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    • Insects and diseases are a natural part of forest ecology. They can become pests during outbreaks or when they interfere with management goals, such as timber production, wildlife habitat, water quality, recreation and views. 

    Field Guide to the Diseases and Insect Pests of Northern and Central Rocky Mountain Conifers is a useful resource for identifying insects and diseases.
    Host: Ponderosa pine

    Ecology: Diplodia shoot and tip blight is a fungus that causes lesions on branches of ponderosa pine. Diplodia produces spores at the end of the summer that are spread by rain splash, insects, and wind. These spores infect young shoots the following spring, if conditions are sufficiently temperate and moist.

    Identifying Infected Trees:
    • Dead branches appearing as orange spots of foliage throughout tree crown
    • Grey needles remaining attached to the branch after death
    • Resinous lesions on branches
    • Small black spore-producing structures on needles and cone scales, often appearing as minute, black pustules
    Damage:
    Diplodia rarely causes mortality but can lead to death of individual branches. Diplodia infections can also be latent in trees and only erupt when triggered by drought or hail injury.

    Treatment:
    • Promote tree vigor to keep latent infections asymptomatic.
    • Avoid pruning trees during wet, temperate conditions.
    • Thin stands to reduce competition for water.
    • Do not fertilize susceptible trees; excess nitrogen can exacerbate infections.
    • Infections can reside in a shoot without symptoms for years therefore, removing infected material does not control disease.
    Additional Resources: 

    Host: DOUGLAS-FIR, with preference for trees that are mature, fire-scorched, infected with root disease, drought-stressed, windthrown or otherwise compromised.

    Life Cycle: Adults work in unison to attack single trees in mid-May. They bore into the inner bark and lay eggs along a long, vertical gallery. The eggs hatch into small, white larvae that bore outward from the main gallery and pupate into adults. They require one year to develop, with offspring ready to emerge and attack new trees the following spring.

    Identifying infested trees:

    • Reddish, brown boring dust accumulating in bark crevices
    • Distinct galleries etched into inner bark
    • Presence of brown adult beetles that are approximately the size of a grain of cooked rice, or presence of small white larvae (less than ¼ inch)
    • Crowns of infested trees may be green. Trees often turn red after beetles have left the tree.
    Damage: Douglas-fir beetle feeds in the inner bark and directly kills attacked trees.

    Treatment:
    • THIN STANDS to reduce competition and promote individual tree vigor.
    • BROOD TREE REMOVAL - Identify and remove beetle-infested trees. Trees may appear healthy but closer inspection will reveal boring dust in bark crevices.
    • PROMOTE non-host species such as ponderosa pine and western larch.
    • REMOVE or DESTROY infested material. Beetles will continue to develop in logs and firewood and, if left on site, can emerge the following spring to attack nearby trees. Preemptively remove windthrown, fire-scorched, or mature trees to limit brood material in stand.
    • Apply MCH pheromone capsules to stands by April 15th.
    Additional Resources:
    Host: Douglas-fir, grand fir, subalpine fir and spruce

    Life Cycle: Eggs hatch into caterpillars and begin feeding on needles in the spring just as buds burst. They pupate in late summer and emerge as adults August through September. Female moths do not fly; they emerge from their pupal case, mate, lay eggs, and die. Eggs overwinter in a mass encased with hairs from the mother’s body.

    Identification:
    • Hairy caterpillars with distinct tussocks along back
    • Gray, hairy pupal cases persistent on main stem of trees and at branch nodes Pupal cases may also be attached to buildings, furniture and other outdoor structures
    • Crowns appearing thinned, scorched or stripped of foliage
    Damage: Caterpillars feed on current and previous years’ growth, often defoliating the entire tree. Understory trees can be killed within one year of severe defoliation.

    Treatment
    • VIRUSES naturally develop in the population and control outbreaks within 3 years of onset.
    • THIN STANDS to reduce competition and promote individual tree vigor. Defoliation may still occur, but trees will be more likely to rebound after outbreak subsides.
    • PROMOTE non-host species such as pines (lodgepole and ponderosa) and western larch.
    • INSECTICIDES may be useful for short-term control of high value trees. Follow the pesticide label and use proper personal protective equipment. A licensed pesticide applicator is encouraged.
    Additional Resources:
    Host: Ponderosa pine 

    Ecology: Elytroderma needle cast is fungus that causes branch deformities, needle discoloration, and needle loss. Needles are infected by wind-born spores in late-summer/fall. The Elytroderma fungus enters the needles and grows into the twigs, becoming systemic within the tree and causing symptoms the following spring. Spores are produced mid to late summer the year after infection.

    Identifying Infected Trees: 
    • Brooms of bushy, round bunches of needles (commonly referred to as “witches brooms”)
    • Needles that are red/orange at the tips but green at the base.
    • Thin, black, elongated, spore structures growing at the base of needles
    • Shedding of needles from within the broom.
    Damage: 
    Elytroderma needle cast is the only needle cast in Montana that is lives within the twigs and reinfects every year. Death is uncommon and usually only occurs on small, young trees. The most common impacts of Elytroderma are growth loss and deformity.

    Treatment: 
    • Increase spacing between trees to promote drying of foliage between rains.
    • Remove trees with severe Elytroderma infections.
    • Shift to non-host species, such as Douglas-fir and larch, in areas where infections are chronic and severe.
    • Thin stands to increase residual tree vigor.
    Additional Resources:  
    Host: Grand fir, with preference for trees that are windthrown, drought-stressed or infected with root disease

    Life Cycle: Adult beetles emerge from trees and fly between July and August. Adults attack trees by boring under the bark and excavating a horizontal gallery, laying eggs along the margins. Eggs hatch into larvae that feed on the phloem until cold winter temperatures initiate dormancy. They resume feeding in the spring before emerging as adults later in the summer. Fir engraver beetles complete one generation per year.

    Identifying infested trees:
    • Light brown boring dust accumulating in bark crevices
    • Distinct galleries etched into inner bark
    • Presence of brown adult beetles that are approximately the size of a grain of cooked rice, or presence of small white larvae (less than ¼ inch)
    • Crowns of infested trees may be green. Trees often turn red after beetles have left the tree.
    Damage: Fir engraver beetles feed in the inner bark. They typically mass attack and kill trees outright but occasionally, small numbers of beetles attack trees and cause top kill, branch dieback, or patches of damage that the tree successfully heals over.

    Treatment:
    • PROMOTE non-host species such as ponderosa pine and western larch.
    • THIN STANDS to reduce competition and promote individual tree vigor.
    • BROOD TREE REMOVAL - Identify and remove beetle-infested trees. Trees may appear healthy but closer inspection will reveal boring dust in bark crevices.
    • REMOVE or DESTROY infested material. Beetles will continue to develop in logs and firewood and, if left on site, can emerge the following summer to attack nearby trees. Preemptively remove windthrown trees to limit host material in stand.
    Additional Resources:
    Host: Four species specific to particular host: lodgepole pine, Douglas-fir, larch, limber pine

    Ecology: Dwarf mistletoes are parasitic plants that grow on coniferous trees. Dwarf mistletoes cause bushy growths on branches called witches’ brooms. The brooms are lush and healthy while the rest of the tree competes for water and nutrients. Dwarf mistletoe plants produce seeds in the spring that are explosively shot up to 50 feet away.

    Identifying Infected Trees: 
    • Witches’ brooms on branches
    • Small mistletoe plants growing on infected tree branches (size and color vary slightly based on species)
    • Tapered branch swellings, often with plants growing from swollen area
    Damage: 
    Dwarf mistletoe plants draw nutrients from the tree and reduce growth. Severe infections can cause top-kill or ultimately kill the entire tree.

    Treatment: 
    • CLEAR a buffer strip of all host trees at least 30 feet from any infected tree(s).
    • PROMOTE non-host tree species. Note that each dwarf mistletoe is specific to a particular tree species. Shift stand composition to non-host species.
    • If regenerating stand to susceptible species, REMOVE infected trees from overstory within 7-10 years.
    Additional Resources:  
    Host: All pines including lodgepole, ponderosa, western white, limber and whitebark.

    Life Cycle: Adult beetles emerge from trees between June and August. Adults attack trees by boring under the bark and excavating a vertical gallery, laying eggs along the margins. Eggs hatch into larvae that feed under the bark until cold temperatures trigger dormancy. They overwinter as larvae and pupate into adults the following summer. Mountain pine beetle completes one generation per year in Montana, although at higher elevations they may require two years to reach maturity.

    Identifying infested trees:
    • Creamy orange masses of pitch, approximately the size and shape of popcorn, up main bole of tree (versus red turpentine beetle which is only the first 6 feet)
    • Orange-red boring dust accumulating in bark crevices
    • Distinct vertical galleries with a “J” at the base etched into inner bark
    • Presence of brown adult beetles that are approximately the size of a grain of cooked rice, or presence of small white larvae (less than ¼ inch)
    • Crowns of infested trees may be green. Trees often turn red after beetles have left the tree.

    Damage: Bark beetles feed in the inner bark and introduce blue-stain fungi which, together, girdle the tree.

    Treatment:

    • THIN STANDS to reduce competition and promote individual tree vigor. Trees “pitch out” beetles so adequate moisture is important for successful defense.
    • BROOD TREE REMOVAL - Identify and remove beetle-infested trees. Trees may appear healthy but closer inspection will reveal pitch tubes and boring dust in bark crevices.
    • DIVERSIFY age and size classes of pine in stands. Mountain pine beetle outbreaks are most severe in continuous stands of pine 80-120 years old.
    • REMOVE or DESTROY infested material. Bark beetles will continue to develop in logs and firewood and, if left on site, can emerge the following summer to attack nearby trees.
    • Apply verbenone pheromone capsules to stands by July 1st.
    Additional Resources:
    Host: Various fungi cause needle diseases on every conifer species in Montana.

    Ecology: Needle diseases are caused by fungi growing in needles, causing them to discolor, die, wilt or entirely fall off. Spores are produced on dead needles during cool wet springs and then splashed by rain onto new needles. Spores will not proliferate if conditions are not suitably wet and cool.

    Identifying Infected Trees: 
    • Spore-producing structures apparent on needles
    • Spotted or banded patterns of discoloration on individual needles
    • Discoloration of needles concentrated near base of crown.
    • Thinning foliage
    Other factors can cause needle discoloration:
    • Seasonal color change (western larch) or senescence
    • Insect feeding
    • Abiotic factors such as drought, heat, frost
    • Chemical damage including herbicides and magnesium chloride
    Damage: 
    Needle diseases are usually cosmetic and do not cause mortality. Subsequent years of infection may decrease tree vigor and growth.

    Treatment: 
    • Thin stands to increase air flow and reduce moist conditions in crown. Dense crowns retain moisture and promote needle diseases.
    • Remove heavily impacted trees during thinning.

    Additional Resources:  
    Bark beetles must mass-attack to overcome a tree’s natural defenses. They communicate via chemical messages, called pheromones, to recruit additional beetles. As the tree becomes too full to sustain additional beetles, they switch to an anit-aggregation pheromone that essentially signals “no vacancy” in the tree. These chemicals are commercially available in capsules to protect trees and stands from beetle attack. Pheromones are only effective in preventing beetle attacks; they cannot save a tree after it has been attacked.

    Pheromones to protect individual trees or in a grid for stand-level protection. Capsules last for only one season and will need to be replaced each year that protection is desired.

    Montana law requires a valid pesticide applicator/dealer license to sell or charge to apply pheromones. If you are interested in applying for an applicator or dealer license, please contact the Montana Department of Agriculture. Homeowners and landowners applying verbenone on their own property do not need a license to purchase or apply pheromones. Follow the pesticide label and use proper personal protective equipment!

    VERBENONE works against mountain pine beetle. Be sure to accurately identify mountain pine beetle versus western pine beetle or red turpentine beetle.
    MCH only works against Douglas-fir beetle.
    Host: All pine, particularly ponderosa and lodgepole pine

    Life Cycle: Adult pine engraver beetles attack fresh slash (or logs) in the spring. Offspring develop into adults within 2 months. These adults then emerge from slash/

    Identifying infested slash and trees:
    • Accumulations of fine red-brown boring dust on top or underneath infested slash (or logs).
    • Distinct multi-branched galleries with NO boring dust.
    • Presence of brown adult beetles approximately the size of a grain of cooked rice, or presence of small white larvae (less than ¼ inch)
    Damage: Pine engraver beetles generally inhabit slash or logs, only causing damage when the second generation emerges to attack small-diameter trees or tops of large trees.

    Treatment:

    • Create slash ONLY during August-December months, if possible.
    • Inspect slash piles, log decks, and windthrown trees for infestation. Chip, burn, or remove infested material.
    • Traps and interspersed slash piles might be useful in managing beetles on site. Contact your local service forester to determine whether these options could be effective in your specific situation.
    Additional Resources:

    Host: Preferentially attack ponderosa and lodgepole pines that are fire-scorched, diseased, or injured. 

    Life Cycle: Red turpentine beetle adults typically attack new trees in the spring. Development of offspring can vary in timing and overlapping generations can be present in single tree.  

     

    Identifying infested trees: 

    • Masses of red-orange pitch in lower 6 feet of main bole of tree 
    • Chamber excavated under bark (versus distinct galleries typical of other bark beetles) 
    • Presence of brown adult beetles approximately ¼” inch long (larger than other bark beetles), or presence of small white larvae (less than ¼ inch) 
    • Crowns of infested trees remain green.  

    Damage: Red turpentine beetles generally do NOT kill healthy trees.  

    Treatment: 

    • Avoid tree damage, particularly fire scorch, mechanical injury and soil compaction.  
    • THIN STANDS to reduce competition and promote individual tree vigor.  

     

    Additional Resources: 

    Host: Various root diseases infect conifers in Montana. Some of the most common diseases and their most susceptible hosts include: 

    Tomentosus – lodgepole pine, Engelmann spruce 

    Armillaria – Douglas-fir, grand fir 

    Laminated – Douglas-fir, grand fir 

    Heterobasidion (fir-type) – Douglas-fir, grand fir subalpine fir 

    Heterobasidion (pine-type) – Ponderosa pine 

     

    Ecology: Root diseases are caused by fungi that rot the root system of trees. The fungi spread underground through root contacts between trees. Conks can also disperse fungi as wind-borne spores and infect stands over long distances.  

    Identifying Infected Trees and Stands: 

    • Patches of trees that have died over a period of time, including older dead stumps and more recently dying trees.   
    • Crowns thinning from the interior; yellowing or dead needles  
    • Swelling at base of the trunk, sometimes accompanied by excessive resin production 
    • Upturned trees with roots broken near trunk. Trees that have fallen over with roots broken close to the trunk 
    • Abundance of cones produced as a response to stress 
    • Rounded, flat or shrubby crowns due to reduced growth 
    • Above-ground symptoms are usually not apparent until the root system is 50% or more colonized.  

    Damage: 

    Root diseases affect root systems and degenerate the tree’s mechanical support, making it more susceptible to windthrow. The fungi also compromise nutrient and water uptake, resulting in significant growth loss and mortality of susceptible species. 

    Treatment: 

    • Remove susceptible tree species and favor more tolerant species 
    • Plant only tolerant species, expect some mortality of young trees of all species and overplant if losses are unacceptable 
    • Avoid treatments that leave susceptible species on site 
    • Broadcast burns will not eliminate root disease on a site but can be useful in long-term species conversion 

    Additional Resources:  

    Host: Ponderosa and lodgepole pines 

    Ecology: Western gall rust produces round swellings on the branches or cankers on the main stem. Dry, powdery spores are produced on galls in spring. These spores are carried by the wind to other pines or other branches in the same tree. 

    Identifying Infected Trees: 

    • Round swellings/galls on branches  
    • Bright orange spores on galls in spring  
    • Dead branches, visible from a distance as orange or brown needles 
    • Flared cankers on main stem, commonly referred to as “hip cankers” 

    Damage: 

    Western gall rust induces the growth of galls on branches and sometimes stems of trees. Branches with galls eventually die. Severely infected trees might die from excessive branch loss. Galls on stems create a weak point on the trunk, making it more susceptible to snap at the gall.  

    Treatment: 

    • THIN STANDS to reduce competition and promote individual tree vigor 
    • Promote non-host tree species 
    • Prune out infected branches to allow space for healthy branches to grow.  

     

    Additional Resources:  

    Host: Ponderosa pine, only known to occur west of Continental Divide. 

    Life Cycle: Western pine beetle can produce 2 generations per year in Montana. Adults attack trees in June and late August. Eggs hatch into larvae that initially fee in inner bark but then migrate toward outer bark.  

     

    Identifying infested trees: 

    • Creamy orange masses of pitch up main bole of tree (versus red turpentine beetle which is only the first 6 feet). Not always present, especially when tree is drought-stressed.  
    • Orange-red boring dust accumulating in bark crevices 
    • Circuitous galleries etched into inner bark  
    • Presence of brown adult beetles that are approximately the size of a grain of cooked rice, or presence of small white larvae (less than ¼ inch) 
    • Bark flaked off tree by woodpeckers feeding on beetles 
    • Crowns of infested trees may be green. Trees often turn red after beetles have left the tree.  

    Damage: Bark beetles feed in phloem and bark, and introduce blue-stain fungi. 

    Treatment: 

    • THIN STANDS to reduce competition and promote individual tree vigor. Trees “pitch out” beetles so adequate moisture is important for successful defense.  
    • BROOD TREE REMOVAL - Identify and remove beetle-infested trees. Trees may appear healthy but closer inspection will reveal pitch tubes and boring dust in bark crevices.  
    • Avoid soil compaction in stands with mature ponderosa pine.  
    • REMOVE or DESTROY infested material. Bark beetles will continue to develop in logs and firewood and, if left on site, can emerge the following summer to attack nearby trees.  

     

    Additional Resources: 

    Host: Douglas-fir, grand fir, subalpine fir, spruce and western larch (occasionally feeding on pines during outbreaks). Outbreaks are most severe on warm, dry sites with dense, multi-layered canopies of host trees. 

    Life Cycle: Caterpillars feed on needles, and sometimes buds, in the spring. Later in the summer, they spin silken shelters in which they feed and pupate. Copper-colored moths emerge in August, mate, and lay eggs on the underside of needles. Eggs hatch in late summer and young caterpillars overwinter in bark crevices. 

    Identification:  

    • Smooth olive-brown caterpillars with pairs of cream-colored dots along back 
    • Small, copper-colored moths most visible in August 
    • Chewed needles bound together by silk on branch tips and terminal leader 
    • Crowns appearing thinned, scorched or stripped of foliage 

     

    Damage: Caterpillars primarily feed on new needle growth which limits tree vigor but does not necessarily cause long-term damage. Subsequent years of heavy defoliation can cause branch dieback and top kill. Severely defoliated trees in the understory can be killed within a single year.  

     

    Treatment 

    • THIN STANDS to reduce competition and promote individual tree vigor. Defoliation may still occur but trees will be more likely to rebound after outbreak subsides.  
    • CREATE a single layer canopy to interrupt dispersal of caterpillars from tree tops to understory.  
    • PROMOTE non-host species such as pines (lodgepole and ponderosa) and western larch. 
    • INSECTICIDES may be useful for short-term control of high value trees. Follow the pesticide label and use proper personal protective equipment. A licensed pesticide applicator is encouraged.  

    Additional Resources: