Adult Western pine beetles are dark brown and about ¼ inch long. Larvae are small, white, legless grubs. Western pine beetle larvae mine a short gallery in the phloem and then turn into the middle bark to complete their development. Attacking beetles release chemicals called aggregating pheromones that attract other beetles until a mass attack overcomes the tree. Attacks may “spill over” into adjacent trees causing what is known as a “group kill.”
Western pine beetles typically fly from late spring through late October. One to three generations may occur in a single year. The adult gallery pattern of the western pine beetle is winding and crisscrosses in several directions. It is the only bark beetle that makes this particular type of gallery.