Mountain Pine Beetle
The Colorado State Forest Service is monitoring outbreaks of mountain pine beetles on the Front Range and other parts of Colorado. Access resources and guidance for managing this native insect in your trees.
Mountain pine beetles (Dendroctonus ponderosae) are native to the forests of western North America. They are beneficial ecologically when they attack and kill diseased and dying pine trees, clearing the way for healthy, young trees to thrive. When forests are dense, unhealthy and stressed by drought and warm temperatures, however, populations of these deadly insects can explode to epidemic levels and be detrimental to forest health.
From the late 1990s through 2013, mountain pine beetles affected 3.4 million acres of forests in Colorado, predominantly attacking lodgepole pines. They killed up to 80-90% of trees in some lodgepole pine forests in Summit, Grand, Eagle and Routt counties during the epidemic. Along the Front Range in the early 2010s, mountain pine beetles began transitioning into ponderosa pine forests. Heavy rains and flooding ended the epidemic there, leaving many ponderosa pine forests susceptible to future bark beetle infestations.
In recent years, oscillating precipitation and warm temperatures have left pine forests that largely escaped the prior epidemic susceptible to outbreaks of mountain pine beetles.
About mountain pine beetles
Mountain pine beetles complete a generation within one year from egg to adult. Adults typically fly to seek new host trees in July through August, though a small proportion of adults emerge earlier and some later through the summer months. Attacking females initially prefer large-diameter trees until they are depleted from the forest. However, they may attack smaller-diameter trees during an outbreak as populations explode and fewer host trees are available.
Mountain pine beetles predominantly infest ponderosa pine (Pinus ponderosa), lodgepole pine (P. contorta) and limber pine (P. flexilis). However, numerous species of pines – and all pines found in Colorado except for piñon pine – are susceptible to attack when mountain pine beetle populations explode, including ornamental pines.
Signs and symptoms of infestation
Residents should look for these tell-tale signs and symptoms of mountain pine beetle infestation:
- Popcorn-shaped masses of resin, called pitch tubes, on the trunk that may be brown, pink or white
- Boring dust in bark crevices and on the ground immediately adjacent to the tree’s base
- Woodpecker damage, where the birds have stripped portions of the bark in search of larvae, leaving accumulations of bark at the base of the tree
- Presence of live mountain pine beetles (eggs, larvae, pupae and/or adults)
- Exit holes on the bark’s surface where adult beetles have emerged
- Foliage turning yellowish to reddish throughout the entire tree crown, which usually occurs eight to 10 months after a successful attack
Management options
Small infestations can be eliminated through quick action, such as the removal of infested trees. Residents should inspect pine trees in the fall after mountain pine beetle adults fly to new host trees. After the beetles infest a tree, the best practice is to remove the tree by the following spring, ensuring infested wood is disposed of properly to avoid spreading the beetles.
Large outbreaks of mountain pine beetles are difficult to control. One of the best ways to mitigate the effects of beetle outbreaks is to manage for overall forest health and resiliency. Thinning overly dense stands of trees to reduce competition and promote tree age and species diversity is the most successful forest management strategy to increase tree vigor.
Options for Managing Mountain Pine Beetles
- Removal of infested trees and slash from thinning or pruning will prevent beetles from emerging and flying to new trees. Continual removal of trees already affected by other insects or diseases, as well as trees in overly dense areas, should be prioritized, as beetles most frequently attack unhealthy trees. Residents should contact their local transfer station or landfill to dispose of infested wood and slash, or search online for local wood drop-off locations or businesses that may utilize the material. Do not move infested wood unless you intend to drop it off at a secure location as it could spread the beetles.
- Solar treatments can be used to reduce mountain pine beetle populations in small, infested stands. These treatments involve felling infested trees, stacking cut logs off the ground in an area with full sun and covering them with clear plastic. The solar treatment of infested trees creates conditions unsuitable for the survival of the beetle’s larvae.
- Trap trees serve as traps for recently emerged, adult beetles. After the trap trees become infested with beetles, the trees are removed and destroyed, usually during the fall or winter while all of the beetles are still inside.
- The use of pheromone packets containing verbenone, known as semiochemicals, disrupts the attraction of incoming beetles and can be used to reduce attacks on pine trees where no other management options are effective (e.g., in proximity to water or near structures where overspray of preventive insecticides may be undesirable). The effectiveness of verbenone has been much debated in the scientific literature, though recent advances in altered concentrations have proven effective when beetle pressure is low. Consult the CSFS Forest Health Team for more detailed information on using pheromone applications.
- The use of preventive insecticide sprays may prove effective in preventing mountain pine beetle infestation. Specific formulations of pyrethroids that are registered and tested for effectiveness are the primary preventive insecticidal sprays used to help reduce the likelihood of attacks on individual trees. The Colorado State Forest Service recommends spraying only high-value trees, such as those near homes, businesses or recreation sites. Follow the label of any chosen product.
Outbreaks in Colorado
The Colorado State Forest Service monitors the spread of mountain pine beetle outbreaks in Colorado through aerial surveys and on-the-ground reports.
Updated October 2025: Ponderosa pine forests in portions of Larimer, Gilpin, Clear Creek, Boulder, Jefferson, Park and Douglas counties are experiencing elevated levels of infestation from mountain pine beetles. In addition, lodgepole pine forests in Park and Gunnison counties that evaded prior bark beetle outbreaks are experiencing elevated mortality of trees as populations of mountain pine beetles expand.
Explore where aerial observers detected mountain pine beetle activity in Colorado’s forests over the past three years. This map shows data from an annual aerial survey conducted by the Colorado State Forest Service and U.S. Forest Service.
Frequently asked questions (FAQs)
About mountain pine beetles
What is the mountain pine beetle?
Mountain pine beetle (Dendroctonus ponderosae) is a native bark beetle that attacks pine trees, especially ponderosa, lodgepole, limber and occasionally bristlecone pine. This beetle plays a natural role in forest ecosystems, clearing out diseased, stressed and dying trees and serving as a sanitizer of the forest. However, mountain pine beetles can cause widespread death of trees when their populations explode due to drought stress, dense stands and warmer winters and reach epidemic levels.
Mountain pine beetle is one of many species of bark beetles in Colorado. Other species in Colorado include pine engraver beetle, fir engraver beetle, spruce beetle, Douglas-fir beetle, roundheaded pine beetle and numerous others. Most species are host-specific, meaning each species typically attacks certain kinds of trees.
How does the mountain pine beetle kill trees?
Adult beetles bore through the bark to lay their eggs in the phloem, the layer between the bark and wood that moves water and nutrients through the tree. When the eggs hatch, the larvae feed on the inner bark and create tunnels that cut off this flow. The beetles also introduce blue-stain fungi, which spread through the tree and further block the movement of water and nutrients. Most trees die within a year of being successfully attacked.
What is the mountain pine beetle’s life cycle?
The mountain pine beetle has a one-year life cycle:
- Summer (July – mid-September): Adults emerge and infest new trees.
- Fall and early winter (September – December): Eggs hatch and larvae partially develop under the bark.
- Late winter and spring (January – June): Larvae complete their development, and they pupate in late May through June.
- Summer (July – mid-September): New adults emerge to seek green trees to begin the cycle again.
When do mountain pine beetles attack trees?
Throughout Colorado, timing may vary slightly, but in general, adults emerge and fly from July through mid-September, seeking new host trees. A tiny number of adults may survive the winter and emerge in early spring, but this early emergence doesn’t contribute to a significant number of tree deaths.
How do cold temperatures affect mountain pine beetle survival?
Larvae can survive temperatures as low as -31°F by producing antifreeze compounds, known as glycols. However, cold snaps that exceed this threshold can kill the larvae and reduce the severity of outbreaks. The vulnerable periods for larvae occur in the fall and spring, when the glycols (antifreeze within the larvae) haven’t fully developed late in the fall or are broken down or synthesized for energy in the spring.
About management options
What tree species do mountain pine beetles attack?
Both 2-3 needle pines and 5-needle pines are susceptible to attack from mountain pine beetles, except for piñon pine which is not considered a host. Mountain pine beetles predominantly infest ponderosa pine (Pinus ponderosa), lodgepole pine (P. contorta) and limber pine (P. flexilis). All pines found in Colorado except for piñon pine (P. edulis) are susceptible to attack when populations of mountain pine beetles are high, including Scots pine (P. sylvestris), bristlecone pine (P. longaeva) and ornamental pines. Mountain pine beetles attack pine trees in both backcountry and urban areas.
How can I tell if my trees are infested with mountain pine beetles?
Regularly inspect trees for early signs of mountain pine beetle infestation, such as pitch tubes, boring dust or fading crowns, particularly in the fall (after mid-September) after adult beetles are no longer flying to new host trees for the year. More signs and symptoms of mountain pine beetle infestation, including photos, are available higher up on this webpage.
Can infested trees be saved?
Once a tree is infested with mountain pine beetles, it cannot be saved. The focus should shift to preventing beetles from spreading to nearby healthy trees by removing the infested trees through chipping, burning, debarking or burying them before July when adult beetles start to emerge from infested trees and fly to new host trees.
How should I safely dispose of infested wood?
Cut and dispose of infested trees before adult mountain pine beetles emerge and fly to new host trees, which typically occurs from July through mid-September. It is critical to do this far enough in advance of the adult beetles’ flight season to ensure these activities will effectively prevent the spread of mountain pine beetles to new host trees.
Proper disposal methods for infested wood include the following:
- Chip infested material to destroy beetle larvae under the bark.
- Debark infested wood to remove the protective layer where larvae live.
- Apply solar treatments to infested material by covering cut logs with clear plastic for four to six weeks during hot months, which kills the larvae.
- Burn infested material where it is safe and permitted by local regulations.
If landowners are unable to properly dispose of infested wood and other materials on-site using these methods, they should contact their local transfer station or landfill to dispose of it, or search online for local wood drop-off locations or businesses that may utilize the material. Landowners should not move infested wood unless they intend to drop it off at a secure location as it could spread the beetles.
Can I use infested wood once it is treated?
Once larvae in infested material have been killed through chipping, solar treatment or drying, or adult beetles have emerged and the material is no longer infested, the wood can be used for firewood, lumber or mulch. While wood that is treated, dried and checked may be safe to move, it is extremely difficult to ensure all beetle larvae and eggs have been killed. Residents should not move treated material unless they are absolutely certain the wood is no longer infested to avoid spreading the beetles.
In Colorado, mountain pine beetles typically emerge in July through mid-September; however, secondary wood-infesting beetles have multiple adult flight periods. Identifying beetle species within wood is critical to prevent the inadvertent movement of insects.
Don’t Move Firewood offers further guidance on how to use wood while preventing the spread of pests.
How can I prevent mountain pine beetles from attacking my trees?
Landowners who have identified pine trees on their property can take several actions to prevent mountain pine beetles from infesting their trees:
- Thin dense stands to improve tree vigor and reduce beetle spread.
- Remove and dispose of infested trees before adult beetles emerge and fly to new host trees starting in July. Get rid of these trees by chipping, burning, debarking or burying the infested material.
- Apply preventive semiochemicals (products containing verbenone) on high-value trees before adult beetles fly to new host trees.
- Apply preventive insecticides, such as carbaryl, permethrin or bifenthrin, on high-value trees before adult beetles fly to new host trees.
- When planting trees or reforesting an area, plant a variety of tree species to increase diversity, so no one insect can infest all or most trees.
What are semiochemical products, and do they work against mountain pine beetles?
Semiochemical products containing verbenone are synthetic versions of the natural chemical signals that bark beetles use to communicate. In nature, verbenone signals to other bark beetles that a tree is already infested with beetles, making it an unsuitable host. When applied to trees, these products can help discourage new attacks by mimicking that natural “no vacancy” signal.
Verbenone can be effective when beetle populations are low or when used as part of an integrated management strategy. However, as beetle populations increase and attacks become widespread, the effectiveness of verbenone declines. In those situations, preventive insecticide sprays may be a more reliable option for protecting high-value trees.
Can preventive insecticides protect trees from mountain pine beetles?
Preventive insecticides can protect individual trees from mountain pine beetle attack when applied before adult beetles fly to new host trees starting in July. This treatment option is typically used to protect high-value trees around homes and viewsheds, not large forest areas. It is not feasible or responsible to apply these pesticides across an entire property or landscape.
Effective active ingredients for preventive insecticides include carbaryl, permethrin and bifenthrin. Applications must completely cover the bark surface up to where the trunk tapers to less than 4 inches in diameter. Multiple applications may be necessary during the summer to be effective against particular beetle species.
Landowners should consult a licensed pesticide applicator or arborist before using any chemical treatment and follow all product labels and Colorado Department of Agriculture regulations. They should contact the CDA for names of local pesticide applicators that are currently licensed through their office.
Can tree injections protect trees from mountain pine beetles?
Tree injections are sometimes used to protect individual, high-value trees from attack by bark beetles such as mountain pine beetles, but their effectiveness is limited. Injectable insecticides deliver the chemical directly into the tree’s vascular system, providing internal protection rather than surface protection, such as what would be provided through a preventive insecticide spray.
Currently, tree injection products are inconsistently effective against preventing attack by mountain pine beetles or other aggressive bark beetles in Colorado’s forests. Because bark beetles attack quickly and in large numbers, injected trees often cannot move the chemical throughout their phloem fast enough to stop an infestation. Preventive surface sprays applied before the adult beetles fly to new host trees starting in July remain the most reliable chemical option to protect trees from attack.
Landowners should consult a licensed pesticide applicator or arborist before using any chemical treatment and follow all product labels and Colorado Department of Agriculture regulations. They should contact the CDA for names of local pesticide applicators that are currently licensed through their office.
Front Range mountain pine beetle-infested wood disposal
The map below displays locations within nine counties along the Front Range accepting bark beetle-infested wood. Locations are delineated by small loads (pickups and trailers from residents) and large loads (semi-truck loads from contractors) and by private or municipality-owned facilities.
Locations are subject to change availability and restrictions by residency. Please follow up with each entity prior to drop-off for current status.
Additional resources
Mountain Pine Beetle (Fact Sheet 5.528) (10.7 MB PDF)
The CSFS offers additional guides and educational materials about mountain pine beetles.