Southwestern Region

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Natural Resources

FOREST HEALTH: Bark Beetle Outbreak

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BARK BEETLES

[Photograph]: Close-up of bark beetle The six species of bark beetles involved in this outbreak area all tiny, only around 1/8 of inch long or about the size of a match-head. They are named "bark beetles" because they live and feed underneath the tree's bark. People often wonder how such a small insect can kill a tree. A single beetle alone cannot kill a tree. The key is the condition of the tree and number of beetles that attack it. The first beetles that arrive at a tree emit a scent that attracts other adult beetles. While stressed trees are the most susceptible, even healthy trees can be overcome by a "mass attack" from many beetles. The adults bore through the bark, mate and lay their eggs. When the eggs hatch, the numerous larvae then begin feeding on the inner bark, girdling the tree. Additionally, as the adult beetles colonize the tree, they introduce a blue stain fungus that blocks the water conducting cells. This prevents water from getting to the tree crown. As a result, the foliage begins to fade from green to light green or yellow and finally changes to bright red as the needles die. Eventually the red color fades as the needles begin to fall off the tree.


Numerous piñon ips bark beetles exposed after cutting into the bark of an infested piñon tree. Click on the image for a larger version.		Three bark beetles (locations shown with arrows) in relation to size of quarter. Note the small size of the beetles. Click on the image for a larger version.

SIGNS OF BARK BEETLE ATTACK

How do you tell if a tree has been attacked by bark beetles? There are a few signs, which can reveal the presence of bark beetles. Initial evidence of a beetle attacked tree includes: sawdust at the base of the tree or in crevices in the bark from the beetles boring into the tree, small popcorn like masses of sap (called "pitch tubes"), and small boring holes where beetles have entered the tree. A month after initially being attacked, the needles on an infested tree will begin to change color or “fade”.


Example of a pitch tube with a quarter for size reference. Some highly stressed trees may not produce pitch tubes. Click on the image for a larger version.		Boring dust that has fallen on the top of a branch. This dust results from the beetles boring into the tree. Click on the image for a larger version.

AFTER A BARK BEETLE ATTACK

The larvae mature in the tree and upon completion of their development, the adult beetles emerge from the infested tree and travel to a new tree. Typically, they travel short distances, but are capable of flying up to half a mile or more. Infested wood can pose a threat to other trees as a source of infestation, but once the beetles have left a tree, it no longer poses a threat to other trees. The following photographs show evidence of bark beetle activity in dead piñon trees where the bark has been removed.


Bark beetle galleries that result from the larve burrowing and feeding on the inner bark. Click on the image for a larger version.		Blue stain fungus and beetle galleries. The fine material in some of the galleries is frass, a combination of excrement and wood particles. Click on the image for a larger version.

FURTHER INFORMATION ON BARK BEETLES

Brochure on engraver beetles in southwestern pines (PDF, 74 kb)
IPM guide on Arizona five-spined ips and pine engraver (PDF, 2.3 mb)

For more in-depth information on some specific bark beetles, please see the following Forest Insect & Disease Leaflet publications:

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