Tree Insect Calendar
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.
Have you got bugs in your trees? Some are harmful, some beneficial. The Ross Tree Insect Calendar helps homeowners identify tree insects feeding on their trees. Please download the tree insect calendar to determine the bugs affecting your trees. 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.
Homeowners should be vigilant for tree pests when Denver has a wet spring. Trees in Denver are usually in full pitch by March and April, 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. As eggs hatch, 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, tree mites love hot late summer days. So it is best to pick a company like Ross Tree with accredited professionals who understand how to control tree pests and protect helpful tree insects and the environment.
We based the Ross Tree Insect Calendar 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. Let’s hope the dreaded Spotted Lanternfly wreaking havoc in the East does not make its way to Colorado.
Harmful Tree
Insects Found
in Denver
Ash Sawfly
The larvae feed on ash leaves and can defoliate entire trees. This insect is active along the Front Range.
Evergreen Tree
Pest Blog
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.
Tree Borer Blog
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.
Tree Aphid Blog
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.
Aspen Tree Care Blog
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
Tree Aphid Blog
Boxelder Bug
The bug does little harm to Boxelder trees but is considered a nuisance by some.
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 is most likely a Codling Moth larvae. They also attack the fruit of crabapples and pears.
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
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.
Evergreen Tree Pest Blog
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
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.
Elm Leave Pest Handbook
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.
EAB Blog
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.
Elm Leave Pest Handbook
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.
Gall Wasps
Gall wasps in produce a wide range of galls, from woody, rounded galls on stems or leaves to woolly or mossy galls. Common leaf galls found in Colorado look like pale-colored balls, reddish cup-like swellings, bright-red raised areas along the veins, or woolly patches.
Giant Conifer Aphid
Interestingly, Colorado is not included in this insect’s distribution list but was found in Arvada on Elms in 2023 by one of our arborists. It is the largest aphid found in North America.
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
Pine Tree Care Handbook
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
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 Podgall Midges
Honeylocust Podgall Midges emerge in the spring and fly to Honeylocust trees to lay eggs on developing leave buds. The best time to treat is during egg-laying time in early spring when trees are budding.
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
Spider Mite Blog
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.
Ips Beetle Blog
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.
Japanese Beetle Blog
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
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.
Mountain Tree Beetle Blog
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
Its adults look like wasps. Larvae burrow into the sapwood of the tree trunk of Peach trees. Girdling injuries weaken and frequently kill trees.
Tree Borer Blog
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
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.
Pine Tree Care Handbook
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.
Pine Tree Care Handbook
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
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
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
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
Spirea aphids feed on plants in the Rosaceae family. Attacks apple and pear. Their feeding causes leaf curling and stunted plant growth.
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.
Pine Tree Care Handbook
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
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.
Spider Mite Blog
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
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
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
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
The first symptom of injury is the appearance of pitch masses on the wound sites. Austrian pines are particularly susceptible.
Tree Borer Blog
Harmful Tree
Diseases Found
in Denver
1000 Canker Disease
The Walnut Twig Beetle is the vector of this disease. It has killed most Walnut trees in Denver.
Anthracnose
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.
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.
Tree Canker Blog
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.
Tree Canker Blog
Cytospora
Cytospora canker is a common Aspen disease. Some more aggressive species infect and kill Aspen, Cottonwood, and Mountain Ash planted in Denver
Tree Canker Blog
Fireblight
Fireblight is a highly infectious and widespread disease caused by a bacterium. It attacks Apple, Crabapple, Pear, Peach, Mountain Ash, and Hawthorns.
Fire Blight Handbook
Leaf Spot
Fungi cause most leaf spots, but bacteria cause some. Some insects cause damage that appears like a leaf spot disease.
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.
Tree Canker Blog
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 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 Canker Blog
Invasive
Tree Insects
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.
Spotted Lanternfly Blog