Insects & Disease

Contact us for more information about protecting your trees from insects and diseases common in our mountain environment:

Mountain Pine Beetle
Ips Beetle
Dwarf Mistletoe
Common Insects & Diseases of Aspen
Insect & Disease Publications from CSFS

Meet the Beetles: Two Species of Concern

By Colorado State Forest Service Staff (reprinted from )

Mountain Pine Beetle
The mountain pine beetle (MPB), Dendroctonus ponderosae, is a bark beetle native to western North America. Its range extends west from the Great Plains, from British Columbia to Mexico.

Mountain Pine Beetle prevention and treatment

Mountain Pine Beetle

The MPB usually takes 1 year to complete its lifecycle, developing through four stages: egg, larva, pupa, and adult. Except for a few days during the summer when adults emerge and fl y to new trees, all development takes place under the bark of infested trees. Female beetles lay tiny, pearl-white eggs under the bark of pine trees in the late summer and early fall. The eggs hatch in 10 to 14 days. The white larvae overwinter in galleries under the bark and begin to mature the following spring. By July, most pupae have transformed into adults. Adults are black and 1/8- to 1/4-inch long.

Adults feed under the bark during the summer and emerge through an exit hole, with peak emergence occurring from mid-July to mid-August. Within a few days, the beetles enter other trees—there are usually enough insects emerging from one tree to attack several additional trees. In Colorado, MPB can attack all native species of pine, including lodgepole, ponderosa, limber, and bristlecone, and even some urban pines like Scotch and Austrian. Beetles carry the spores of blue-stain fungi on their bodies and introduce them into pine trees. The combination of the feeding beetles and spreading fungi kills the tree within a year.

Ips Beetles

Eleven species of Ips beetles are native to Colorado; Ips pini, the pine engraver beetle, is the most common. Its primary hosts are the same as MPB—lodgepole pine and ponderosa pine—but it can attack most other species of pine in its range, which extends across the United States and much of North America. Generally, Ips beetles attack stressed or dying pine trees and do not cause a lot of damage. But, Ips beetle populations have been increasing due to the effects of the MPB epidemic, drought, and the availability of wounded trees. In some counties, Ips beetles have recently killed dozens to hundreds of healthy trees in a single year. Like the MPB, Ips beetles introduce blue-stain fungi to trees; the fungi quicken tree death by blocking the fl ow of water up the tree.

Ips Beetle prevention and treatment

Ips Beetle

Ips beetles complete their four-stage lifecycle in only 40 to 55 days, and can produce two to four generations of beetles per year. This makes them more challenging for the homeowner to control than MPB. In the spring, beetles begin to emerge as early as March, when consistent daytime temperatures reach 50 to 60°F, and flight can continue into November. Beetles can attack trees throughout the flight period; however, the first spring flight appears to be the most damaging. Ips beetles are about the size of a grain of rice (1/8- to 3/8-inch long), reddish-brown to black, and can be distinguished from MPB by the depressed cavity and spines at the rear end of the body.

Example Of Blue Stain and Pitch tubes

Example Of Blue Stain and Pitch tubes


Example of Blue Stain Fungus

Example of Blue Stain Fungus


MPB pitch tubes and boring dust (sawdust) are evident on infested trees

MPB pitch tubes and boring dust (sawdust) evident on infested trees


Ips beetle galleries often form a Y, X, or H shape

Ips beetle galleries often form a Y, X, or H shape


Examples of pitch tubes

Examples of pitch tubes


MPB galleries often form an I or J shape

MPB galleries often form an I or J shape


Fall Is a Great Time to Survey Your Trees and Determine Treatment Options

In the fall, most new beetle attacks have already occurred, except for those by a few late-season Ips beetles. There is no snow on the ground, the days are beautiful, and there are many treatment options, including saving newly infested wood for firewood. As the flight time of the beetles approaches during the spring, the number of treatment options decreases. Experts recommend that people carry out their forestry practices (cutting trees for beetle sanitation, fi re mitigation, or forest health) during the fall and winter months. In addition to the reasons noted earlier, this is because Ips beetles are attracted to the smell of newly cut wood and slash. Restricting your cutting to the cooler season will help prevent new infestations. If it is not possible to perform your forestry work during this time, it is still better to cut infested trees or work on fi re mitigation during the spring or summer than not to do it at all. Be sure to follow the slash management guidelines, and store any newly cut wood away from living trees.

Treating Infested Trees

Trees must be treated to destroy live beetles after they are cut down.

Here are some options:

1. Chip the entire tree and/or slash pile to destroy the beetle. Spread the chips out in a thin layer so they will dry quickly and not attract Ips beetles.

2. Take the entire tree to a sort yard for disposal. Visit for information on sort yard locations.

3. Peel the bark from the trunk with a chain saw, draw knife, or Log Wizard® (a chainsaw attachment that peels logs). You can store peeled logs for fi rewood without a problem.

4. Use solar treatment: In the fall, lay logs in a single layer in a very sunny, south-facing location. Cover the logs with clear (not black), 6 mm plastic and leave the logs covered for at least 8 weeks. If you choose not to cover the logs with plastic, leave them for at least 3 months and partially roll the logs every few weeks to ensure all sides of the log are exposed to direct sunlight.

This treatment often is not as effective at higher altitudes because it requires a lot of heat to kill the beetle. A common misconception is that the plastic is used to trap beetles, but in fact, the beetles can easily chew through it. Plastic is used to raise the temperature under the bark enough to “cook” the beetles to death. 5. Use infested wood for firewood. In the fall, cut wood to firewood length and split—this will allow the wood to dry out enough to burn in the spring. Mark your infested pile and ensure that the entire pile is burned before July, when remaining beetles could fly to live trees.

Dwarf Mistletoe

(Reprinted from Colorado State Forest Service fact sheet: )

Dwarf mistletoes are small, leafless, parasitic flowering plants that are a common problem in western forests. Dwarf mistletoes spread slowly from tree to tree and are mostly found on lodgepole, limber, pinyon, ponderosa pines, and Douglas fir.

Mistletoes spread slowly from tree to tree. In closely spaced trees of about the same height, this spread is 1 to 2 feet per year. The spread from large to small trees can extend 60 feet, but the average usually is less than 30 feet. Most dwarf mistletoes are specific to a particular type of tree (i.e., lodgepole pines) and do not infect other tree species.

General Life Cycle

Seeds are dispersed in August and early September.

1. Seeds land on conifer needles.
2. Rain washes them onto twigs.
3. Seeds germinate and infect the twig.
4. A swelling appears two years later.
5. Shoots appear after another two years.
6. Fruits mature after two more years.

dwarf mistletoe colorado

Dwarf mistletoe colorado

Dwarf mistletoes are parasites of native conifer forests that can cause severe damage. These plants are host-specific, parasitic flowering plants that spread by forcibly ejected seeds.

Large witches' broom on dwarf mistletoe-infected lodgepole pine

Large witches’ broom on dwarf mistletoe-infected lodgepole pine

Dwarf mistletoe infections can retard growth and reduce seed production and wood quality; heavy, long-term infections can kill trees.

Some dwarf mistletoe species induce abnormal tree growth at the point of infection, and produce a structure known as a witches’ broom, which disrupts the typical branching structure.

Dwarf mistletoes are small, leafless parasitic flowering plants that kill by slowly robbing the tree of food and water.


Dwarf mistletoes are not quick killers, so long-term management options are feasible. However, dwarf mistletoe infected trees may attract various types of bark beetles that may breed and kill parts or the whole tree. These beetles may then attack nearby trees.

Management Options

  • Pruning and removing infected trees is the best management measure available to reduce or eliminate dwarf mistletoe infestations.
  • Plant resistant trees under infected ones to replenish the forest after infected ones are removed.
  • Dwarf mistletoes are not quick killers.Therefore, long-term management practices such as pruning, removing infected trees, and planting nonhost species are the best solutions.

Pruning and Tree Removal

Pruning and removing trees is the best management measure available to reduce or eliminate dwarf mistletoe infestations in ornamental trees or urban forests. First, remove severely infected trees (rated 5 and 6) or those with only a few live branches. Trees with high, unreachable mistletoe infections will continue to shower seeds on nearby trees if not cut down. However, it is not necessary to completely eradicate the mistletoe – that may require removal of all trees. Prune and remove a few heavily infected trees and keep a green forest on the property. The parasite can be removed from lightly infected trees (rated 1 to 3). Prune off all infected branches for healthier trees. Prune the entire branch at the branch collar near the trunk. Examine trees every two or three years, and remove any newly infected branches.

Mistletoe shoots die as soon as the tree branch is cut. Burning pruned-off branches is not necessary. When pruning, keep 30 to 40 percent of the branches (from the top down), even if that means leaving some infected branches.

Trunk infections are not as detrimental as branch infections, so their removal is not necessary. If space allows, create 50-foot buffer zones between infected trees and healthy trees by cutting or by planting resistant trees

Plant Resistant Trees in Heavily Infected Areas

Plant resistant trees under infected trees to replace trees when infected ones are removed. Site and moisture availability will determine what trees can be planted. NOTE: Scotch pine is susceptible to both ponderosa and lodgepole pine dwarf mistletoe.

• Ponderosa pine areas: Replant to Douglas-fir, white fir, blue spruce, pinon pine, limber pine, Rocky Mountain juniper, bristlecone pine, gambel oak and pea shrub.

• Lodgepole pine areas: Replant to Englemann spruce, subalpine fir, Douglas-fir, bristlecone pine and limber pine.

• Douglas-fir areas: Replant to aspen, ponderosa pine, lodgepole pine and Englemann spruce.

Hardwoods, such as ash, birch and aspen, also can be planted in affected areas because dwarf mistletoes do not attack hardwood trees.

Insects and Diseases of Aspen

Reprinted from


Aphids are small insects that feed by sucking plant sap from leaves and excrete a sweet, sticky substance called honeydew. The damage is mostly unsightly but, in the long-term, may kill the branches they feed on.

Aphids on Shade Trees and Ornamentals Fact Sheet (43 KB PDF)

Ink Spot Disease

Ink spots are a result of the fungus Ciborinia and are commonly found in dense aspen stands. This disease causes leaf discoloration and early leaf drop, which may reduce tree growth.

Aspen and Poplar Leaf Spots Fact Sheet (203 KB PDF)

Marssonina Blight

The Marssonina fungus causes this most common disease on aspen foliage. Although there is leaf discoloration, this condition usually is not damaging. Heavy infestations cause early leaf drop.

Aspen and Poplar Leaf Spots Fact Sheet (203 KB PDF)

Aspen Leaf Miners

Adult leaf miners cut tiny slits on aspen leaves and lay their eggs inside. The larvae live inside the leaf and feed by “mining” chlorophyll from plant cells; this is not harmful to the tree’s health.

Leaf Miners Fact Sheet (128 KB PDF)


Sawflies are closely related to wasps. The larvae are plant feeders and look like hairless caterpillars. Sawflies often feed in groups and can quickly defoliate portions of their host plant.

Elk Scarring

Elk browse on the shoots and stems of aspen trees, creating wounds and allowing the introduction of diseases.

Western Tent Caterpillar

This defoliating caterpillar feeds on the leaves of aspen, causing aesthetic damage. Consecutive years of defoliation, however, may kill the tree.

Tent Caterpillar Fact Sheet (88 KB PDF)

Oystershell Scale

Scale, a common and destructive pest, overwinter on trees and harm them by sucking sap. Branch and tree death are possible with long-running infestation.

Oystershell Fact Sheet (78 KB PDF)

Poplar Twiggall Fly

These common galls are caused by tiny black flies. While they will continue to grow years after they are produced, they do not threaten the health of the tree.

Poplar Twiggall Fly Fact Sheet (144 KB PDF)

Blank Canker

This slowly developing canker is caused by the fungus Ceratocystis fimbriata and is easily recognized. The canker rarely kills the tree due to its slow development.

Sooty Bark Canker

Sooty bark canker is caused by the fungus Encoelia pruinosa and is the most lethal canker on aspen in Colorado. The dead bark falls off and exposes the crumbly black, sooty inner bark.

Epidermal Bark-Mining Fly

This curious spider-shaped track is made by the larvae of a fly and is not harmful to the health of the tree.

Trunk Rot

Phellinus igniarius decay fungus enters through old branch stubs or other wounds. Affected trees often are used by hole-nesting birds.

Poplar Borer

This wood-boring beetle lays eggs on the bark of the aspen. The larvae then tunnel, weakening the wood. Entry and exit holes of the beetle invite fungi, which can result in limb breakage.

Shade Tree Borers Fact Sheet (244 KB PDF)

Keeping Your Aspen Healthy

  • Maintain a proper watering schedule – aspen will suffer if over- or under-watered.
  • Prevent direct sprinkling of leaves by lawn watering systems.
  • Unwanted aspen sprouts that appear in the lawn may be mowed. DO NOT spray the sprouts, as they are connected to the mother tree.
  • Avoid wounding the main trunk with movers or weed wackers.
  • Trim out cankers that are less than half the circumference of the aspen.
  • Clean up heavy scale-insect infestations.

CSFS Insect and Disease Publications

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