Mountain Pine Beetles

Quick Facts About Mountain Pine Beetles in Colorado

  • Mountain pine beetle is the most important insect pest of Colorado's pine forests. Pine beetles kill large numbers of trees annually during outbreaks.
  • Trees that are not growing vigorously due to old age, crowding, poor growing conditions, drought, fire or mechanical damage, root disease, and other causes are most likely to be attacked.
  • For a long-term remedy, thin stands of susceptible pine trees with an emphasis on leaving well-spaced healthy trees.
  • For short-term controls, spray, burn, and peel attacked trees to kill the beetles.
  • Preventive insecticide sprays can protect only green, uninfested trees. The typical treatment window is from mid-May to late July.
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    Group of ponderosa pines killed by mountain pine beetle.

    Mountain Pine Beetle larvae under the bark of a Ponderosa pine

    About the Mountain Pine Beetle

    Mountain pine beetle (MPB), Dendroctonus ponderosae, is an insect native to the forests of western North America. Previously called the Black Hills beetle or Rocky Mountain pine beetle, periodic outbreaks of the insect can kill millions of trees.

    Since the mid-1990s, mountain pine beetle has affected roughly 80%, or about 3.4 million acres, of ponderosa-lodgepole pine in Colorado.

    2020 report on the health of Colorado's forests

    Outbreaks develop irrespective of property lines, being equally evident in wilderness areas, mountain subdivisions, and back yards. Even windbreak or landscape pines many miles from the mountains can succumb to beetles imported in infested firewood.

    Tree Species Attacked by Mountain Pine Beetles

    As the name suggests, mountain pine beetles infest pine trees. Pines most commonly attacked include:

    Bristlecone, Scots (Scotch), and pinyon pine are less commonly attacked.

    However, when beetle populations explode, all pine species found in Colorado are susceptible to attack.

    A related insect, the Douglas-fir beetle (Dendroctonus pseudotsugae), occasionally damages Douglas-fir.

    Signs & Symptoms of Mountain Pine Beetle Infestation

    Look for these signs that mountain pine beetles have attacked.

    • 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 

    Top and side view of adult mountain pine beetles.

    Adult mountain pine beetles. Image courtesy of USDA Forest Service - Region 2 - Rocky Mountain Region , USDA Forest Service, Bugwood.org.

    Which Trees Are Most Likely to be Attacked?

    During the early stages of a mountain pine beetle outbreak, attacks are limited largely to trees in poor health. This includes trees that are under stress from:

    • injury,
    • poor site conditions,
    • fire damage,
    • overcrowding,
    • root disease, or
    • old age.

    Most often, mountain pine beetle outbreaks are associated with previous injury by the western spruce budworm (Choristoneura occidentalis). This highly destructive foliage insect is found in Colorado and the western areas of the U.S. and Canada.

    >> Learn how to identify and get rid of the spruce budworm

    However, as beetle populations increase, mountain pine beetle attacks usually spread to involve most trees in the outbreak area, including healthy, well-cared-for pines on residential properties.

    NOTE - Injured pines also can be attacked by the red turpentine beetle (Dendroctonus valens).

    Timing - When Mountain Pine Beetles Attack

    Adults beetle 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.

    Effects of Drought on Vulnerability to Attack by MPB

    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 the decrease in snow and rain that we've seen in recent years in the Colorado Front Range, trees aren't getting the moisture they need. This makes them more susceptible to mountain pine beetle attacks.

    To make things worse, periods of drought often last several years. During that time, bark beetle populations may build to epidemic levels.

    Adult MPB excavating a tunnel through oozing pitch in a ponderosa pine.

    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 Identify Pine Beetles

    Pitch tube forming on Ponderosa pine

    You can distinguish mountain pine beetles, and other bark beetles in the genus Dendroctonus, from other bark beetles by the shape of the hind wing cover.

    If you look at the hind wing cover from the side, it is gradually curved. In contrast, the wing cover of Ips or engraver beetles, another common group of bark beetles attacking conifers, is sharply spined. And in Scolytus beetles, such as the shothole borer and European elm bark beetle, the area under the wing cover (abdomen) is indented.

    What to do with Trees Infested with MPB

    There is no treatment to "cure" trees that have been infested by mountain pine beetles. The only solution is to cut down the trees and treat the logs to kill off any beetles that are still present.

    The key to preventing further spread is to remove infested trees before adult insects emerge.

    NOTE - Removing infested trees will have no impact overall in an area experiencing a severe mountain pine beetle outbreak. However, beetle-killed trees are a safety and wildfire hazard so it's always best to remove them ASAP.

    When treating pine beetle-infested logs, the most common treatment used to involve the use of the insecticide "Lindane." Sometimes the logs were covered with plastic as well. However, since the major beetle outbreak in the 1970's, Lindane has been banned in most forms.

    Because of this, and to be more environmentally friendly, we now use a solar treatment on infested logs as outlined by the U.S. Forest Service. Infested trees are cut down and the cut logs are stacked in an area that gets full sun. The logs are then tightly covered with clear plastic. This type of "solar treatment" of infested trees creates conditions unsuitable for the survival of mountain pine beetle larvae.

    Preventing Mountain Pine Beetle Attacks - Insecticidal Sprays

    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. 

    Call us at 303-674-8733 to learn more about mountain pine beetle treatment.

    MPB larvae and tunnels in a lodgepole pine.

    MPB larvae and tunnels in a lodgepole pine. Image courtesty of Dave Powell, USDA Forest Service (retired), Bugwood.org.

    Pine Beetle Identification is Easiest in Winter

    Pitch tubes caused by Mountain Pine Beetle

    The best time to check for mountain pine beetle infestation is in winter.

    Check for "pitch tubes" on the trunks of your trees. If you see pitch tubes, cut a piece of bark off and see if the wood under the bark is discolored bluish-gray.

    If you see this bluish-gray color under the bark, the tree(s) must be removed. Not doing so will allow the beetle to spread rapidly, not only to other trees on your property but to trees throughout the entire area.

    The Certified Arborists at LAM Tree Service are available in winter (and all year round) to inspect your property for beetles. We charge an hourly fee for this service.

    Video - What to Look For

    If you'd rather try to identify these destructive pests yourself (and to see how we do it when we're called to evaluate pines on your property), check out the brief video below. In it, Ryan Reed (owner of LAM Tree Service) shows you how to remove the bark and points out the beetle galleries beneath.