What Are Spruce Beetle Signs?

The presence of woodpeckers and pitch tubes may be some of the first sign of spruce beetle attack of a forest. Rapidly fading spruce needles, and paths and patterns of beetle movement on the inner bark of trees are also indication.

NPS Illustration / M. Perez

WOODPECKERS: As a spruce beetle outbreak progresses, the forest attracts more woodpeckers and other predators that prey on the fattening beetle larvae. Woodpeckers can be observed pecking and peeling away the outer bark to access beetle larvae and adults—a tasty treat during the winter months. In Denali, local residents have noted woodpeckers arriving a season or two before widespread beetle-caused spruce death was apparent.



PITCH TUBES: Spruce beetles bore into the bark of host trees—leaving a very small, but visible entrance hole. The light-brown boring dust—resembling sawdust—mixes with the tree resin to create reddish coarse globs near the entrance hole. This color can also appear as milky white. White spruce often has a lot of resin, which helps protect trees from pests and acts as a natural defense mechanism. It is the presence of boring dust and entrance holes that indicate beetle infestation, not just sticky resin, which may arise from an unrelated injury to help the tree heal.



DISCOLORED NEEDLES: In the winter months, spruce trees are dormant. As the weather warms and soil thaws, the trees become active and attempt to transport water from roots to needles, and sugar from needles to roots.

Green needles are required for photosynthesis and for trees to remain healthy. Adult beetles bore into spruce bark and begin to disrupt phloem in early summer (late May through June in the Denali area). Depending on the strength of the attack and the initial health of the tree, the host tree may show color changes in needles within the first season of attack. By early summer of the following year, needles may begin to fade into a yellowish green as the tree struggles to move sugars down its trunk. Needles continue to change from yellowish to reddish brown. Eventually these lifeless needles fall and leave nothing to sustain the life of the tree. Without movement of water, sugars, and nutrients through phloem, the tree dies.



GALLERIES: As the tree weakens and dies, it becomes possible to peel back the outer bark and peek at the tunnels left behind by adult spruce beetles and larvae. The tunnels can also be seen in the flakes of bark woodpeckers peck off in search of beetles. Note that there are other types of beetles that leave holes in the bark or tunnels in the phloem. The size and patterns of the tunnels in combination with other evidence (i.e. entrance holes, pitch tubes) is what allows certainty in determining the likely cause of tree death.

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