Bark Beetle Signs: All You Need to Know for Healthy Trees

Bark beetles are tiny insects that bore into tree bark, causing significant damage and even killing trees. Being able to identify the signs of a bark beetle infestation is crucial to protect your trees and manage the problem effectively.

One common sign of bark beetles is the presence of small holes in the bark, often accompanied by sawdust-like material called frass.

Additionally, you might notice the tree’s foliage turning yellow, then orange, and finally red as the infestation progresses.

In some cases, woodpeckers may be seen flaking away the bark in search of beetle larvae, which can be an indirect indication of an infestation.

By understanding these signs, you can take action to safeguard the health of your trees and the surrounding environment.

Identifying Bark Beetle Signs

Holes in the Bark

Bark beetles create small, round holes in tree bark as they tunnel inside, which are signs of their infestation.

For example, holes from the spruce beetle are often red-brown filled with boring dust. These holes vary in size depending on the species and could be about the size of a pencil tip.

Pitch Tubes and Sawdust

Pitch tubes are blobs of resin that trees produce in an attempt to expel invading beetles.

Bark beetles also create sawdust-like material called frass as they tunnel into trees.

Bark beetle frass often accumulates around the trunk base or in bark crevices, and its color varies between species.

Comparison between species:

Species	Pitch Tube Color	Frass Color
Spruce Beetle	Clear or white	Red-brown
Pine Beetle	Pinkish-yellow	Cream
Fir Beetle	Grayish-green	Brown

Woodpeckers and Other Wildlife

Woodpeckers and other birds feed on bark beetle larvae, creating visible signs of their presence.

Flaking bark by woodpeckers, for example, is an indication of spruce beetle attack.

The presence of other wildlife, such as squirrels or bluejays, is also a potential sign of bark beetle activity in trees.

Keep an eye for:

• Flaking and damaged bark
• Woodpeckers and blue jays presence
• Squirrels feeding on infested bark

Damage and Effects of Bark Beetle Infestations

Cypress, Elm, and Other Tree Species

Bark beetle infestations can affect various tree species, including cypress and elm trees. Some signs of infestation include:

• Boring dust around the tree base
• Bark flaking from woodpeckers feeding on beetles
• Egg and larval galleries on the tree bark¹

Infestations can lead to weakened trees, making them more susceptible to other pests and diseases.

Infestations in California and U.S. Forests

The bark beetle outbreak has caused significant damage to forests across the United States, especially in California². Factors that contribute to infestations include:

• Warming temperatures
• Drought
• Weakened trees from previous infestations

As a result, forests experience increased tree mortality and reduced overall health.

Drought, Fungus, and Weakened Trees

Drought-weakened trees suffer from reduced natural defenses, making them more prone to bark beetle attacks².

Furthermore, bark beetles often carry fungi, which can:

• Infect trees and disrupt their water transport system
• Exacerbate stress from the drought
• Accelerate tree decline and mortality

ConnectionState	Severity of Infestations
Healthy	Low risk of infestation
Drought-affected	Moderate to high risk
Weakened by fungus	High risk of infestation

Bark Beetle Lifecycle and Galleries

Eggs

Bark beetles lay eggs in the inner bark, typically near the base of the tree. Female beetles create tunnels, called galleries, where they lay eggs. Some examples of galleries include:

• Winding patterns for individual species
• Tunnels filled with boring dust, called “frass”

Once the eggs hatch, the larvae begin to feed on the inner bark.

Larvae, and Pupa Stages

Larvae feed on the inner bark, creating more galleries as they grow. These galleries can become visible on infested trees.

After feeding and growing, the larvae enter the pupa stage, during which they transform into adults.

Adult Bark Beetles and Girdling

Adult bark beetles emerge from their pupal cases and start girdling the tree.

Girdling is the process by which a beetle chews around the circumference of the branch, disrupting the flow of nutrients and water within the tree.

Different Types of Bark Beetles

There are several different types of bark beetles within the Scolytinae subfamily:

• Mountain pine beetle
• Western pine beetle
• Red turpentine beetle
• Dutch elm disease vector

These beetles can have varying effects on trees, and management strategies may include insecticides or prevention measures.

Bark Beetle Type	Primary Host Trees	Damage
Mountain Pine Beetle	Pine trees	Kills large numbers of trees by feeding on phloem
Western Pine Beetle	Ponderosa Pine	Causes distinctive serpentine galleries
Red Turpentine Beetle	Pine trees	Attacks the base of the tree
Dutch Elm Disease Vector	Elm trees	Spreads Dutch Elm Disease, killing elm trees

Prevention and Treatment Strategies

Tree Care and Watering

Proper tree care is essential to prevent bark beetle infestations. Maintain tree health by ensuring they receive adequate nutrients and water. For instance:

• Regularly watering conifer trees, particularly during dry periods
• Applying slow-release fertilizers to improve nutrient uptake

Healthy trees can naturally defend against certain pests, including engraver beetles and mountain pine beetles¹.

Insecticides and Chemical Treatment

Chemical treatments and insecticides can be used to control bark beetle infestations. For example:

• Using preventative insecticides containing pyrethroids to protect uninfested trees
• Applying pheromone-baited traps to attract and kill adult beetles²

However, chemical treatments aren’t always effective against established infestations.

Pros:

• Prevent infestations in healthy trees
• Attract beetles away from susceptible trees

Cons:

• May not work for advanced infestations
• Potential harm to beneficial insects and the environment

Tree Removal and Infested Wood Disposal

Removing dying or infested trees can help prevent further infestations in nearby trees.

Additionally, disposing of infested wood prevents emerging beetles from spreading³. A few guidelines include:

• Removing trees showing signs of bark beetle infestation, such as exit holes and dying leaves
• Chipping, burning, or burying infested wood to destroy pupa and adult beetles

	Tree Care and Watering	Insecticides and Chemical Treatment	Tree Removal and Infested Wood Disposal
Effectiveness	Helps prevent infestation	Can control, but not guarantee prevention	Helps slow down infestation spread
Application	Regular maintenance	Preventative and/or during infestation	During infestation
Benefits	Healthier trees, better defenses	Protection of uninfested trees, attract beetles away from susceptible trees	Prevent damage and infestation spread

Conclusion

Bark beetles, although diminutive in size, can inflict significant damage to trees. Recognizing the signs of a bark beetle infestation is paramount for the health of your trees.

Common indicators include small holes in the bark, accompanied by sawdust-like frass, and a change in the tree’s foliage color as the infestation advances.

Woodpeckers flaking away the bark in search of beetle larvae can also hint at an infestation. Various tree species, including cypress and elm, can be affected, with symptoms like boring dust and bark flaking.

The bark beetle outbreak has notably impacted forests, especially in California, exacerbated by factors like drought and weakened trees.

Proper tree care, including regular watering and nutrient provision, alongside chemical treatments, can help manage and prevent infestations.

Footnotes

1. US Forest Service ↩ ↩²
2. Yale Environment 360 ↩ ↩² ↩³
3. UMN Extension ↩

Reader Emails

Over the years, our website, whatsthatbug.com has received hundreds of letters and some interesting images asking us about these insects. Scroll down to have a look at some of them.

Letter 1 – Bark Gnawing Beetle

Subject: What is this beetle??
Location: San Diego
January 7, 2017 10:50 pm
Can you identify this bug/beetle
Signature: Thank you

This appears to be a Bark Gnawing Beetle in the genus Temnoscheila and according to BugGuide:  “can inflict a painful bite if handled carelessly.”  Despite its name, Bark Gnawing Beetles are not injurious to trees, but rather they are predators. 

Of the family Trogossitidae, BugGuide notes:  “Many are predatory on other beetles and their larvae.”  The Myrmecos Blog states:  “This colorful insect arrived to a blacklight in my backyard a couple of years back, right when I first moved to Tucson. Previously I’d encountered Temnoscheila only under the bark of dead trees, where they apparently prey on the larvae of other beetles.

I’ve always wondered why a beetle that spends most of its time secluded in the dark would need such a brilliant metallic sheen, if the color serves a purpose or is just a spandrel.”

Ok I found 2 in my house…… so do you think it came In on some wood? We cut down and split our own…

That is a distinct possibility.  We read on iNaturalist that pine is a preferred host.

Letter 2 – Bark Beetles or Cigarette Beetles

Subject:  What is this beetle?
Geographic location of the bug:  League City, Texas
Date: 10/22/2017
Time: 12:24 AM EDT
These beetles have infested my towel closet in the bathroom (20-50 bugs).  I’ve fogged the room and it killed some, but they showed up again in 48 hours, primarily in the towels, but some are crawling along the floor. 

They are tiny, about 1/5 the size of rice grain. What are these things?
How you want your letter signed:  CR and bugs

Dear CR,
Based on the University of Minnesota Extension site, these look like Bark Beetles in the family Scolytidae, that are described as “small (1/8-¼ inch long), robust reddish brown to black insects.

They are very common in the landscape, and can emerge from many types of wood brought into homes.”  Is your towel closet made of cedar?  Though there are many species of similar looking Bark Beetles, your individuals resemble the Cedar Bark Beetle pictured on BugGuide.

Update:  Cesar Crash of Insetologia has suggested these might be Cigarette Beetles, but BugGuide indicates they eat:  “Dry plant matter of any sort, including spices and tobacco.”  We wonder what might be their food in the towel closet.

Letter 3 – Bark Gnawing Beetle

What’s this bug?
June 2, 2010
This bug was about an inch long and metallic blue, with purple, depending on how you looked at it. I can’t find what it is. Picture attached. Thanks!
Elizabeth
OK

Hi Elizabeth,
We believe this is a Bark Gnawing Beetle in the genus Temnoscheila.  Surprisingly, we came to that conclusion very quickly while browsing BugGuide.  Despite the name, BugGuide also indicates that they are predators and states:  “Larger species can inflict a painful bite if handled carelessly.“

Letter 4 – Bark Gnawing Beetle

Subject: Bark-Gnawing Beetle?
Location: Delmar, MD
October 26, 2015 3:45 am
Could this be a Bark-Gnawing Beetle? Thanks…
Signature: G Robinson

Dear G,
You are absolutely correct that this is a Bark Gnawing Beetle in the genus Temnoscheila as the images on BugGuide will verify.  BugGuide also notes:  “can inflict a painful bite if handled carelessly.” 

Letter 5 – Bark Gnawing Beetle

Subject:  Is this a Cottonwood Beetle?
Geographic location of the bug:  AZ  Tucson
Date: 01/24/2020
Time: 07:56 PM EDT
Your letter to the bugman:  I found this bug on my bar which I had the top replaced with a large Mesquite slab. 

This the second one I’ve found.  There was a lot of small wood chips and sawdust falling out of holes.  I was worried it was termites.
How you want your letter signed:  Mason

Dear Mason,
Based on this BugGuide image, this is a Bark Gnawing Beetle in the genus Temnoscheila.  BugGuide does not describe the food preferences in its genus page, but of the family Trogossitidae, BugGuide notes:  “Many are predatory on other beetles and their larvae.”  

Myrmecos states:  “Previously I’d encountered Temnoscheila only under the bark of dead trees, where they apparently prey on the larvae of other beetles.” 

We have not located any information on the larvae, and we can speculate that if the larvae are also predatory on wood boring beetles, the two individuals you found may have emerged after spending their larval stages feeding in the mesquite slab, and they emerged after metamorphosis.