Borers are destructive insects that burrow into the bark of a tree, leaving behind small, round holes. Many species of boring beetles attack aspen trees, often leaving them with "bleeding" wounds. Aspen borers emerge from beneath the bark in June, and the insects can spread rapidly from tree to tree. Killing aspen borers requires a systematic spraying of specially formulated pesticides.
Examine the trunk of the aspen trees for signs of holes, which indicate the presence of borers.
Fill a garden sprayer with an insecticide approved for use on aspens that is formulated for boring insects. These pesticides typically contain the ingredient permethrin or Sevin (carbaryl).
Put on gloves, a dust mask and safety glasses. Place a ladder next to the affected aspen tree trunk, and climb it with the garden sprayer.
Spray the entire trunk with a light layer of the pesticide, as well as the leaves and branches.
Move the ladder to any nearby aspen trees, and spray them with the pesticide as well. Spray the closest trees, even if no evidence of holes is seen, to prevent the borers from moving to those trees.
Wait one to two weeks, or the time specified on the pesticide label, and repeat the treatment of spraying the aspen trees.