Insect Control and Prevention for Trees

Guide to Controlling and Preventing Insects on Deciduous, Coniferous and Fruit Trees.

Aphid Control on Trees

Boulder and Fort Collins Tree Care Services

Every year many trees in Colorado get attacked by insects. These damaging tree insects include aphids, curl aphid, mealy bug, leafminers, bronze birch borer, elm leaf beetle, elm scale, honeylocust plant bug, juniper scale, kermes scale, cottony maple scale, spider mites, oystershell scale, ash sawfly, emerald ash borer and many other insects. Most of these tree pests won't kill a tree, but they can cause severe stress to the tree. Additional stress reduces the trees ability to withstand biotic or environmental conditions like heat and drought stress.

A tree that suffers from insect pressure will grow much slower than a healthy tree. Insect pressure on trees can reduce the growth rate by as much as 70%. It is rare, but trees that are left untreated over several years can even be killed by severe insect pressure.

If you are concerned about insect pressure on your trees please call Organo-Lawn to speak with one of our local tree care experts.

Boulder (303) 499-2000 or Fort Collins (970) 225-9425.


Common Tree Pests of Colorado

Common Damaging Tree Insects of the Front Range of Colorado

Aphids on Trees

These tiny, soft-bodied, pear-shaped insects can be almost any color from green to blue to black. Aphids suck the moisture and sugars from tree foliage and shoots. They feed on trees by sucking fluid through a slender mouth part called a proboscis. They usually feed in large clusters, on the underside of leaves. Aphids prefer tender young shoots and buds. As the aphids feed, they secrete a sticky fluid called honeydew which is a shiny and sticky substance. When enough aphids are feeding on the tree, this honeydew will begin dripping from the leaves. If you have ever parked under a tree in the summer and then you had a sticky residue on your windshield this is honeydew.

Aphid Treatment

Aphids can be found on almost all types of trees. Safari is a wonderful reactive treatment and Criterion is a wonderful preventative treatment for aphids on trees.

Black Locust Borer

This adult beetle is a mostly black beetles with yellow cross bands on the thorax and "W" shaped bands on the wing covers. The black locust adults are about 18-22 mm in length and are most commonly noticed when they are visiting flowers in late summer and early fall. This is an introduced species to Colorado and widely established. This insect is abundant and very damaging to black locust trees. In recent years it has been particularly damaging to the popular cultivar “Purple Robe” locust.

Black Locust Treatment

This insect is difficult to control after an attack. They prefer attacking trees that are drought stressed and following the 1-2-3-2-1 lawn watering technique can help prevent black locust borer pressure. Preventative treatment is possible with Criterion or Safari but reactive treatment is difficult and not usually effective. Due to the destructive nature of this insect, it is highly recommended to have annual preventative applications applied to prevent black locust trees from dying.

Bronze Birch Borer

The bronze birch borer is a serious pest known to attack white, paper, and cut-leaf weeping birches. Bronze birch borer adults are slender, dark, iridescent, sometimes greenish-bronze beetles that are 7-12 mm long. In late April or early May the larvae molt into the pupal resting stage. During early June adults chew their way out of the bark and emerge as adults This is when they leave the characteristic “D”-shaped hole from the trunk of the birch. The first indication that a tree is infested with borers is wilting and dying of the upper crown. Closer examination may reveal ridges and bumps on limbs and branches as well as “D”-shaped adult emergence holes in the bark.

Bronze Birch Borer Treatment

Birch trees prefer cool, moist and shaded environments. They do not grow well in sunny or exposed areas. Bronze birch borers prefers to lay eggs on trees growing in full sunlight. Preventative treatment is possible with Criterion or Safari but reactive treatment is difficult and not usually effective.

Brownheaded Ash Sawfly

Some years the brownheaded ash sawfly is a major problem along the Front Range and others it isn't even noticeable. Larvae damage ash trees by chewing the leaves typically in late May and early June. The larvae are pale green ‘worms’ with some light banding. During severe outbreaks, they can defoliate the entire tree in a matter of a few days. This can cause significant stress to the tree, particularly when it occurs repeatedly. Weather is the most important natural control and late spring frosts and strong winds can kill large numbers. The larvae are delicate and strong winds can easily dislodge them from ash trees. If they are dislodged, the larvae rarely re-establish themselves back on the tree.

Brownheaded Ash Sawfly Treatment

Ash sawflies are easily controlled by insecticidal soaps and many can also be dislodged by a strong jet of water from a hose. Criterion and Safari are both very effective at controlling the ash sawfly.

Cooley Spruce Gall

Cooley spruce galls are found on the new growth of spruce trees. The galls are caused by an insect called Cooley spruce gall adelgids which is a wooly aphid. The galls are light green during late spring, but dry out and become brown starting in mid-July. People often mistake the galls for seed cones. Cooley spruce galls are common and conspicuous on blue spruce trees; however, they do little or no harm to the tree. In the spring, females mature and lay several hundred eggs near developing buds. Eggs hatch at about the time of bud break and young nymphs migrate to the new spring growth. There they feed at the base of growing needles. Saliva introduced into the plant causes changes in plant development and produces galls..

Cooley Spruce Gall Treatment

Control of Cooley spruce gall is not necessary because it does not cause harm to the tree; however, insects may be controlled to prevent aesthetic injuries that can detract from trees appearance. Control must occur in the fall using either Criterion or Safari, because spring applications occur after the damage has already been caused.

Elm Leaf Beetle

Elm leaf beetles are common insects that chew holes and damage leaves of elm trees. The dark grub-like larvae chew on the underside of leaves, but they avoid the larger leaf veins, producing a type of injury pattern known as skeletonizing. Leaves damaged by elm leaf beetle larvae look lacy, turn brown and may prematurely drop from the trees. English and Siberian elms are favored by the elm leaf beetle. Elm leaf beetles also can be important as a nuisance pest in homes, because they often enter buildings in autumn when seeking winter shelter. Fortunately elm leaf beetles are strictly a nuisance invader type of insect that does not feed on nor damage anything inside a home.

Elm Leaf Beetle Treatment

The most effective form of control of the elm leaf beetle are Criterion or Safari. Both of these systemic insecticides can be applied to the soil and they will move to the leaves where the elm leaf beetles feed.

Emerald Ash Borer

The Emerald Ash Borer (EAB) is probably the most well know insect in Colorado. EAB was first identified in Michigan in 2002; since then it has spread to 22 states, including Colorado. It was introduced from East Asia likely through infested shipping or packing material. The most likely route of introduction to Colorado was probably in firewood or nursery stock. EAB kills ash trees, so it is extremely to protect high value trees. The larvae feed under the bark, eventually girdling the tree and cutting off nutrients. Ash trees are killed within 2-4 years of first symptoms, even if the tree was previously healthy. Trees of all size can be attacked, from 1/2 inch saplings to mature trees. This insect is very difficult to detect, because it is under the bark and the adults are only around from May to September. Due to the destructive nature of the EAB, we have made its own informational EAB page.

Emerald Ash Borer Treatment

For trees that are in Boulder, Lafayette, Longmont, Louisville, Lyons and Superior we recommend treatments using TREE-age. For trees that are in neighboring cities we recommend preventative treatments using either Criterion or Safari.

Honeylocust Plant Bug

Honeylocust plant bugs commonly attack honeylocust in late spring around the end of May. The honeylocust plant bug is a very damaging species of plant bug and is generally green in color. When immature, it superficially resembles an aphid but unlike aphids it can jump long distances, often jumping onto people. Injured honeylocust foliage turns yellow or brown spotting. Leaves become twisted, and twig dieback can occur following heavy infestations. Heavy midge infestations often collapse when new growth ceases and the trees are no longer attractive to the egg-laying adult midges.

Honeylocust Plant Bug Treatment

Honeylocust plant bugs are easily controlled with Safari as a reactive treatment or Criterion as a preventative treatment. Honeylocust are susceptible to many types of insect pressures and Safari and Criterion will prevent the majority of attacks. Safari and Criterion will not prevent spider mites from attacking a honeylocust tree.

IPS Bark Beetle

Ips beetles, also called “engraver beetles,” are bark beetles that damage pine and spruce trees. Ips beetles are small (1/8 to 3/8 inch long), reddish-brown to black beetles. They develop under the bark and produce girdling tunnels that can cause dieback and sometimes they can even kill trees. There are 11 different species of IPS beetles in Colorado. Prolonged drought stress increases the susceptibility of an IPS beetle attack and watering via the 1-2-3-2-1 lawn watering technique will greatly reduce the likelihood of an attack. Woodpeckers are common predators of ips beetles. Their presence may also indicate bark beetle activity. Woodpeckers often remove the tree bark in an effort to obtain this food source.

Ips Bark Beetle Treatment

To prevent ips beetle attacks, use practices that promote vigorous tree growth like deep root tree fertilization and watering according to the 1-2-3-2-1 lawn watering technique. Ips beetles can be treated proactively with a soil drench of Safari or reactively with a trunk injection of TREE-age.

Leafminers

Leafminers are insects that feed on the insides of leaves or needles. Their feeding produces tunneling injuries that sometimes make the leaf tissue semi-transparent. Several different types of insects have developed this habit, including larvae of moths, beetles, sawflies and flies. Most of these insects feed for their entire larval period from inside the leaf. Elm leafminers are the most problematic species in Colorado and are found in multiple Front Range cities. Elm leafminers attack American, English and Siberian elms.

Leafminer Treatment

Leafminers don't usually pose a significant threat to the health of trees or shrubs. The injuries caused by leafminers are usually cosmetic and treatment decisions are based on plant appearance. Leafminer insects may be controlled with Safari or Criterion to prevent aesthetic injuries that can detract from trees overall appearance.

Lilac / Ash Borer

The lilac borer or ash borer is a serious pest of ash trees planted throughout Colorado. Larvae of the borers feed under the bark and into the wood weakening and sometimes killing the tree. The moths emerge during the spring, mate and lay eggs, before the larvae bore into the trunks of ash trees. They construct galleries in wood beneath the bark causing severe damage the tree. The ash borer is more severe when plants are growing under stressful conditions like heat and drought stress. Proper care, deep root fertilization, and proper lawn watering can greatly limit attacks. Freshly pruned wounds are highly attractive to egg-laying moths. It is important to avoid pruning prior to periods when the moths fly.

Lilac / Ash Borer Treatment

To kill and prevent the lilac borer from returning, we recommend watering according to the 1-2-3-2-1 lawn watering technique and having one of our tree care experts perform a tree trunk injection called TREE-age. Proper lawn watering is the best way to control the lilac borer, because it is extremely unlikely for an attack to occur to a healthy tree.

Pinion Pitch Mass Borer

The piñon pitch mass borer is a serious pest of piñon trees in landscapes throughout much of Colorado. Though piñon trees are the main host, ponderosa pines and occasionally other pines may also be attacked. Like most borers, the piñon pitch mass borer takes advantage of trees that are stressed or weakened. Over watering and over crowding is a huge stress to piñon pines. Adjusting irrigation schedules so they water according to the 1-2-3-2-1 lawn watering technique is extremely important to preventing borers from attacking trees. Damage from borer attacks can disfigure large branches and will severely weakens the tree. Heavily infested branches may even completely break off the tree.

Pinion Pitch Mass Borer Treatment

Pinion pines do not need as much water as other trees. They are extremely heat and drought resistant and they prefer soils that dry out between watering. It is important to follow the 1-2-3-2-1 lawn watering technique. If an attack occurs we recommend a trunk injection control called TREE-age.

Scales on Conifers

Scales are some of the most problematic pests affecting conifer trees in Colorado. Scales feed on bark and needles, they remove sap and may damage the plant's cells. This can lead to needle drop, decreased vigor and dieback. Trees that are stressed due to insect pressure are more susceptible to other insects or disease. Some scales also excrete sticky honeydew, which further detracts from plant appearance and attracts nuisance bees and wasps. There are many types of scales that affect conifers including: Black pineleaf scale, juniper scale, spruce-bud scale, pine tortoise scale, pine needle scale, pinion pine scale and striped pine scale,

Scales on Conifers Treatment

The most effective control for scales on conifers is annual soil drenches with Safari. Note: If a tree is suffering from scale pressure it will take two seasons of Safari applications to fully control the scale population.

Scales on Ornamental Trees and Shrubs

Scales are notoriously hard to control. These sap-sucking insects prey on most kinds of trees and plants. Some species can cause a great deal of damage to the foliage on which they feed, while others are harmless. Scales resemble small bumps on twigs and branches, scales don’t look like most insects; therefore, they can easily go undetected. In Colorado there are a number of types of scales on deciduous trees that are problematic. Oysternshell scales are common on shrubs and trees, including aspen, ash, cotoneaster, poplars, willow and lilac. Kermes scale is a serious insect pest of pin and red oak. European elm scale is a serious pest of large leaf elms

Scales on Ornamental Trees and Shrubs

The most effective control for scales on conifers is annual soil drenches with Safari. Note: If a tree is suffering from scale pressure it will take two seasons of Safari applications to fully control the scale population. Europen elm scale has become resistant to Safari and the best treatment is with a direct trunk injection using TREE-age.

Spider Mites on Trees

It’s surprising that spider mites that are incredibly tiny creatures can have such a negative impact on trees. Even the largest tree can sustain serious damage. Spider mites are among the most commonly found pests on trees and shrubs. Spider mites are members of the tetranychidae family, which are more closely related to spiders and ticks than insects. There are literally hundreds of different species are categorized as spider mites. These tiny pests can damage plants when the suck on the plan's tissues, mainly in the leaves. Mites often cluster on the undersides of leaves. Each adult female can lay around 100 eggs and, in warm weather, they can have up to 30 generations in a year.

Spider Mite Treatment on Trees

Spider mites are not related to insects so products like Safari and Criterion do not kill these pests. For spider mite control on deciduous trees we use a product called Orthene. For fruit and coniferous trees our organic insect control called Ecotrol is highly effective. Eriophyid mites and NOT controllable with either product.

Thousand Cankers Disease - Black Walnut

Thousand caners disease is a newly recognized disease that was discovered in Boulder Colorado in 2008 and occurs in black walnut species. It is caused by a fungus that is vectored by the walnut twig beetle. Thousand cankers disease has produced widespread death of walnuts in many western states during the past decade. Thousand cankers disease kills trees from the cumulative effects of numerous coalescing cankers (and tunneling) that develop around individual entry wounds made by walnut twig beetles. The end result is that the tree loses the ability to push fluids up and down the trunk.

Thousand Cankers Disease Treatment

The use of soil applied systemic insecticides like Criterion (imidacloprid) and Safari (dinotefuran) is the most effective control for preventing a bark beetle attack. Anecdotally these treatments are not 100% effective and if the beetles do attack in large enough numbers they can kill the tree even if it has been treated. Treatments made after symptoms begin to appear are ineffective.

Tussock Moth Caterpillar

Douglas fir tussock moth caterpillar is a native defoliator of spruce, Douglas fir and true fir trees. The tussock moth rarely feeds on Colorado blue spruce trees that are planted in urban areas. When outbreaks occur they typically develop rapidly and then subside abruptly. The reason for this is due to a virus that spreads throughout each isolated population. In Colorado, isolated outbreaks tend to be cyclical in nature and typically occur in intervals of every seven to 10 years. Needle defoliation is the most obvious symptom of Douglas-fir tussock moth infestation. Trees can be stripped of all of their foliage in a single season. Defoliation typically begins at the top of the tree and progresses downward. In addition to destroying trees, the tussock moth caterpillar hairs can cause an irritating and painful rash in humans that are exposed to them.

Tussock Moth Caterpillar Treatment

Tussock moths are control preventative and reactively with a soil drench application of Safari.

Zimmerman Moth

Zimmerman pine moth has become well established along the Front Range of Colorado. Austrian pines are the tree that is most commonly infested. Scotch and ponderosa pines are also potential hosts. Zimmerman moth larvae damage the branches of the pine trees, typically causing them to break at the crotch area where they join the trunk. Symptoms include dead and dying branches, most often in the upper half of the tree and popcorn-like pitch masses at wound sites. The pitch masses may reach golf-ball size and ultimately resemble clusters of small, pale grapes.

Zimmerman Moth Treatment

Zimmerman moth will only attack trees that are stressed due to over watering. If a tree is showing signs of Zimmerman moth pressure we recommend a sprinkler audit to make sure the lawn is being watered according to the 1-2-3-2-1 lawn watering technique. Then after the water is corrected we will apply a trunk injection using an insecticide called TREE-age.


FAQ - Insect Control for Trees

How to keep insects from attacking trees?

Each tree species is different because they have different types of insects that attack them at different times of year. We have three main insect controls that kill and prevent insects from attacking trees. Depending on the type of tree and the type of insect we are trying to prevent will depend on what insecticide we use. Our three main insect controls for trees are a preventative insecticide called Criterion, a preventative and reactive insecticide called Safari, and an insecticide for trunk boring insects called TREE-age. A combination of these three insecticides can control about 99% of the insects that attack urban trees in Colorado. Please call and speak with one of our tree care experts so we can help you develop a tree care program that is right for your property.

Are tree insect controls for trees safe for bees?

Criterion and Safari are in the neonicotinoid family of pesticides which are extremely toxic to bees. At Organo-Lawn we greatly care about the safety of honeybees. We have researched the use of Criterion and Safari as a soil injection and have found no evidence that that the honeybees come in contact with the insecticides Imidacloprid or Dinotefuran.


The reason that we feel comfortable using these insecticides is because the active ingredient cannot penetrate the abscission layer of the tree, therefore, it cannot get into the flower or pollen of the tree. If we find scientific evidence that states otherwise we will discontinue the use of these insecticides.

Tree Pest Control Boulder