I used to apply these things for the USFS. Lawnboy is right. It's a pheromone that tells bark beetles the trees are occupied, move on. It's not a pesticide, in that it doesn't kill anything. They are not 100 percent effective, but they do work. Usually they use them for high-value stands. Wasn't a bad job, walking through the woods, stapling these things to trees, then coming back later to see if they worked or failed.