Module 3 – Bark Beetles

Learning Objectives

  • To learn what bark beetles are and how they damage trees.
  • To learn bark beetle groups in Alaska.
  • To learn signs and symptoms of bark beetle activity.
  • To learn potential bark beetle threats to Alaska.
  • To become familiar with the vocabulary of bark beetles.

What are bark beetles and how do they damage trees?

Bark beetles live and feed between the bark and wood of trees and shrubs. Different species of bark beetles may be found on the trunk, twigs and branches, or roots of trees. The most destructive bark beetles attack the main stem of living trees. Bark beetles are found in forests throughout the world and in a wide variety of host species.

There are several important pest species of bark beetles that, in outbreak conditions, will attack and kill otherwise healthy trees. These are considered primary pests. In Alaska, the most notorious is the spruce bark beetle, Dendroctonus rufipennis. However, like wood borers, most bark beetles attack weakened, dead, or dying trees and are considered secondary pests.

Even primary pest species prefer trees stressed by drought, disease, injury, or improper care. Wind damage is a common disturbance factor in forests. Wind damaged trees are commonly attacked by bark beetles, and large wind events that down trees often result in increased bark beetle populations in an area due to the large-scale availability of host material. There are other factors that contribute to bark beetle population increases and often many of these factors are working together.

Downed trees as a result of wind storms serve as good habitat for bark beetles and can contribute to population increases. Photo by Milan Zubrik, Forest Research Institute – Slovakia, Bugwood.org

Damage from bark beetles is the result of tunneling and feeding between the bark and wood of the tree. A diagram of the components of a tree can be found here. This area contains phloem, tissue that is responsible for conducting sugars produced by the tree and used for food within the tree. Disruption and damage of this tissue impacts the tree’s ability to feed itself and is referred to as girdling. Tree mortality is due to starvation. In a living tree, many years of repeated attack may be required before the tree is actually killed. Mass attacks during outbreak conditions can kill trees in less time.

2181027-Darren Blackford_USDA Forest Service_Bugwood org

A bark beetle gallery beneath the bark damages the phloem and disrupts the tree’s ability to feed itself. The vertical galleries are egg laying galleries while the horizontal galleries are the result of larval feeding. Photo by Darren Blackford, USDA Forest Service, Bugwood.org

Bark beetles can also vector fungi to trees. The beetles may feed on the fungi in order to gain nitrogen in their otherwise nutrient poor diet. Fungi may also aid beetles by exhausting tree defenses more rapidly than beetle feeding alone. The fungi can also be damaging to the tree by clogging xylem tissue, the tissue responsible for upward transport of water and nutrients, contributing to the girdling of trees or by degrading wood that may be used for wood products or timber.

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Blue stain fungi in the sapwood of a Norway spruce. Photo by Louis-Michel Nageleisen, Département de la Santé des Forêts, Bugwood.org

Bark beetles are members of the weevil family, Curculionidae. They are small and cylindrical and can range in size from 1mm to 8mm in length. Adults are typically dark in color, ranging from reddish-brown to black. Because of their size, it can be very difficult to see distinguishing features on bark beetles without magnification. Because of their habit of living under the bark, beetles themselves (any lifestage) are not likely to be seen before the damage they cause is observed.

Two features that can help distinguish bark beetles from other groups of beetles are the antennae and the eyes. Bark beetle antennae are elbowed (bent at a 90° angle) with the last few segments enlarged like a club. This characteristic is common to all members of the weevil family (Curculionidae). Within the weevil family, bark beetles can be distinguished from their next closest look-alikes by the shape of their eyes. Bark beetles eyes are elongated and oval-shaped while similar beetles’ eyes are round. More information and images on distinguishing bark beetles from other closely related beetles can be found at this site.

The most common tree-killing bark beetles in Alaska are in the genera Dendroctonus and Ips.

Dendroctonus: Dendroctonus species are considered the most destructive group of bark beetles in North America. There are three species of Dendroctonus in Alaska. The most common and most damaging is Dendroctonus rufipennis, the spruce bark beetle. To learn more about this native species see these resources from the US Forest Service and Cooperative Extension.

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Spruce bark beetle adults are about ¼ inch long and ⅛ inch wide. Photo by Steven Valley, Oregon Department of Agriculture, Bugwood.org

Ips: Ips species are considered the second most destructive group of bark beetles after Dendroctonus. There are five species of Ips in Alaska. The most common and most damaging is Ips perturbatus, the northern spruce engraver. To learn more about Ips see this resource.

NSE adult

Northern spruce engraver adults are about ⅛ inch long. Photo by Pest and Diseases Image Library, Bugwood.org

 


Life history

Bark beetles have diverse and varied life cycles depending on the species and location of the pest populations. Adults bore through the bark and make an initial gallery between the bark and the wood where the eggs are laid.

Bark beetle larvae are off-white and grub-like. It is very difficult to distinguish bark beetle species in the larval stage.

Bark beetle eggs in a gallery. Eggs are creamy white and very small. Exact size depends on the species. Photo by William M. Ciesla, Forest Health Management International, Bugwood.org

Eggs hatch and the larvae mine galleries that branch out from the initial egg gallery. The larval feeding is the primary cause of damage from pest species. Different genera and species of bark beetles may produce distinct galleries.

When larvae are mature they pupate and emerge as adults. The majority of bark beetles’ lives are spent beneath the bark. Only adults emerge from the tree and usually it is in the spring to find a new host. Some species emerge as adults in the fall and drop to the soil where they spend the winter insulted from the cold and emerge in the spring to find new host trees.

Bark beetles will attack conifer or hardwood trees depending on the insect species. The native bark beetle pests in Alaska and the potential invasive species of concern are all conifer, mostly spruce, pests. All of our native and ornamental spruce trees are potential hosts for invasive bark beetles.


Signs and symptoms of bark beetles

When we talk about signs and symptoms in forest health, we are referring to the clues used to help diagnose a tree health problem.

Signs are physical evidence of the pest itself. Examples of signs include:

  • Insect or animal evidence
    • Holes in bark or chewed in leaves
    • Frass
    • Actual insects present causing damage
  • Fruiting bodies of fungi
    • Conks
    • Mushrooms
  • Bacterial ooze

Signs of bark beetle activity include:

  • Boring dust accumulating in bark fissures or at base of tree (finer than that of wood borers, also usually reddish-brown in color)
  • Galleries beneath the bark
  • Actual beetles (any lifestage)

 

Symptoms are the plant’s response to a pest. Examples of symptoms include:

  • Leaf discoloration, spots, or distortion
  • Branch dieback
  • Stem lesions
  • Galls
  • Dead stems

Symptoms of bark beetle activity include:

  • Fading needle color. Color usually changes from green to yellow to reddish-brown.
  • Pitch tubes on main stem
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A pitch tube resulting from a bark beetle attack. Photo by Darren Blackford, USDA Forest Service, Bugwood.org

Symptoms for multiple pests/disorders can be similar. Never confirm a diagnosis on symptoms alone!


Invasive bark beetles

Bark beetles can be unintentionally moved from place to place in several ways. Common pathways for introduction include:

  • In solid wood packing material
  • In living plants
  • In firewood
  • In bark-on wood products

Invasive bark beetles are challenging in similar ways as other invasive species. Invasive bark beetles may overlap in host preferences and timing with native bark beetles and other pests making it difficult to distinguish or isolate damage between different pests. Invasive bark beetles may also vector potentially invasive pathogens, and invasive bark beetles lack natural enemies in their new location that would help to keep their populations in check. Additionally, bark beetles are very small and invasive ones may be difficult to distinguish from our native bark beetle species.

A few invasive bark beetles threaten Alaskan forests. While none have been detected or currently exist in Alaska, they all pose a threat and could be introduced through travel or trade.

An important bark beetle of concern for Alaska’s forests is the European spruce bark beetle, Ips typographus.

Ips typographus infestation (the yellow-ish tan trees) in Norway spruce. Photo by Daniela Lupastean, University of Suceava, Bugwood.org

The European spruce bark beetle (ESBB) is considered one of the most destructive spruce pests in Europe. While ESBB is not established in the United States, it has been detected at ports and is a target of Early Detection Rapid Response (EDRR) trapping efforts in the state.

ESBB attacks spruce, pine, larch, and fir hosts, though they prefer spruce, and attacks mainly occur in the lower to middle parts of the stem.

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Ips typographus adult. Adult beetles are between ⅛ and ¼ inch long. Photo by Jim Stimmel, Pensylvania Department of Agriculture, Bugwood.org

ESBB adults are around 3/16 inch long, cylindrical, brown beetles. The larvae are white, legless grubs similar in size to the adults. Because of their size and the general difficulty of identifying bark beetles, species-level identifications must be made from adult specimens by trained taxonomists. Several species native to Alaska may look similar to the European spruce bark beetle.

Signs and Symptoms of ESBB include:

  • Pitch tubes and boring dust on bark surfaces
  • Fading foliage in the crown (green to yellow to reddish-brown)
  • Galleries under the bark
  • Blue-stain in woody tissue under bark
  • Round exit holes (~1/10 inch diameter)

These signs and symptoms are fairly consistent for most bark beetles. Native species producing similar symptoms include the spruce bark beetle and the northern spruce engraver.

Our native spruce bark beetle will differ significantly by size. Spruce bark beetles are longer and wider and will not have the distinct declivity (scooped out rear end) that is characteristic of Ips and a few other closely related genera.

ESBB is most likely to be confused with one of our native Ips species. We have 5 Ips species in Alaska. Northern spruce engraver, Ips perturbatus, is the species most likely to cause damage and is common in interior Alaska, though it can be found throughout the state where white spruce grows. Northern spruce engraver is smaller than ESBB, between 5/32 and 3/16 inches long.

While ESBB attacks middle parts of the main stem, Northern spruce engraver attacks and kills the upper portion of the main stem. Northern spruce engraver will also attack and kill small diameter trees.


Scouting for invasive bark beetles

It will be difficult to distinguish invasive bark beetles from their native counterparts. The key for scouting for invasive bark beetles will be to note unusual features of damage such as

  • Rapid and aggressive spread
  • Unusual timing of damage
  • Damage to newly planted trees that may have originated out of state.

When assessing tree health there are some things to keep in mind that could help to find or identify invasive bark beetles early. For bark beetles, look for symptoms in the late spring and early summer resulting from an infestation in the previous year. Look for signs later in the summer.

Beetles: Larvae, pupae, and adults are the most likely lifestages to be noticed. They can all be found beneath the bark of the trees. The exact lifestage present may vary depending on the time of year. Adults can also be found flying or boring into trees in the summer. The native spruce bark beetle typically flies from May to July but that can change depending on the weather. If you notice actively boring or flying beetles and are unsure of the identity of the beetle it is best to collect a specimen and submit it for identification.

Pitch tubes/Entrance and Exit holes: Look for pitch tubes in the middle to lower part of the trunk and boring dust collecting around the base of the tree or in fissures of the bark. Exit holes may also be visible in the bark and will be round and approximately 1/10 inch in diameter.

Crown symptoms: Crown symptoms will be similar for invasive bark beetles as they are for our native pest species. Needles on infested trees will fade from green to yellow to reddish-brown. These symptoms may be noticed early in the growing season and result from damage done the previous year, or they may be noticed later in the growing season if an infestation is severe enough and damage to the tree happens quickly.


Reporting and submitting specimens

If you find insects that you do not recognize as native pests, it is best to collect the specimen and submit it for identification. Insects are quick and can be elusive. If you don’t think you can capture the specimen, try photographing it. For physically collected specimens, please kill them before submitting for identification.

Insects can be killed by placing them in the freezer for a night or two or by drowning them in alcohol (70% rubbing alcohol will work). Insects can then be placed in a tightly sealed, sturdy container and brought or mailed to your local Cooperative Extension office.

Please group specimens by the location/tree they were collected from. Do not put specimens from different locations in the same container. Please include collection information with each container submitted.

An important aspect of insect specimens that people often forget is the importance of collection information. When you collect a specimen, please make note of the following data:

  1. Location information – Country, state, location
  2. Latitude/Longitude – If possible collect this with a GPS unit and include +/- accuracy. If you do not have a GPS unit, a physical address is second best.
  3. Habitat description, host plant, collection method used (e.g. hand collected, sweep net, trap)
  4. Date and collector

Example:

  1. USA: Alaska: Anchorage
  2. 61.205825, -149.88631 +/- 10m
  3. Ornamental landscape, lilac shrub, hand collected
  4. 13JUL 2015 M.J. Moan

If you do not have physical specimens, photos of insects or suspicious damage are welcome and can be submitted through our online Pest Portal or to uaf-foresthealth@alaska.edu.

Digital photos can be great tools for insect identifications. Keep in mind, though, that photos are not without their limitations. The tips below can help improve the likelihood of getting an accurate and quick diagnosis when submitting digital photos.

  • Check photos for clarity before submitting
  • Submit original, unedited photos
  • Include a size reference in your photo (coins and pencils/pens work great if you don’t have a ruler available)
  • Include as much information about the subject as possible
  • Include the same collection information as if you were submitting a physical specimen.

Summary

  1. Bark beetles live and feed between the bark and wood of trees and shrubs.
  2. Larval feeding damages the phloem and limits the tree’s ability to transport food and energy to where it is needed within the tree.
  3. Bark beetles typically attack weakened, dead, or dying trees; however some species can attack otherwise healthy trees if the conditions are right.
  4. Invasive bark beetles are a threat to Alaska’s forests because:
    • They lack natural enemies
    • Are able to exploit resources
    • They have known pathways for introduction
  5. Signs and symptoms of bark beetles include:
    • Changing needle color: green to yellow to reddish brown
    • Presence of any lifestage of the insect
    • Pitch masses on the trunk
    • Boring dust collecting at the base of the tree or in bark fissures
  6. Invasive bark beetles can reach Alaska through:
    • Live plants for the plant trade
    • Firewood
    • Bark-on wood products
  7. Invasive bark beetle threats to Alaska include:
    • European spruce bark beetle, Ips typographus

Quick guide

Download and print this summary of bark beetle information for reference.

2016 Fact Sheet_ESBB


Vocabulary

  1. Boring dust: Sawdust-like material that results from the activity of the pest boring within the tree.
  2. Frass: Insect excrement, for boring insects excrement is often mixed with wood fragments.
  3. Gallery: a tunnel created by an insect usually directly beneath the bark, though sometimes deeper into the interior of the tree, for the purpose of egg laying of as a result of larval feeding.
  4. Girdle: the act of damaging the cambium completely around the circumference of the stem, root or branch, typically causing death of the tree or tissue beyond the point of girdling.
  5. Phloem: vascular tissue that conducts synthesized food through the plant, located adjacent to the outside of the cambium in trees, essentially the inner bark.
  6. Pitch tube: tube-like or mass-like accumulation of pitch around a bark beetle entrance hole on the bark of a tree.
  7. Signs: physical evidence of a pest.
  8. Symptoms: a plant’s response to a pest.
  9. Xylem: Vascular tissue that conducts water and mineral salts, taken in by roots, throughout the plant, essentially the woody part of the stem or trunk.

Sources and more information

  1. Furniss, R. L. and V. M. Carolin. 1977. Western Forest Insect. USDA Forest Service Misc. Pub. 1339, 654 pp.
  2. Dreistadt, S. H. 2004. Pests of Landscape Trees and Shrubs: An Integrated Pest Management Guide. Publication #3359, ANR Publications, University of California, 6701 San Pablo Avenue, Oakland, CA 94608-1239. 501pp.
  3. Holsten, E., P. Hennon, L. Trummer, J. Kruse, M. Schultz, and J. Lundquist. 2009. Insects and Diseases of Alaskan Forests. USDA Forest Service, Alaska Region R10-TP-140, 246 pp.
  4. Noma, T., M. Colunga-Garcia, M. Brewer, J. Landis, A. Gooch, M. Philip. 2010. European spruce bark beetle Ips typographus. Michigan State University’s invasive species fact sheets. Last accessed February 26, 2016, from http://www.ipm.msu.edu/uploads/files/Forecasting_invasion_risks/europeanSpruceBarkBeetle.pdf.
  5. Burnside, R. E., E. H. Holsten, C. J. Fettig, J. J. Kruse, M. E. Schultz, C. J. Hayes, et. al. 2011. Forest Insect and Disease Leaflet 180: Northern Spruce Engraver. USDA Forest Service, Pacific Northwest Region. R6-RO-FIDL#180-11/008. Last accessed July 27, 2016, from http://dnrc.mt.gov/divisions/forestry/docs/assistance/pests/fidls/180.pdf.
  6. Ips typographus (European Spruce Bark Beetle) – Fact Sheet. January 23, 2012. Canadian Food Inspection Agency. Last accessed February 26, 2016, from http://www.inspection.gc.ca/plants/plant-pests-invasive-species/insects/european-spruce-bark-beetle/fact-sheet/eng/1327356236249/1327365288030.
  7. Cavey, J., S. Passoa, and D. Kucera. 1994. Screening aids for exotic bark beetles in the Northeastern United States. USDA Forest Service, Northeastern Area. NA-TP-11-94.
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