The Truth Behind Bug Trails

Photos & story by Kim Brown

You’ve likely clambered over a barkless tree trunk across the trail and come face to face with a weird, artistic pattern carved in the wood. The artists in this case are beetles. Yes, beetles, and the natural, artistic display is known as a beetle gallery.

The gallery is carved into the tree’s phloem, or inner bark, by female beetles and their larvae. Some species of beetles carve a specific pattern, while others carve random grooves. The galleries are often beautiful but beetle attacks are deadly. Typically, beetles attack trees already weakened by disease, wind damage, fire (beetles can sense fire from afar) or other reasons, but attacks often spill over into healthy trees. A beetle attack destroys the tree’s ability to transport food and nutrients, killing the tree.

The female beetle bores a straight tunnel into the phloem, while the male follows. After mating, she lays her eggs. When the eggs hatch, the larvae bore their own tunnels to create cozy nests to spend the winter in while they metamorphose into pupae and finally, adults. When ready to leave the nest, they munch tiny escape holes in the tree, then flutter out to make their own way in beetle life, leaving behind the galleries of tunnels in the tree’s phloem, which are eventually exposed when the bark falls away.

• The Beetles
• Some of the native beetles that attack trees in Washington State:
• Pine beetles: Affect ponderosa, lodgepole, whitebark and jack pine trees.
• Fir engraver beetle: Affect all species of true fir; in Washington, grand fir and noble fir are the species most attacked.
• Spruce beetles: Affect Engelmann and Sitka Spruce.
• Douglar-fir beetles: Attack Douglas-fir and downed (not live) western larch.

Beetles are drawn to a particular tree by pheromones emitted as other beetles bore their tunnels. A pheromone emitted from a beetle signals to others in order to attract large enough numbers to overcome the tree’s defense system: a sticky resin or sap that exudes from the tree, either in rivulets or as a plug, covering the entrance hole bored by beetles. Once inside the tree, beetles “click-click-click” to each other within the tree so others know where they are. Later, the beetles issue a different pheromone to signal would-be attackers that the tree is full—no more room. If no more weakened trees are available, then the swarm might attack a healthy stand of trees.

Depending on the type of tree, evidence of a beetle attack can be globules of resin or long tubes of sap dripping down the tree. It’s the tree’s defense mechanism. It clogs holes with a sticky mess so that beetles cannot get in. Other attacks can show up as a yellow or red canopy, woodpecker activity as the birds peck away bark to eat the beetles, or the presence of sawdust spilling from the bark as beetles push it out of the trunk.