Records dating back as far as 1750 indicate a consistent record of intermittent outbreaks of aggressive bark beetles in various forest types across the West. Over the past 10-15 years, however, the frequency, severity, and extent of bark beetle outbreaks have increased.
The current bark beetle outbreaks differ from previously recorded infestations because of:
Their intensity — bark beetles are killing trees in larger numbers, at a faster pace, and over longer time periods
Their extent — bark beetle outbreaks are occurring in numerous ecosystems from Alaska to northern Mexico
Their synchroneity — bark beetle outbreaks are occurring concurrently across western North America
The Characters: all beetles in the subfamily Scolytinae
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| Figure from Bentz, 2005. |
Dendroctonus sp (Bark beetles) Outbreaks are usually associated with drought (Page, 1981). The beetle also responds to fire-damaged trees, but not those killed by fire.
Ips sp. (Pine engraver beetles): Species within the genus Ips, such as the piñon ips and
Arizona fivespined ips, also can kill their hosts, although typically they are not considered major disturbance agents. In recent years, however, elevated population levels of a number of Ips species have coincided with drought, resulting in large areas of mortality, particularly in piñon and ponderosa pine forests of the southwestern U.S.
The above beetles are distinguished from Ambrosia beetles as they only attack and consume live trees, whereas Ambrosia beetles can bore into heartwood even after a tree is dead and dry.
Ambrosia beetles (several species): Classed as wood borer beetles that attack weakened, dying, and
recently cut or killed trees. They can attack freshly cut lumber and lumber in decks before it is dried, and they can cause pinhole defects and dark staining in the outer wood. Galleries are formed in the sapwood or heartwood and damage the wood. Because ambrosia beetles tunnel into the wood, they are considered wood borers rather than bark beetles.
Management Considerations
When live trees are blown down, their phloem can remain suitable for bark beetle development up to a year later. Stressed pine trees emit volatile compounds (terpenes). Bark beetles have evolved to detect these compounds and use them to identify suitable host trees.
Bark beetles are also attracted to freshly cut wood. Freshly processed chips emit the same volatile compounds (terpenes) as susceptible host trees. These chips will attract bark beetles during their active periods. The bark beetles cannot utilize the chips as a food source, but the attracted bark beetles may then colonize suitable host trees adjacent to the chips piles.
Fresh dead and down material (slash) can be a refuge for bark beetles, allowing them to breed, and possibly colonize nearby healthy trees. Lop and scatter can let slash dry out, killing any beetles in the bark and preventing new beetles from feeding on it.
Wood that has been debarked is not suitable for colonization by bark beetles. Only freshly cut, logs or slabs that have not been debarked are at risk of colonization.
Dead trees that do not have bark beetles in them and that do not pose a safety hazard can be left in the forest to be used by wildlife. Dead trees do not necessarily pose more of a fire hazard than live trees.
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| Figure from Bentz, 2005. |
Additional Resources
Diana L. Six, Ryan Bracewell, Bark Beetles, 2015
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