Forest Health Update: Bark Beetles FAQs
By Erika Eidson
IDL Forest Health Specialist
What are bark beetles?
Bark beetles are a group of insects that spend almost their entire life beneath the bark of trees. They tunnel in the moist inner bark, lay eggs and these develop into larvae or grubs. The tunneling kills trees by girdling them (cuts off the supply of nutrients). Adults emerge later to infest other trees in late spring or early summer. For more information, please see the IDL bark beetle fact sheet.
Certain bark beetles can reproduce in green logs, uprooted trees and green firewood if the inner bark is moist in April and May.
Bark beetles are cold blooded, so they will develop faster during warm weather. Drought and hot, dry summers are stressful for trees and increase the success of bark beetles.
What bark beetle species cause the most problems?
Pine engraver: prefers damaged ponderosa and lodgepole pine trees and slash. It has multiple generations per year. Pine engraver overwinters as an adult, and flies early in the spring as temperatures warm. It infests green pine logs > 3” diameter in April and May and lays eggs. These eggs develop into adults approximately 6 weeks later.
The emerging adults will infest more down material if it is available, if none is available they will attack standing trees in June and July. Normally, the eggs laid in these standing trees develop into adults that overwinter. However, in warm and dry years, a third generation of beetles can attack and kill additional trees later in summer and fall. For more information, please see the IDL pine engraver fact sheet.
Douglas-fir beetle: can infest damaged Douglas-fir or down western larch. This species has one generation per year. It overwinters as an adult, and flies early in the spring and prefers green, down material. Beetles tunnel in the bark, lay eggs and one generation of larvae develop in the logs or down trees. Adults then overwinter under the bark or in the forest litter. For more information, please see the IDL Douglas-fir beetle fact sheet.
Spruce beetle: can attack damaged spruce and takes one or two years to develop in infested logs, depending on temperature. Spruce beetle outbreaks can be very severe if many host trees are present. Needles on fatally-attacked trees may remain green for longer.
What is the best way to avoid bark beetle problems?
The best course of action is to NOT have down, green material available in spring when the bark beetles emerge. If logs become infested, remove or destroy them before beetles can emerge.
What if I live in urban or suburban areas? Some counties or municipalities will accept woody material at transfer stations or landfills. The material is often chipped to be used for other purposes such as mulch or industrial fuel.
If you can’t remove the damaged trees or slash, try to make them unsuitable for bark beetle reproduction. If salvage is not practical, damaged trees can be safely burned if allowed, debarked or chipped. Planer chainsaw attachments, such as the LogWizard, can be effective tools for bark removal. If this is not practical, broken tops or uprooted trees can be cut into smaller pieces and the limbs removed. Placing them in sunny areas will assist the drying process. The idea is to help the logs to dry out as quickly as possible.
Can I save the wood for use as firewood?
If the species is not pine, firewood cut into 16” pieces and split may be infested by beetles this spring, but it is unlikely that any beetles will emerge in 2022. This is not the best option for pine because beetles will infest the wood and emerge around June, 2021. Never stack green firewood next to live standing trees. This is inviting bark beetles to kill the standing trees when they emerge.
Decks of green logs stored through the winter are very likely to become infested in the spring if the inner bark is still moist. Snow cover and shade will increase the drying time.