What are spruce beetles and what do they look like?

Spruce beetles are small cylindrical beetles that are reddish-brown to black in color. Spruce beetles belong to a large group of beetles called bark beetles who get their name because they live and feed beneath the bark of trees. Adult spruce beetles are about a quarter-inch long and and eighth-inch wide. Immature stages of spruce beetles are rarely seen because they occur beneath the bark of trees.

Spruce Beetle Adult Sideview
Spruce beetle adult. M. O’Donnell and A. Cline, Wood Boring Beetle Families, USDA APHIS ITP, Bugwood.org
Spruce Beetle Adult Top View

Where do spruce beetles live within trees and how do they damage trees?

Spruce beetles live and feed just beneath the bark of the tree within the phloem tissue. Phloem tissue carries the sugars created in the needles during photosynthesis to other parts of the tree to be used for energy. When this tissue is damaged, it interrupts the tree’s ability to feed itself.

Female adult beetles bore through the bark of the tree to reach the phloem. Once there, they chew a vertical path, called a gallery, beneath the bark and sends a chemical signal to attract a mate. After mating, the female lays eggs along the sides of the gallery. When the eggs hatch, the larvae feed perpendicular to the vertical gallery, radiating out and severing the phloem tissue. The attacked tree dies when these larval galleries occur around the entire circumference of the tree, severing the trees ability to feed itself.

Spruce beetle attacks are primarily concentrated in the lower portions of the tree trunk, but may extend into upper portions of the trunk.

What trees/forests are vulnerable to spruce beetle?

Spruce beetles are pests of spruce trees; all native spruce species in Alaska can be hosts. This includes white, Sitka, Lutz (a white spruce-Sitka spruce hybrid), and black spruce. Spruce beetle attacks in black spruce are less common but may occur when spruce beetle populations are high. Additionally, ornamental spruce species can also be hosts. In Alaska, our most common ornamental spruce are Colorado blue spruce, Norway spruce, and Engelmann spruce.

Under normal population levels, spruce beetles live and feed in recently downed trees such as those blown down during wind events. If enough of this material is present and other conditions are right, populations can increase and the beetles will start to attack standing trees. Larger diameter and slower growing trees are usually attacked first. Trees that are stressed by climate conditions, such as drought, and trees that are damaged by factors such as other insects or fire are typically more susceptible to spruce beetle than otherwise healthy trees.

Spruce beetles are also attracted to spruce that have been recently cut down. This material can act as a lure for spruce beetles and put standing live trees at risk in future seasons. See the Management section of this website for steps to take to minimize spruce beetle risk when cutting trees.

Where do spruce beetles come from?

Spruce beetles are native to Alaska and are always present in the environment. In times of low beetle populations, their activity may largely go unnoticed. When populations are low, spruce beetles play an important part in maintaining a healthy forest by removing declining trees and creating gaps for new trees.

What environmental conditions contribute to spruce beetle population increases?

  • Abundance of host material – such as after a wind event or large-slow growing trees under stress
  • Successive years of unusually warm summer temperatures
  • Drier than normal conditions

How do I know if I have spruce beetles/How can I recognize a spruce beetle-infested tree?

There are a few things to look for to diagnose a spruce beetle attack.

  • Boring dust: a fine sawdust-like material created when the beetles bore through the bark of trees. Boring dust usually accumulates in cracks and crevices of the bark or around the base of the tree. Boring dust from spruce beetle is typically red and granular
  • Pitch tubes: A mixture of tree sap and boring dust that accumulates on the outside of the tree trunk at beetle attack sites. Pitch tubes can range in size and color and may not always be present on beetle-attacked trees.
  • Needle discoloration: As damage from spruce beetle activity progresses, needles will start to change color, progressing from green to yellow to red before falling off the tree. This color change typically occurs the second year after the tree has been attacked.

It is important to know that not all trees will respond exactly as expected so it is important to look at all aspects of tree health before diagnosing a cause.

For images on how to identify a spruce beetle attack, visit the Identification portion of this website.

When are spruce beetles active?

Spruce beetle adults are most active from May to July during which time they are actively looking for new host trees. May to July is considered their “flight period” and the time when most new attacks occur. It is important to plan management activities outside of this time frame. See the next two questions below for guidelines for management practices.

What can be done to protect individual, high-value trees?

Good tree health and maintenance can help keep spruce trees resilient to spruce beetles but won’t guarantee protection from beetle attacks. Provide supplemental water in spring and early summer and avoid pruning spruce between May and July. Avoid damaging the trunk and roots of the tree by keeping equipment away and avoiding grade changes or other ground disturbances.

Individual trees can be protected from spruce beetle through the use of pesticide applications. Pesticides can be used to prevent a spruce beetle infestation in a tree but are not effective for treating trees that are already infested and cannot be used to treat firewood that may have spruce beetle individuals in it.

Spray applications of a registered pesticide are historically the most common method for treating trees. Sprays should be applied only to uninfested trees and should be made prior to May. Carbaryl and permethrin are effective active ingredients against spruce beetles and are sold under a few product names.

Newer pesticide formulations for treating trees for spruce beetle include injectable pesticides with the active ingredient emamectin benzoate or abemectin.

For more information and details on pesticide use for spruce beetles, please visit the Management section of this website.

What can be done to protect spruce forests?

Pesticide applications aren’t logistically or financially practical on a forest level, so sound silvicultural practices that increase the overall health of the forests are important.

Maintaining good tree health and vigor as well as swift cleanup and processing of dying or recently downed spruce trees are keys to reducing the likelihood of spruce beetle population buildups in your spruce forests. In times of high spruce beetle populations though, these efforts may not be sufficient to prevent spruce beetle attacks.

Preventive tactics for spruce beetle include pruning individual trees and thinning of dense forest stands. These tactics can increase the overall vigor of the remaining trees while allowing more light and air movement around the lower portions of the trees. Preventive tactics are most effective when spruce beetle populations are not elevated and are unlikely to prevent spruce beetle attacks during an outbreak.

For more information and details on forest-level management, please visit the Management section of this website.

What do I do with an infested tree?

Homeowners with infested trees on their property should consider having the tree(s) removed. Infested trees should be felled before the spring and debarked or processed into firewood or chips to avoid having beetles emerge and attack surrounding trees. Tree stumps should be cut as low to the ground as possible. Stumps can also be ground down.

Landowners with spruce beetle infested trees will want to take measures to help protect remaining trees on the property while also considering long term forest management goals. Infested trees can be felled and used for future firewood, chipped, debarked, or burned on site. If burning, make sure you know about any burn restrictions in your area. Material that is to be used as firewood should be processed quickly and treated following the firewood best management practices available from the Alaska Division of Forestry.

Can spruce beetles get into my dry firewood?

No. Spruce beetles are not interested in attacking well-seasoned dry firewood, nor can they survive in it. When spruce beetles are found in firewood, they were most likely in the material before it was split. Spruce beetles will never bore any deeper into the tree than just below the bark. Tunnels going deep in the firewood pieces are caused by other wood-boring beetles, not spruce beetles.

Is it safe to move spruce beetle infested wood?

Moving spruce beetle infested material long distances should be avoided when possible. Logs that have not been properly processed may have live spruce beetles under the bark that could emerge to infest nearby spruce at their new location. If you plan to move spruce material from an area with a known spruce beetle infestation, be very cautious and consider practices such as cutting logs into shorter lengths, splitting them, or debarking the material before moving it. Stacking the firewood to maximize drying and away from other susceptible spruce trees is recommended.