Foliar Feeding Insects & Mites


Aphids feed on the foliage of host plants, sucking the sap. Hundreds of species exist, varying in size, color and preferred hosts. Damage can include nuisance amounts of honeydew (sticky excretion), curling of foliage and a reduction in plant vigor.

Honeylocust Plant Bug

Honeylocust plant bugs feed on the developing leaves and buds of honeylocust trees. Heavy infestations result in significantly deformed, stunted growth and occasionally twig die-back.

Spider Mite

Spider mites feed on the foliage of host plants, sucking the sap. There are several species of mites that we typically see in our region; aspens, honeylocusts and spruce are common hosts. Damage includes yellowing/bronzing of foliage, defoliation and a reduction in plant vigor.

Gall Making Insects & Mites

Cooley Spruce Gall Adelgid

Cooley spruce gall adelgids feed on new needles of spruce trees, which causes a cone-like gall to form. Death of the terminal growth results. This damage, while unsightly, typically doesn’t create serious health concerns.

Eriophyid Mite Galls

Eriophyid mites are tiny mites that feed on plants creating discoloration, pouches and/or galls. There are many species of Eriophyid mites and each prefers a different host plant. Typically, damage is of an aesthetic nature only.

Poplar Twiggall Fly

Poplar twiggall fly primarily attacks Aspens, but is found on Cottonwoods from time to time. Damage consists of round galls on the twigs of host trees. This damage is considered mostly aesthetic, however, heavily infested trees do have a higher incidence of fungal infection. This secondary infection is the primary health concern associated with poplar twiggall fly damage.

Poplar Vagabond Aphid

Poplar vagabond aphids feed on the developing leaves of aspens, which results in distorted leaf galls. The damage appears as thickened, folded masses of leaves. While heavy infestations are unsightly, the damage is aesthetic only.

Scale Insects

Black Pineleaf Scale

Black pineleaf scale is an armored scale that feeds on the needles of various pine species. Locally, Austrian and mugo pines are the most common hosts. Damage includes discoloration of foliage and defoliation.

Cottony Maple Scale

Cottony maple scale is a soft scale species that feeds on the twigs of maples, honeylocusts, lindens and other hardwood trees. Damage results in the die-back of twigs and limbs. Cottony maple scale can easily be identified by the conspicuous white egg sacks that form in late spring and the abundance of honeydew.

Kermes Scale

Kermes Scale is a soft scale that feeds on the twigs of various Oak species. The primary host in our region is Gambel oak. Heavier infestations cause twig dieback to infested trees.

Oystershell Scale

Oystershell scale is an insect that feeds on the trunks and branches of many different types of trees and shrubs. Aspens, ash and lilacs are very common hosts. Damage results in the dieback of branches and a weakened tree, making it vulnerable to additional insect/disease infestations.

Pine Needle Scale

Pine needle scale is an insect that sucks sap from the needles of pines, spruce and fir trees. The scale covering is white, elongate and can easily be seen on the needles of host plants. Resulting damage includes defoliation and the death of individual limbs or entire trees.

Pinyon Needle Scale

Pinyon needle sale is an insect that feeds on the older needles of pinyon pines, leading to the yellowing and dropping of these needles. Heavy infestations stress host trees, making them vulnerable to Ips beetle attack.

Spruce Bud Scale

Spruce bud scale is an insect that feeds on the twigs of spruce trees. The resulting damage includes defoliation and twig die-back. Spruce bud scale also produces an abundance of honeydew, giving host trees a blackened appearance. Mature female scales look similar to a spruce bud, making it easy for the infestation to go unnoticed.

Striped Pine Scale

Striped pine scale is an insect that feeds on the twigs of various pine species. Damage includes defoliation, stunted growth and the death of limbs. This pest produces heavy amounts of honeydew which causes black sooty mold to grow and gives the needles and branches a black sooty look.

Willow Scale

Willow scale is a non-native insect that feeds on the limbs and trunks of host trees. Aspens and cottonwoods are the species most commonly attacked. Damage results in the die-back of branches and a weakened tree, making it vulnerable to additional insect/disease infestations. Cytospora canker, in particular, is associated with heavy infestations of willow scale.

Tip Moths, Twig & Terminal Feeding Insects

Pinyon Tip Moth

Pinyon tip moth feeds on the tips of the branches of pinyon pines. This feeding results in girdling wounds that kill twigs.

Southwestern Pine Tip Moth

Southwestern pine tip moth affects several species of Pines in our region. Common hosts are Austrian, mugo and bristlecone pines. The larvae develop by tunneling under the soft bark of the new shoots in the spring. They excavate the inner tissues of the newly expanding growth and cause them to “shepherds crook” and turn brown.

White Pine Weevil

White pine weevil feeds under the bark of the terminal leader of spruce trees, which girdles and kills it. This results in a bushy, deformed tree. While this damage isn’t life threatening, the aesthetic damage can be significant.

Bark Beetles

Douglas Fir Beetle

Douglas-fir beetle is related to the mountain pine beetle but only attacks Douglas-fir trees. Damage from feeding on the inner bark results in tree death. Clear, runny sap and boring dust are tell-tale signs of attack by Douglas-fir beetles.

Engraver Beetle (Ips)

Ips beetles tunnel the trunks and limbs of host trees causing girdling wounds. These wounds damage the vascular system which prevents the uptake of water. Bluestain fungi, which further disrupt the vascular system, may also be introduced by Ips beetles. Pines and spruce trees are the most common hosts, especially trees weakened by drought, construction damage or other stresses.

Fir Engraver

Fir Engraver is an insect that attacks true fir trees, as opposed to Douglas-firs. Stressed trees are most often attacked. Damage includes larval galleries that deeply score the wood and possibly the introduction of a brown-staining fungus that disrupts the vascular system. Attack by fir engravers often results in the death of the host tree.

Mountain Pine Beetle

Mountain pine beetle tunnels the trunks of certain species of pines (lodgepole, ponderosa, Scots and limber are the most common hosts) causing girdling wounds. These wounds damage the vascular system of infested trees which prevents the uptake of water. Bluestain fungi, which disrupt the vascular system, may also be introduced. Attack by mountain pine beetles results in the death of the host tree.

Spruce Beetle

Spruce beetle tunnels the trunks of spruce trees, causing girdling wounds. These wounds damage the vascular system of infested trees which prevents the uptake of water. Attack by spruce beetles results in the death of the host tree.

Western Balsam Bark Beetle

Western Balsam Bark Beetles damage true Firs in our area. This includes Sub-Alpine Fir, Corkbark Fir, and White Fir. It tends to prefer Fir stands near the southern reaches of the host range, which tend to have longer summers and higher average temperatures.

Western Cedar Bark Beetle

Western Cedar Bark Beetles are common on native stands of juniper species. Some level of activity is normal but when Juniper/Cedar trees suffer from root disturbance or damage the level of activity is usually higher. It bores into the trunk as similar to other bark beetle species and lays eggs in galleries under the bark.

Insects that Bore into Trunks & Large Branches

Bronze Birch Borer

Bronze Birch Borers cause damage to birch species in our area by tunneling in to the trunks and meandering through the bark as they feed. The caused raised areas on the outside of the bark as they tunnel through the bark resulting in a zig-zag or curve pattern of raised bark visible on the outside of the bark. They often transmit secondary decay fungi and or fungal cankers to the soft wooded trees.

Honeylocust Borer

Honeylocust borer larvae tunnel and feed beneath the smooth bark of honeylocust trunks and large branches. Trees damaged by poor pruning cuts, wounds, and environmental stressors are especially susceptible. Common diagnostics include heavy oozing at larval entrance holes, and small dry D-shaped exit holes where metallic greenish-black adult beetles emerge. Repeated attacks can result in dieback in the crown.

Lilac/Ash Borer

Lilac/Ash Borer is another moth species whose larvae feed under the bark of both White and Green Ash trees. They prefer stressed trees in less than optimal planting sites and damage is often seen near the crotches of the lower branches or along the lower trunk. They are also commonly attracted to injuries to the trunk, especially if drought stressed.

Peach Tree Borer

Peach Tree Borers affect cherry, peach, and plum, and other stone fruits. Another moth species, the larvae of this insect bores in to the lower trunks, usually at or below the soil level but often seen higher up on the trunk. Boring damage will produce amber to clear gumming mixed with particles of the chewed bark. Secondary decay fungi are commonly associated with the wounds and gummosis.

Pinyon Pitch Mass Borer

Pinyon Pitch Mass Borer is a common pest of Pinyon Pines along the front range. It is a moth species that lays its eggs in cracks in the bark and the larvae do all of the damage as they feed on the cambium layer under the bark. A pink cast to the pitch can be helpful in diagnosing.

Poplar Borer

Poplar borers tunnel the sapwood of trees, causing girdling wounds. Infestations often result in an abundance of sawdust and a varnish-like stain oozing from the point of attack. Infested trees are vulnerable to disease that enters through the boring wound. Untreated infestations usually lead to the decline and death of the tree.

Zimmerman Pine Moth

Zimmerman Pine Moth is a less common pest of Austrian, Scots, and sometimes Ponderosa pines in our area. Popcorn like pitch masses can be seen at the point where the upper branches meet the trunk. Upper branches can begin to die back and break off at the trunk from the boring wounds under the bark. Look for dead branches at or near the top of the tree.

Insects that Feed on Flowers, Fruits & Seeds

Boxelder Bug

Boxelder Bugs are the red and black, fairly conspicuous insects that group in large numbers on Boxelder Maple trees. These are common shade trees in our area. They often move from the host trees to porches, houses, and other structures and become a nuisance.

Codling Moth

Codling Moth is commonly known as the “worm in the apple,” though they also attack pears, crabapples, and less frequently apricots, peaches, and English walnuts. They are the most common pest affecting fruit trees in the West. Larvae are pale with a dark head, while adults are small grey moths with copper wing tips. There are many management options, both biological and chemical.

Western Cherry Fruit Fly

Western Cherry Fruit Fly is a pest of fruiting Cherrie’s in southwest and western Colorado. It lays eggs in the cherry fruit after mating and damages the surface of the fruit. The larvae feed on the inside of the cherry fruit causing heavy damage and unusable fruit. It can very rarely attack Apples but this has only been seen in Utah.

Fungal Diseases


Anthracnose is a foliar fungus that affects mainly Sycamore trees in our area. We do not see a lot of issues with anthracnose in higher elevations.

Apple Scab

Apple Scab is a fungal disease that affects Apple and Crabapple trees in our area. It causes olive green to brown spots on the upper surface of the leaves near the leaf veins as it grows. In wetter areas it can infect the fruits as well, however our humidity levels in this regioin are not usually high enough for fruit damage. ‘Radiant’ Crabapples have the highest level of Apple Scag incidence in our area.

Cytospora Canker

Cytospora canker affects many different species of trees. Symptoms vary depending on the host tree, but usually include discoloration of bark and pimple-like fruiting structures. There is no chemical treatment for cytospora; trimming out diseased limbs is the recommended remedy. Left untreated, cytospora often leads to the decline and death of the host tree.

Marssonina Blight

Marssonina blight is common to aspens, especially when we experience wet spring weather. Damage includes brown, blotchy areas on the leaves that are often surrounded by a yellow halo and premature leaf drop. Damage is mostly aesthetic, but infestations can stress host trees, especially when they are attacked several years in a row.


Needlecast diseases affect all types of conifers. Symptoms are usually expressed as discolored needles and premature death and shedding of needles. Needlecast fungi symptoms are easily confused with abiotic disorders such as frost damage, nutrient deficiencies, salt damage, herbicide damage, or other symptoms caused by environmental factors. Needlecast is commonly confused with normal fall needle drop as well.

Powdery Mildew

Powdery Mildew is a foliar fungus common in cool shady areas. It can affect many different trees, shrubs, and perennial plant materials. It usually will not kill the host but heavy infections greatly reduce the plants ability to photosynthesize. It causes a gray, ghosty, powdery cast to the affected leaves of the host plant. Powdery Mildew overwinters in leaf debris from the previous season.

Septoria Leaf Spot

Septoria leaf spot is one of the more common foliar fungi on Aspen and Cottonwood trees in our area. It causes light brown to tan spots on Aspen and Cottonwood leaves which is diagnostic of this particular fungus in comparison with Marssonina Blight which causes black to dark brown spots on the leaves. Damage is mostly aesthetic, but continued infestations can stress the host tree and cause other issues to move in. Usually results in premature leaf drop as with other foliar blights.

Shoot Blight

Shoot blight is a fungus that typically affects Aspen trees in our area. It enters the leaves at the soft new tips of Aspen branches and moves through the new growth rapidly. The leaves turn black, beginning with splotches and eventually the whole surface. It causes a “Sheperds Crook” to the tips of the branches. Warm temperatures and longterm rain cycles cause this fungus to be more of a problem and it usually affects smaller, younger Aspen closer to the ground as it prefers higher humidity.

Bacterial Diseases

Fire Blight

Fire Blight is a bacterial disease that mainly affects Apple, Crabapple, Pear, and Quince. It can also infect Mountain Ash and Serviceberry in our area. The infection causes the branches to curl and turn black from the tips back. This creates the indicative “Shepherds Crook” look to the tips of the infected branches. It is difficult to control and aggressive but removal of infected plant parts by sanitarily pruning is the best option for control.