Top Tree Pests

Oystershell scale is a hard-shelled immobile scale insect with a shape very similar to its namesake: oyster shells. Commonly found on aspens, cottonwoods, lilacs, and other thin-barked deciduous trees, oystershell scale can severely weaken its hosts, eventually draining them completely....

Emerald ash borer, an invasive insect responsible for the death or decline of tens of millions of ash trees in 21 states, has been recently found in Colorado. This small, green metallic beetle, found on September 23, 2013 in Boulder County, attacks all ash species. Some signs of infestation are...

White pine weevil feeds underneath the bark of the top leader of both Blue Spruce and Engelmann Spruce trees, girdling and killing the top most growth. This damage results in a bushy, deformed tree that is aesthetically undesirable. White pine weevil can be treated with a spring spray or a fall...

Fire blight is a bacterial disease commonly found in apples, crabapples, pears, hawthorns and European mountain ashes. Symptoms include leaves that turn brown or black and remain on the tree as well as dead twigs that form a shepherd’s crook at their tips. Fire blight infections result in dead...

Spider mites feed on the foliage of plants by breaking open the upper cell layers and sucking the sap. Populations develop rapidly in drier conditions and stressed plants are particularly vulnerable. Symptoms include discolored (bronze to black) and permanently weakened foliage and premature...

Aphids feed on the foliage and twigs of plants by sucking sap. There are over 350 different aphid species in Colorado and almost all plants are potential hosts. Symptoms vary depending on the type of aphid and the host plant, but include curling leaves, wilting leaves, premature defoliation and...

Marssonina blight is a fungal disease that infests aspens and cottonwoods. The infection starts as dark brown spots with yellow halos. As the disease progresses numerous spots may grow together forming large black dead patches. Infected trees typically drop their leaves early. Remedies include...

Cytospora canker is a fungal disease that affects many different species of trees, particularly aspens and cottonwoods. Symptoms vary depending on the host tree, but usually include discoloration of bark and pimple-like fruiting structures. There is no chemical treatment that effectively...

Pine needle scale feeds on the needles of most species of spruce, pines and fir. The small white flecks on infested spruce needles can easily be seen, but scale insects between pine needles are easily overlooked. Heavy infestations result in defoliation, twig dieback and...

Willow scale, whose primary host is aspen and cottonwood, is a relatively new insect to Colorado. These grayish-white, oval shaped, immobile insects suck sap through the thin bark of the host tree's branches and trunk. Damage often results in the death of infected trees, either directly due to...

Ips beetles are a type of bark beetle that typically infest pines and spruce, especially those that are stressed due to drought or construction damage. Pitch masses on the trunk and/or limbs and reddish-brown boring dust will often be produced when a tree is attacked. Unlike mountain pine beetle...

Mountain pine beetle has killed thousands of acres of trees throughout Colorado and the Rocky Mountain region. This insect attacks several species of pines; lodgepole, ponderosa, Scots and limber pines are the most common. Popcorn-like masses of pitch on the trunks of trees sometimes accompanied...