The mountain pine beetle is making a comeback along Colorado’s Foothills – right as wildfire season begins. After killing more than 80% of the state’s pine forests between 1996 and 2014, this native pest is once again showing up in high numbers from Evergreen to Fort Collins.
And it couldn’t be worse timing. According to the latest forest health report from the Colorado State Forest Service, 2024 was one of the warmest, driest years on record. Trees already stressed can’t produce the resin they need to defend themselves, making them easy targets for beetles. The result? More infestations, more dead trees, and more fuel on the ground just as fire risk peaks.
Key Takeaways:
- Mountain pine beetle (MPB) is the most destructive insect pest of Colorado’s pine forests.
- Outbreaks have killed millions of trees, especially during prolonged drought and warm winters.
- Trees under stress from drought, overcrowding, poor soil, fire damage, age, or disease are the first to be attacked.
- If a tree is infested, it needs to be removed, and the wood must be chipped or solar treated right away to stop the beetles from spreading.
- Preventive insecticide sprays can protect uninfested trees, but only if applied before beetle flight (usually mid-May through July).
About the Mountain Pine Beetle
The mountain pine beetle (MPB), Dendroctonus ponderosae, is a small bark beetle native to the forests of western North America. It used to be called the Black Hills beetle or Rocky Mountain beetle – but whatever the name, it’s known for doing serious damage.
Under normal conditions, these beetles play a natural role in forest health by targeting old, overcrowded, or weakened trees and making room for new growth. But when their populations surge – often after several warm, dry years – they don’t stop at stressed trees. They’ll overwhelm even healthy pines, killing them from the inside out.
Mountain pine beetles don’t respect property lines, either. They show up in wilderness areas, mountain subdivisions, and backyard windbreaks. And thanks to infested firewood, they can travel far beyond the forests – sometimes showing up in landscape pines miles from their usual range.
Why Drought Fuels Mountain Pine Beetle Outbreaks
Pine trees defend themselves from bark beetle attacks by essentially pushing the pests out of the tree with resin. The force of the resin, and the chemical compounds it contains, can protect healthy trees from occasional beetle attacks.
However, pine trees need moisture to produce enough resin to protect themselves. With less snow and rain across the Colorado Front Range saw in 2024, trees haven’t been able to build up their natural defenses. Drought-stressed trees can’t fight back, which makes it easier for beetles to burrow in, lay eggs, and multiply.
This isn’t a one-and-done kind of problem. When drought stretches across multiple seasons, beetle populations don’t just go away – they grow. That’s when small, manageable outbreaks can start to snowball.
Adult mountain pine beetles. Image courtesy of USDA Forest Service – Region 2 – Rocky Mountain Region , USDA Forest Service, Bugwood.org.
Which Trees Do Mountain Pine Beetles Attack?
Mountain pine beetles will attack any pine tree if the conditions are right – especially when the tree is already stressed. That includes pines dealing with drought, overcrowding, root disease, fire damage, poor soil, or just old age. They’re opportunists, and vulnerable trees give them an easy entry point.
One of the biggest contributors to that stress? The western spruce budworm (Choristoneura occidentalis). It’s the most widespread forest pest in Colorado right now – its impact grew from 202,000 acres in 2023 to 217,000 acres in 2024.
That said, some species are hit harder than others. In Colorado, the ones they attack most often are:
- Ponderosa Pine (Pinus ponderosa),
- Lodgepole Pine (P. contorta), and
- Limber Pine (P. flexilis).
When beetle populations explode, all pine species found in Colorado are susceptible to attack, including bristlescone, scots (scotch), and pinyon pine.
How Do Mountain Pine Beetles Kill Trees?
Mountain pine beetles don’t chew through wood the way termites do. Instead, they burrow between the bark and the inner wood (the phloem), where they carve out winding tunnels (called galleries) to lay their eggs. This is the layer responsible for moving water and nutrients throughout the tree – so once it’s compromised, the tree starts to decline fast.
But the beetles don’t stop there. They also carry a fungus known as blue-stain fungus, which spreads through the tree’s vascular system. This fungus clogs the tissue even further, blocks water flow, and interferes with the tree’s natural defenses.
A healthy pine might be able to fight off a few beetles by pushing them out with resin – but if the tree is stressed or the attack is large enough, that defense system gets overwhelmed.
Once a beetle attack is successful, it’s game over. The tree will usually die within a year, often turning reddish-brown the following season. From there, it becomes standing fuel – dry, brittle, and quick to ignite when fire moves through.
What’s the Active Season for Mountain Pine Beetles?
Adult beetles typically emerge from infested trees in July and August (although some may emerge earlier or later, depending on local conditions). They then fly to find new host trees in which to lay eggs. Female beetles prefer larger diameter trees (which is why those tend to be killed first) but will settle for smaller trees if no large ones are available.
Adult MPB excavating a tunnel in a ponderosa pine. Although this tree is still producing pitch, it has been heavily attacked by mountain pine beetle. This adult beetle is excavating a larval gallery, treading the oozing pitch. Image courtesty of Whitney Cranshaw, Colorado State University, Bugwood.org.
How to Tell It’s a Mountain Pine Beetle
If you’re trying to tell the difference between mountain pine beetles and other bark beetles, look at the back end of the insect – specifically the hind wing covers.
- Mountain pine beetles (and other beetles in the Dendroctonus group) have a smooth, rounded shape when viewed from the side
- Ips beetles – also called engraver beetles – have a sharply spined wing cover
- Scolytus beetles, like the shothole borer, have a sunken or indented abdomen under the wings
Signs & Symptoms of Mountain Pine Beetle Infestation
Not sure if your trees have been hit? Here’s what to watch for. Some signs show up right away, others take time – but the sooner you spot them, the better.
- Pine needles turning yellowish-red and then becoming a rusty brown color
- Needles dropping (this usually happens in the 2nd summer after a tree has been infested)
- Boring dust (it looks like fine sawdust) in bark crevices and on the ground near the tree
- Popcorn-shaped masses of brown, pink, or white resin, called “pitch tubes,” on the tree trunk
- Woodpecker damage – in their search for pine beetle larvae, woodpeckers will strip pieces of bark from infested trees, leaving piles of bark at the base of trees
- Exit holes on the bark surface where the adult beetles emerged
Watch: How to Spot Mountain Pine Beetle Damage
Want to see exactly what to look for? In this quick video, Ryan Reed, owner of LAM Tree Service, shows you exactly how we check for signs of mountain pine beetle activity.
You’ll see how we carefully remove a section of bark above the pitch tube and point out the telltale beetle galleries underneath – two of the most reliable signs a tree has been infested.
[blockquote] Not comfortable inspecting the tree yourself? Our Certified Arborists are available year-round to help. We offer detailed on-site inspections and charge a flat hourly rate for this service.
What to Do with Trees Infested by Mountain Pine Beetles
If you’ve got a tree that’s already been infested, there’s no reversing it. Once the beetles are inside, it’s too late to save the tree. The best thing you can do is have it removed – and do it before the next generation of beetles emerges and spreads to nearby pines.
But removal alone isn’t enough.
The wood needs to be treated properly, or the beetles can still finish their life cycle and spread. Even after a tree has been cut down, live beetles and larvae remain under the bark. If the logs are left sitting, those beetles can emerge weeks later and attack healthy trees across your yard or neighborhood.
At LAM Tree, we follow treatment recommendations outlined by the Colorado State Forest Service to make sure the beetles don’t spread after removal. There are several ways to treat MPB-infested logs – each designed to kill larvae before they emerge in the summer:
Chipping
This is the most practical and widely used option, especially for residential or smaller-scale removals.
- Chipping breaks apart the bark and inner wood where beetles develop
- It kills larvae quickly and eliminates the need for storage or additional treatment
- It’s efficient, effective, and can be done on-site right after removal
Solar Treatment
In some cases – especially with larger trees or remote locations – solar treatment may be used.
- Logs are stacked in a sunny location and covered tightly with clear plastic
- For it to work, temperatures under the bark must reach at least 110 degrees Fahrenheit
- This method is labor intensive and isn’t suitable for all sites or seasons
Other Forestry-Approved Methods
While not commonly used in residential areas, these options are also effective:
- Peeling the bark by hand or machine to expose larvae to the elements
- Burning or scorching the logs in a controlled pile (best done when snow is on the ground and after checking local fire restrictions first)
- Burying logs under at least 8 inches of soil
Important: There are currently no approved insecticides for treating mountain pine beetle larvae in cut logs. That means physical destruction is the only effective approach.
How to Prevent Mountain Pine Beetle Attacks
It is possible to treat healthy trees that have not yet been attacked with a highly effective insecticidal spray.
This preventive insecticide is sprayed on pines before beetle flight occurs. The treatment stops beetles from feeding and laying eggs, effectively stopping infestation in treated trees.
Mountain pine beetle preventive treatments contain pyrethroids that are registered and have been tested for effectiveness. Although they cannot save an infested tree, they do reduce the likelihood of attacks on individual trees. Most residents in Evergreen, CO and the surrounding area choose to spray high-value trees, such as those near homes or businesses.
Always use a commercial pesticide applicator to ensure that your MPB treatment is properly applied.
If You’re Seeing Signs of Mountain Pine Beetle Damage, It’s Time to Call LAM Tree Service
If you’re seeing pitch tubes, boring dust, or blue-stained wood, it’s not something to wait on. That tree has already been compromised – and if it’s not removed and handled the right way, the beetles will keep spreading.
At LAM Tree Service, we know what to look for and how to deal with it. Whether you need confirmation, removal, or just peace of mind, we’re here to help you get ahead of the problem.
Call us at 303-502-2188 to learn more about mountain pine beetle treatment or to schedule an inspection.
