Tree Insect Calendar

Got bugs on your trees? The Denver area is home to many tree insects — some harmful, some beneficial. The Ross Tree Insect Calendar helps homeowners identify tree insects that might harm their trees. Please use the tree insect calendar below to determine the bugs affecting your trees.

Understanding each tree insect’s lifecycle is essential in treatment timing. Maple and other trees in Denver are in full pitch by late March or early April. The trees are busy sending nutrients to their leaf and flower buds, making them swell and ripen. The warm weather also jump-starts harmful tree insects’ mating and feeding cycles. Adults emerge from where they overwintered. After they mate, the females look for places to lay their eggs. Eggs begin to hatch, and their larvae start to feed as trees leaf out. The Tree Insect Calendar gives approximate times for treatment by insect species. Treatment timing varies depending on weather, temperature, location, and adult and egg emergence. Click here to download the Tree Insect Calendar.

Control product applications that are not well-timed waste money and time. Wet springs support the growth of many of the insects listed below. On the other hand, hot late summer days support the development of other pests. So it is best to pick a tree company, like Ross Tree, that has accredited professionals who know how to control tree pests and protect helpful tree insects and the environment.

Ross Tree Company abides by our motto, Integrity in Action, which means residential and commercial property owners can expect the correct use of tree insect control products needed for the job. Ross Tree has three International Society of Arboriculture Certified Arborists on staff to help identify tree pests and four Qualified Supervisors licensed by the Colorado Department of Agriculture to handle pesticide products. If one of these tree pests is listed on our tree insect calendar, please fill out a request service form or call 303-871-9121 to make an appointment.

The Ross Tree Insect Calendar is based on the Colorado Extension Service study titled INSECTS THAT FEED ON COLORADO TREES AND SHRUBS. Colorado State University published the study in 1914. Recent arrivals are the Japanese Beetle, most of the sawflies, and the Emerald Ash Borer.


Harmful Tree
Insects Found
in Denver

Ash sawfly found in Denver

Ash Sawfly

The larvae feed on ash leaves and can defoliate entire trees. This insect is active along the Front Range.

Ash / Lilac Borer

The borer’s larvae tunnel in branches and trunks on ash, excavating galleries several inches long. On lilac and privet, the larvae cause wilt and die back.

Ashleaf Curl Aphid in Denver

Ashleaf Curl Aphid

Feeding by the aphids creates very tightly rolled and thickened leaves, or pseudogalls, at the tips of the twigs. This injury often causes distortion and twisting of the twig next to the damaged leaves.

Aspen Leafminer in Denver

Aspen Leafminers

The leafminer creates winding, serpentine tunnels in Aspen leaves in the spring and summer. Infestations are rarely severe, but some people find their moths a nuisance.

Boxelder & Maple Aphids

Boxelder And Maple Aphids

These aphids produce copious amounts of honeydew covering objects beneath infested trees. The insect causes leaves to drop early and or defoliate a tree—however, outbreaks occurring late in the season cause minor damage

Boxelder Bug in Denver

Boxelder Bug

The bug does little harm to Boxelder trees but is considered a nuisance by some.

Bronze Birch Borer in Denver

Bronze Birch Borer

Its flathead larvae girdle limbs, causing dieback. A large infestation can kill trees. Birch trees stressed by insufficient watering are particularly susceptible.

Codling Moth the worm in an apple

Codling Moth

The worm in an apple is most likely a Codling Moth larvae. They also attack the fruit of crabapples and pears.

Cottony Maple Scale in Denver

Cottony Maple Scale

They get their name from the very large marshmallow-like egg sack extending from the rear of females. They attack Maple, Honeylocust, Linden, and other hardwoods.

Douglas-Fir Tussock in Denver

Douglas-Fir Tussock

The Douglas-fir Tussock moth is a pest of landscape trees along much of the Front Range. Larvae feed on the needles, starting on the top of the tree. Repeated attacks over several seasons can kill trees or make them susceptible to bark beetles.

Elm Bark Beetle vector for Dutch Elm disease

Elm Bark Beetle

The beetle is the principal vector of the Dutch elm disease. Spraying tree crowns with insecticides can kill the beetles. Antifungal injections sometimes save trees.

Elm- Leaf Miner treatment

Elm Leaf Miner

Adults first appear in May laying their eggs singly near the center of the new leaves. Its larvae hatch and mine out the middle layers of the leaf sometimes forming large blotch mines.

EAB in Colorado

Emerald Ash Borer

The EAB arrived in Boulder in 2013 and is now spreading along the Front Range. It is just a matter of time before it comes to Denver. American ash trees have no immunity. Homeowners should treat any high-value Ash now to save them.

European Elm Flea Weevil treatment Denver

European Elm Flea Weevil

Female weevils mine holes partway through the leaf tissue to lay eggs. These hatch into legless, white larvae that tunnel through soft leaf tissue at the leaf tips turning the tips white, then eventually brown. The bugs jump like fleas when disturbed.

European Elm Scale

The European elm scale is one of the most destructive scale insects in Colorado. Prolonged infestations weaken branches causing premature leaf yellowing, leaf drop, and dieback. These scales can produce copious amounts of honeydew which molds.

Giant Conifer aphid treatment Denver

Giant Conifer Aphid

The giant conifer aphids feed on the sap from twigs and branches of Pines, fir, Douglas-fir, and Spruce. Heavy infestations cause yellowing of foliage and needle drop. Copious deposits of honeydew are often associated with sooty mold growth

Hawthorn Mealybug in Denver

Hawthorn Mealybug

The Hawthorn Mealybug feeds on the sap of twigs and small branches. These pests produce honeydew that molds. Heavy infestations weaken trees and cause dieback.

Honeylocust Leafhopper in Denver

Honeylocust Leafhopper

These leafhoppers attack Honeylocust, Elms, and other deciduous trees. These bugs produce honeydew which can be a nuisance. Infested foliage becomes discolored and distorted.

Honeylocust spider mite treatment Denver

Honeylocust Spider Mites

Honeylocust spider mites attack drought-stressed trees. They damage the upper cell layers to remove sap. With heavy infestations, the leaves of the whole tree turn an off-yellow color

ips beetle

Ips Engraver Beetle

These beetles leave brownish boring dust in bark crevices or around the base of trees. Ips beetles usually attack weakened trees injured by construction, drought stress, or overwatering.

japanese beetle

Japanese Beetle

Japanese Beetles eat over 300 types of plants in Denver yards and are voracious eaters. They target Siberian Elms and other elm hybrids. Controlling these beetles with insecticide products is complicated because the pests are active at the same time as flower pollinators.

oak kermes scale

Kermes Scale

Kermes scales attack Red Oaks causing reduced growth and vigor. Symptoms of heavy infestations are leaf flagging, dieback of twigs, and twig abscission.

Longhorn Beetles in Denver

Longhorn Beetles

Longhorned Beetles attack Maple, Birch, Horse Chestnut, Poplar, Willow, Elm, Locust, and Ash trees growing in Denver. After hatching, the larvae feed on the Cambium but sometimes bore deeper, causing significant structural weakening.

Mountain Pine Beetle

Mountain pine beetles develop in pines, particularly ponderosa, lodgepole, Scots (Scotch), and limber pine. The bugs introduce a blue-staining fungus that turns the sapwood grayish-blue and clogs the vascular system. Infected trees usually die.

Oystershell Scale

The Oystershell scale feeds on the phloem of trunks and branches of Ash and Aspen trees. Dieback is a common injury, and weakened trees often succumb to Cytospora canker.

Peach Crown Borer in Denver

Peach Crown Borer

Its adults look like wasps. Larvae burrow into the sapwood of the tree trunk of Peach trees. Girdling injuries weaken and frequently kill trees.

Pine Needle Scale Treatment Denver

Pine Needle Scale

Pine needle scales suck sap, causing needles to discolor around the feeding site. Heavy infestations cause premature needle drop and kill branches. Pine Needle Scale targets Mugho Pines in Denver.

Pine Sawyer Beetle in Denver

Pine Sawyer Beetle

Pine sawyers target Pine, Spruce, and Douglas-fir trees. Larvae bore extensively in sapwood and heartwood of dying and recently killed trees. Adults cause minor injury by feeding on needles and shoots.

Ponderosa Pine Sawfly Denver

Ponderosa Pine Sawfly

The tiny sawfly wasps emerge in the fall and deposit eggs on pine needles to overwinter. Pine Sawfly infestations are hard to miss, with large populations of larvae feeding on pine needles.

Poplar Twiggal Fly in Denver

Poplar Twiggal Fly

The Poplar Twiggall Fly produces lumpy twigs on Aspens. These small, dark flies insert eggs into new twigs in the spring, where they grow protective woody galls. The fly is native to Colorado, and sometimes it is best to let local birds and parasitic wasps control this tree pest.

Red Headed Ash Borer in Denver

Red Headed Ash Borer

Usually attacks only dead and dying wood but will also bore into twigs and branches on Ash trees. Can reduce sapwood to powder.

Red Turpentine Beetle in Denver

Red Turpentine Beetle

Turpentine beetle attacks cause large, pinkish-purple pitch tubes confined to the lower eight feet of the trunk. Trees stressed by fire or construction damage are susceptible. Beetles may be active throughout the warmer months, peaking in mid-summer.

Rose of Sharon Aphid in Denver

Rose of Sharon Aphid

Aphids quickly infest a Rose of Sharon plant and suck it dry, but they also leave behind a sticky honeydew. Aphid honeydew traps fungal spores on their sticky surfaces, leading to sooty mold infections.

Spirea Aphid in Denver

Spirea Aphid

Spirea aphids feed on plants in the Rosaceae family. Attacks apple and pear. Their feeding causes leaf curling and stunted plant growth.

Red Turpentine Beetle in Denver

Spruce Beetles

Spruce beetles are native to Colorado. Adults fly to seek new hosts in late May through July, targeting large trees. Needles on infected trees tend to turn a pale yellowish-green color and fall during high winds.

Spruce Gall Adelgids in Denver

Spruce Gall Adelgid

Closely related to the Wolly Pine Adelgid, problems on pines are usually short-lived. Sometimes the adelgids disperse without causing severe damage.

Spruce Spider Mite in Denver

Spruce Spider Mite

Spruce spider mites are common in Denver and damage Spruce and Juniper trees. They leave tiny light-colored spots at feeding sites. Infested trees become brownish-gray and may defoliate.

striped pine scale denver

Striped Pine Scale

The Striped Pine Scale has become a problem in the metro Denver area. Scale feeding stunts needles, causing many to drop. Heavy infestations kill branches and sometimes young trees.

Walnut Twig Beetle

Walnut Twig Beetle

The beetle is the insect vector for the Thousand Cankers Disease. It is now threatening Walnuts growing in the Eastern part of the country. Symptoms include thinning canopy, leaf yellowing/wilt, tip dieback.

Wolly Pine Adelgid in Denver

Wolly Pine Adelgid

These aphid-like insects do not have cornicles and are covered with a thick coat of white waxy filaments. Pines with a large infestation become yellowish and stunt growth. Feeding can cause shoots to droop and die. Infestations can be found on the bark of trees.

Wood Borer Weevils in Denver

Wood Borer Weevils

The Poplar and Willow Borer Weevils are the most common members of this insect family found in Colorado. These borers attack young trees 1 to 4 inches in diameter. The adults emerge from exit holes plugged with sawdust mixed with excrement. 

Zimmerman pine moth treatment

Zimmerman Pine Moth

The first symptom of injury is the appearance of pitch masses on the wound sites. Austrian pines are particularly susceptible.

Harmful Tree
Diseases Found
in Denver

1000 Canker Disease

1000 Canker Disease

The Walnut Twig Beetle is the vector of this disease. It has killed most Walnut trees in Denver.



Anthracnose is a common fungal disease of shade trees that results in leaf spots, cupping or curling of leaves, and early leaf drop. The fungus attacks Sycamore, Ash, and Maple causing significant stress and potential death.

Peritheica of Black spot Nectria canker

Black Spot Nectria Canker

Black Spot cankers have raised asexual fruiting structures cream or peach in color. Their perithecia are flask-shaped and reddish-brown and grow in solitary or groupings.

Coral Spot Nectria Canker

Coral Spot Nectria Canker

Like Thyronectria and Black Spot cankers, their fruiting bodies from raised smooth sporodochia masses are creamy colored and look like coral. Their perithecia are small bright reddish-brown round flasks.

Cytospora Canker on Aspen Tree


Cytospora canker is a common Aspen disease. Some more aggressive species infect and kill Aspen, Cottonwood, and Mountain Ash planted in Denver

Fireblight Treatment in the Spring


Fireblight is a highly infectious and widespread disease caused by a bacterium. It attacks Apple, Crabapple, Pear, Peach, Mountain Ash, and Hawthorns.

Leaf Spot

Leaf Spot

Fungi cause most leaf spots, but bacteria cause some. Some insects cause damage that appears like a leaf spot disease.

Phomopsis Cankers

Phomopsis Cankers

Typically, Phomopsis usually attacks trees grown in nurseries and tree farms. For some unknown reason, this fungus is now affecting Evergreens found in yards.

Powdery Mildew on trees

Powdery Mildew

Powdery mildew is a disease common in trees that appears as a white powdery substance on the surface of tree leaves. The floury appearance comes from millions of tiny fungal spores, spread by air currents to cause new infections.

Thyronectria canker

Thyronectria Canker

Thyronectria cankers attack Honeylocust trees in Denver. Both asexual and perithecia fruiting bodies start in lenticels or on branches with thin bark. Keeping Honeylocust trees healthy is the best approach to controlling this canker.


Tree Insects

Spotted Lanternfly

Spotted Lanternfly

Ross Tree likes to keep its customers informed about invasive tree insects. In Pennsylvania, New York, Maryland, Delaware, and Virginia, local media outlets feature stories about the Spotted Lanternfly. Some stories are pretty hilarious. However, this sap-sucking bug is no joke and could severely harm Colorado’s fruit and wine industry. Let’s hope the Lanternfly does not make its way to Colorado.