugs, you may also find a lot of ants. Mealy bugs excrete honeydew, a sweet substance that is attractive to ants.
Signs of Mealybug Infestation
- Visible infestations of white, fuzzy-looking mealy bugs
- Wilted, curled, or discolored leaves
- Twig and branch dieback
- Premature leaf drop
- Sticky substance on leaves and beneath trees, possibly with black sooty mold
Having beneficial insects, such as ladybugs or green lacewings, in your yard can help to naturally manage this tree pest. Insecticides applied as a soil drench can help to control some types of mealy bugs as well, although these can also kill the beneficial insects that keep mealybug populations under control. Dormant season oil sprays in the winter may also provide some degree of control.
PRO TIP: Don’t mistake the mealybug destroyer for a mealybug! The mealybug destroyer (Cryptolaemus montrouzieri) is a small, predatory beetle that’s a voracious eater of mealybugs in both its larval and adult stages. Adult mealybug destroyers are brown or blackish with a tan or orange head and tail and look similar to ladybugs but without spots on their wings. The larvae however, look a lot like their prey, although they’re more mobile and are about twice as large. A single larva may consume up to 250 small mealybugs!
Larvae of the pinyon pitch mass borer, photo by Brytten Steed, USDA Forest Service, Bugwood.org
9. Piñon Pitch Mass Borer
These boring beetles aggressively target piñon trees and are also known to attack ponderosa and other pines throughout Colorado. Piñon pitch mass borer larvae tunnel into the vascular tissue of the tree, severely weakening large branches to the point of breaking off.
Infested trees can become a hazard as their structural integrity may be compromised.
Signs of Piñon Pitch Mass Borer Infestation
- Disfigured branches
- Gouges in the tree bark that ooze light, pinkish sap
- Dead branches falling
These pests generally target landscape trees in heavily watered areas. This leads to succulent growth, which is attractive to the pest. If you have piñon pines on your property, be careful not to overwater them.
The borers also target trees with splitting branches which allow the larvae to enter the tree. Regular pruning of weak branches can help prevent infestations.
Close-up look of woolly pine scale insects, photo by Lacy L. Hyche, Auburn University, Bugwood.org
10. Scale Insects
Scales are tiny tree pests that attack conifers throughout Colorado. Various species of scales target a wide variety of conifers. In our part of the Front Range, the most destructive and common scale insects are the oystershell scale, pine needle scale, and striped pine scale. You can learn more about them in our article on scale insects.
Scale insects feed both on bark and needles, removing sap and damaging plant cells. Wounds caused by scales provide an opportunity for pathogens to enter the tree. This leads to deformities and needle loss.
Many scales excrete honeydew, which encourages the growth of black sooty mold (affecting tree appearance), while also attracting nuisance bees and wasps.
Signs of Scale Infestation
- Dropping needles
- Leaf/needle discoloration
- Decreased tree vigor and stunted growth
- Noticeable dieback
- Excess bee and wasp activity
Long-term and/or heavy infestations of scale can be extremely damaging to the trees on which they feed, leading to vulnerability to other pests and diseases, and even tree death. If you notice scales on your conifers, take action to eradicate them quickly to prevent worse damage.
11. Spider Mites
Spider mites are tiny, sap-sucking arachnids (they’re not insects!) and are known to gather on the underside of leaves. Females can lay up to 100 eggs at a time and do so as many as 30 times per year in
warmer weather. Most spider mite activity occurs during the warmer months, though some species are also active in the cooler months of spring and fall.
These are extremely common pests found on many different trees and shrubs throughout Colorado. Evergreen trees are commonly infested by the spruce spider mite (Oligonychus ununguis) on spruce and juniper, Oligonychus subnudus on pines, and Platytetranychus libocedri on arborvitae and juniper. Deciduous trees are attacked by the honeylocust spider mite (Platytetranychus multidigituli) on honeylocusts, and a range of other mites on shade trees, such as elm, mountain ash, and oak. We also see a lot of spider mites on aspen trees, especially those in drier locations.
They primarily feed on leaf tissue, causing bruises. They can cause serious stress to trees and can kill them if left untreated.
Signs of Spider Mite Infestation
- Flecking, discoloration, and scorching of leaves
- Premature leaf loss
Spider mites thrive in dry conditions and on trees stressed from drought. Adequate watering, and occasionally hosing off foliage can prevent or limit spider mite damage.
One of the biggest reasons spider mites multiply is that many pesticides kill their natural predators, allowing mites to reproduce without predation. Be careful not to over-apply pesticides to your yard.
Insects Bugging Your Trees? Call LAM Tree!
This list just scratches the surface of bugs that attack trees in Colorado during the summer. But these are some of the most common and most destructive insects to keep your eyes peeled for. If you notice signs of insect damage, act immediately to ensure the best chances of survival for your trees.
For those near Evergreen, CO, you can call your local arborists at LAM Tree Care Experts at 303-674-8733 to inspect your trees and help you fight off bothersome pests today. Or use this online form to request an inspection and estimate now.
