Conifer Bark Beetles

Insect Profile

What Are Bark Beetles?

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Bark beetles are very small beetles that feed on the inner bark of trees with severe consequences. At less than one-third of an inch or smaller, they are smaller than other beetles such as longhorn beetles and roundheaded/metallic woodboring beetles, which infest the inner wood of trees [1]. In this article, we focus on bark beetles that affect evergreen conifers. Look for another article on bark beetles affecting broadleaved trees.

What types of trees are susceptible?

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In the intermountain west, there are about 18 common species of bark beetles affecting evergreen conifers including pine, pinyon, fir, douglas fir, spruce, juniper, and cypress [1]. Bark beetles prefer trees weakened by stress [1].

Trees in our urban forest are commonly stressed, making them targets for bark beetles [1]. Healthy trees are more resistant to bark beetles because they produce a sap-like substance called resin (or pitch) that inhibits attacks [2]. Stressed trees do not produce as much resin and are more susceptible to bark beetles [2]. However, healthy, unstressed trees can be overwhelmed by “mass attacks” when populations are high [1][2].

Large trees and older trees are usually more susceptible, while trees less than 3” diameter are less susceptible to bark beetles like Mountain Pine Beetle [1][3].

Insect life cycle

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Adult “pioneer” beetles search for stressed trees. When they bore into the bark and begin feeding on the phloem, chemicals are released, signaling to other members of the species that a suitable host has been found [1].

They continue to bore into the phloem-cambial region (inner bark), create mating chambers called “parental galleries”, and lay eggs [1][4]. Larvae hatch from eggs and spread outward, feeding on, and girdling the inner bark [1]. They will continue to exhaust a host tree until its phloem layer is dried up [1]. Adults will lay eggs in parental galleries or bore out of the bark, making exit holes, and move to nearby trees [1].

Most conifers are affected by bark beetles that have multiple generations per year [1]. Many bark beetles also carry a fungus that inhibits water transport [1].

Consequences and Severity

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Bark beetles are a significant cause of tree death in forests and urban forests [1]. Bark beetles girdle part of a tree’s vascular system that is necessary to transport water, nutrients, and carbohydrates [1][5]. Because they are small, damage from individual beetles is not severe, but their mass attacks can overwhelm and quickly kill healthy trees [1]. Most trees are vulnerable to attacks at all times during a growing season due to multiple, overlapping generations of bark beetles [1]. 

Diagnostic Clues

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Usually by the time a tree shows obvious signs of bark beetle damage, it is already dead, even if it still appears to be alive. The most obvious symptom is crown fading, the fading of foliage color from normal green to off-green, yellow, brown, or red [1]. If this is confirmed to be caused by bark beetle infestation, it is irreversible and a tree can be pronounced dead [6].

Crown fading can begin immediately or be delayed by cold weather [1]. Late season attacks may not show crown fading until the following growing season [1]. If there is crown fading, search for other clues to confirm the presence of bark beetles, and check the condition of the phloem [1]. 

Other signs and symptoms of bark beetles include frass in crevices of bark or on the ground, and pitch tubes from where bark beetles bore into bark [1]. Pitch tubes are deposits of resin, a sticky sap-like substance that is produced in the bark of a tree and specifically used to bandage wounds when an injury occurs and to defend themselves against bark beetles [1][7]. Resin is different from regular sap in that it is not water-soluble and starts to harden when exposed to air [7]. Small pitch tubes can be used to confirm the presence of bark beetles, but should not be confused with larger pitch masses created by Sequoia Pitch Moth [1]. Trees may continue to die from the top down such as those attacked by Ips beetles [1]. 

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If frass and pitch tubes are found, check for healthy phloem (inner bark). Loose or detached bark is a bad sign. If it doesn’t detach easily, leave it alone and consider it a good sign. If the entire circumference of the phloem is brown, black, or dried up, then the tree is dead [1]. Feeding tunnel patterns called “galleries” may be present, as well as exit holes, and confirm the presence of bark beetles as well [1].

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