Which pest can kill millions of cubic feet of timber and switches between attacked trees through pheromones, making thinning a preventive measure?

• A. Mountain pine beetle
• B. Pine engraver beetle
• C. Douglas fir tussock moth
• D. Cypress tip moth

Answer: (A)

Pheromone-driven mass attacks by bark beetles explain why thinning can prevent outbreaks. Mountain pine beetle outbreaks kill millions of cubic feet of timber by coordinating attacks with aggregation pheromones that attract more beetles to a host tree; as trees are attacked and stressed, beetles can move to nearby trees, effectively switching hosts within the stand. Reducing stand density through thinning boosts tree vigor and disrupts the beetles’ ability to coordinate a large, synchronized attack, making it harder for them to overwhelm defenses. The other listed pests don’t match this combination of huge-scale, pheromone-coordinated host-switching and reliance on dense stands, so thinning is less clearly preventive for them.