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Conifer Bark Beetles

Insect Profile and Management

Spruce Removal.heic

Spruce tree removal due to bark beetle death

© Aspect Tree Care 2023

 

Insect Profile

 

What Are Bark Beetles?

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?

In Utah, 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

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

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

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]. 

 

If frass and pitch tubes are found, bark should be peeled off to inspect the phloem. A healthy phloem is soft, white, and wet [1]. It is the backing layer on the inner bark where it attaches to wood, and it should feel slimy. If the entire circumference of the phloem is brown, black, or dried up, then the tree is dead, and the bark may detach more easily [1]. Feeding tunnel patterns called “galleries” may be present, as well as exit holes, and confirm the presence of bark beetles as well [1].

Bark beetle frass.heic

Bark beetle pitch tube and frass

© Aspect Tree Care 2023

Spruce Ips Adult.heic

Adult Spruce Ips beetle

© Aspect Tree Care 2023

Spruce Ips Pupal Casing.heic

Bark beetle pupal casing

© Aspect Tree Care 2023

Management

 

Control methods should focus on prevention of bark beetle infestations. Effective prevention utilizes a combination of cultural and chemical controls. In other words, the best way to prevent tree death by bark beetles is maintaining tree health and applying a systemic insecticide before there are visible signs and symptoms, such as crown fading. Crown fading indicates tree death by phloem girdling and no form of treatment will save it [1][4]. 

 

Trees that are confirmed to be killed by bark beetles should be removed immediately to prevent spread [1].

 

Cultural Control

Keep trees healthy and stress-free with proper deep watering and fertilization, especially for older trees [1]. Also, see our articles on mulching and deep watering for more information on keeping your trees healthy. 

 

Chemical Control

Methods of chemical control effective at preventing bark beetles fall into two categories. Systemic insecticides are injected at the base of a tree and transported throughout the entire tree by its vascular system [5]. Contact insecticides are applied by spraying almost all of the bark on an entire tree [1]. Systemics are a superior method using new technology and based on new scientific research. Contact insecticides should now be considered inferior due to difficulties in timing, equipment limitations, and risk to the environment and applicator health [5].

 

Systemic insecticides

 

Research beginning in 2007 has demonstrated that the systemic insecticide emamectin benzoate (EB) is an effective preventative control against bark beetles [6]. 

 

In 2009, The forest service acknowledged the promise of EB in new research, stating that, “Tree injections using EB may be effective against Western Pine Beetle” [2]. In 2010, after a three-year experiment, EB was concluded to protect Ponderosa pines from death attributed to Western Pine Beetle. [6] New scientific evidence led to the registration of Arborjet’s TREE-äge, containing a commercial formulation of emamectin benzoate [6]. In 2014, another study showed TREE-äge was effective for protecting Lodgepole Pine from death attributed to Mountain Pine Beetle [6]. In a research study in Utah over a two-year period from 2013-2016, TREE-äge was effective for protecting our native Engelmann Spruce from death attributed to the Spruce Beetle in Uinta-Wasatch-Cache National Forest [6].

 

Although the USU fact sheet on bark beetles from 2012 states that there are no effective systemic insecticides, this is likely based on early research (from 1991-2006) of other common insecticide formulations which have been determined to be ineffective [1][6].


 

When to Inject

We use Arborjet’s TREE-äge R10, which is the latest formulation of EB and is effective for 2 years when injected 30 days prior to attack [8]. Injections can be made at any time of year when trees are transpiring [6][8]. For most conifer trees such as pines and spruces, it is appropriate to treat anytime from early spring to late fall, so long as trees are not suffering heat stress. Studies have not shown any negative side effects or injury to tree tissue, due to injections, unless trees are heat- or drought-stressed at time of injection [6][8]. If they are currently suffering from drought stress, deep watering should be done prior to injection, or at the same time as injection [8].

Trunk injection pine.heic

Trunk injection of TREE-äge R10 for bark beetle prevention

© Aspect Tree Care 2023

 

Contact insecticides

Contact insecticides may be effective at preventing infestations when correctly applied [1]. However, we feel that their drawbacks are too numerous and should be replaced with systemic injections of EB. 

 

The most effective products are labeled for use on bark and contain active ingredients such as carbaryl, bifenthrin, and permethrin [1], all of which are considered moderately to highly toxic in mammals, and highly toxic to beneficial insects like bees, and aquatic life [3].Carbaryl is considered by the EPA, “Likely to be carcinogenic in humans” [3]. Spray drift can contaminate sensitive areas, as well as harm the applicator through inhalation, and skin and eye contact. Spraying is not recommended on a large scale for ecological reasons [3]. Contact insecticides are susceptible to runoff from precipitation within 2 hours of treatment [3].

 

Contact insecticides work by soaking into treated bark and preventing bark beetles from entry [1]. Contact insecticides must be applied to the bark of the entire circumference of a tree’s trunk, and large branches, from ground level to the top of the tree where it tapers to less than 4-5” [1][3]. Unsprayed areas are open to attack, and tall trees may not be possible to spray effectively at all [3].

 

Effective spraying of contact insecticides for bark beetles must be done by a licensed pesticide applicator [4]. Only commercial formulations, or home-use products with similar concentrates of active ingredients, are effective for bark beetle control [1][4]. Many other insecticides available to home users contain low concentrates of active ingredients and are not effective for bark beetle control [4]. In all cases, high-powered spray equipment is needed for complete coverage on medium and large trees, timing is crucial, and reapplication is needed every 3-4 weeks to prevent new generations of adult bark beetles from attacking [1][4].  Additionally, Insecticide sprays are not recommended against cedar or cypress bark beetles [4].

Biological Control

Biological control is a form of control that utilizes natural predators to control populations and reduce infestations. While natural predators like woodpeckers and other insects can help reduce bark beetle population size and damage in forests, they are not effective at preventing the death of individual trees [4].
 

 

 

 

 

 


 

References

 

1.   USU Extension Fact Sheet: Bark Beetles. Ryan S. Davis, Arthropod Diagnostician, and Darren McAvoy, Extension Forestry Associate. https://digitalcommons.usu.edu/cgi/viewcontent.cgi?referer=&httpsredir=1&article=1849&context=extension_curall

2.   Bark Beetles–They’re Back!! Are Your Trees at Risk?

https://ffsl.utah.gov/wp-content/uploads/BarkBeetleAreYourTreesAtRisk.pdf

3.   Spraying Trees To Protect Against Mountain Pine Beetle Common questions and things for landowners to consider.

Prepared by Irene Shonle, CSU Extension and Ingrid Aguayo, CSFS Entomologist.

https://extension.colostate.edu/docs/pubs/ag_natr/spray-trees.pdf

4.   University of CA IPM

https://ipm.ucanr.edu/PMG/PESTNOTES/pn7421.html

5.   ISA Arborists’ Certification Study Guide 2010

6.   Injections of Emamectin Benzoate Study. Christopher J. Fettig , Darren C. Blackford, Donald M. Grosman, and A. Steven Munson.  https://www.fs.usda.gov/psw/publications/fettig/psw_2017_fettig001.pdf 

7.   Resins. https://www.fs.usda.gov/wildflowers/ethnobotany/resins.shtml

8.   TREE-äge R10 by Arborjet. https://arborjet.com/product/TREE-age-r10/

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