Mountain Pine Beetle

What it does

Mountain pine beetles overcome a tree's defenses by attacking in mass, coordinated by chemical signals so hundreds strike at once. They bore in and cut vertical J-shaped egg galleries in the inner bark, and they introduce blue-stain fungi that block the tree's water transport. The tree's pitch defenses are overwhelmed, and the combined feeding and fungus kill it, usually within a year.

Where it lives

It attacks most western pines, including lodgepole, ponderosa, whitebark, and sugar pine, in forests, mountain communities, parks, and yards. It ranges through the western United States and western Canada, from the Rockies to the Pacific. Dense, mature, or drought-stressed pine stands are most at risk, and warmer winters have allowed outbreaks to expand to higher elevations.

How it breeds

There is generally one generation per year. Adults fly in mid to late summer, mass-attack pines, and lay eggs in vertical galleries. Larvae feed under the bark and overwinter there, protected by natural antifreeze compounds, then resume feeding and pupate before emerging the next summer. Mild winters improve larval survival and can fuel larger outbreaks.

What it eats

Adults and larvae feed on the inner bark (phloem) of pine trees and on the blue-stain fungi they introduce. The beetle does not feed on people, animals, or finished wood; it requires living pines to reproduce and complete its life cycle.

Identification

Adults are small, about 5 mm, hard black cylindrical beetles rarely seen because they live under the bark. Diagnosis relies on tree signs: cream to reddish popcorn-shaped pitch tubes on the trunk, reddish boring dust in bark crevices and at the base, J-shaped galleries under the bark, and needles that fade to yellow and then red about a year after attack.

Signs of an infestation

• Popcorn-shaped pitch tubes on the trunk, often cream or pink
• Reddish-brown boring dust in bark crevices and at the tree base
• Vertical J-shaped galleries in the inner bark
• Whole-tree needle fade to yellow, then red, about a year after attack
• Blue-gray stained sapwood in cut wood
• Woodpecker flecking on the bark

Prevention tips

• Keep pines vigorous and thin overcrowded stands to reduce competition
• Water valued landscape pines during drought
• Remove and destroy infested trees before beetles emerge the next summer
• Consider professionally applied preventive sprays or repellents on high-value pines in outbreak areas
• Work with a forester on stand-level management where outbreaks are active

Professional solutions

Managing mountain pine beetle is a job for foresters and certified arborists. In outbreaks, infested trees are removed and destroyed before the next generation emerges. High-value yard pines can sometimes be protected by preventive insecticide sprays or anti-aggregation pheromone (verbenone) applied by professionals before attack. A tree that has been successfully mass-attacked generally cannot be saved.