Top 10 Tree Pests

There are certain tree pests that we see every day. The following list consists of ten of the most common insect and disease problems that affect trees in the mountains of Colorado.

1. 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 hosts. Popcorn-like masses of pitch on the trunks of trees sometimes accompanied by a reddish boring dust is evidence that a tree has been attacked by mountain pine beetles. The most effective strategy to control mountain pine beetle is to reduce beetle populations by removing infested trees and spraying high value trees to protect them from attack. The spray should be done in the summer prior to beetle flight, which typically occurs in July/August. Likewise, infested trees should be removed prior to beetle flight.

2. Ips beetles are a type of bark beetle that infest pines and spruce. We commonly see this insect on spruce and pinyon pines, especially trees 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, infestation by ips beetles won’t necessarily kill the entire tree. However, repeated attacks over the course of several years often kill the host tree. Maintaining healthy trees is key to prevent ips attacks. Also, spruce and pines shouldn’t be trimmed in the warmer months of the year because pruning wounds can attract ips beetles. Controlling ips entails removing badly infected trees and spraying vulnerable trees to keep them protected during periods of ips activity.

3. Willow scale is a relatively new insect to Colorado. The primary hosts are aspens and cottonwoods. Willow scale can be identified by a grayish-white, oval shaped scale on the trunk and/or limbs of the host tree. Damage often results in the death of infected trees, either directly due to the scale or through secondary infection by cytospora canker. Treatment depends on the severity of the infestation. Options range from sprays with a horticultural oil to applications of a systemic insecticide.

4. Pine needle scale feeds on the needles of most species of spruce, pines and fir. The small white flecks on infested conifer needles can easily be seen. Heavy infestations result in defoliation, twig dieback and occasionally the death of the entire tree. Sometimes pine needle scale is kept in check by natural predators. When treatment is necessary, sprays, soil injections and/or trunk injections are recommended depending on the severity of the infestation.

5. Cytospora canker is a fungal disease that 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 that effectively controls cytospora canker. The best way to avoid infection is to maintain healthy trees that are free from stress. In particular, stress from drought, over-watering and infection by other pathogens should be avoided. If a tree does contract cytospora canker, pruning out infected limbs is the recommended treatment.

6. 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 cleaning up leaf litter, throwing away infected leaves and doing preventive sprays in the spring.

7. 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 sometimes even dieback of shoots and buds. In addition to the negative impacts to your plants, aphids can be a significant nuisance due to their production of honeydew. Honeydew is the sticky waste material excreted by aphids, and it covers everything that lies beneath infested plants. Black sooty mold grows on the honeydew, which detracts from the appearance of decks, sidewalks and plants. Several predator insects feed on aphids and sometimes keep their populations in check. When these natural controls are not sufficient, treatment options include spraying, soil injections and trunk injections.

8. Spider mites feed on the foliage of plants by breaking open the upper cell layers and sucking the sap. Populations develop rapidly under dry conditions and stressed plants are particularly vulnerable. Symptoms include discoloration of foliage (bronze to black) and defoliation. Severity of spider mite outbreaks depend largely on the weather and the ability of natural predators to keep mite populations in check. Treatment options include spraying, soil injections and trunk injections.

9. Fire blight is a bacterial disease commonly found on apples, crabapples, pears, hawthornes and European mountain ash. Symptoms include leaves that turn brown or black and remain on the tree and dead twigs that form a shepherd’s crook at their tips. Fire blight infections result in dead twigs and limbs and can kill the entire tree if left untreated. Pruning out diseased portions of infected trees is the most effective remedy.

10. White pine weevil feeds underneath the bark of the top leader of spruce trees, which girdles and kills it. 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 soil injection.