What are they?

• The word "borers" applies to all wood-boring insects, including moths, beetles, sawflies, horntails and flies. Most of the damage is done when they are in the larval stage.
• Borers attack urban and rural forest trees, causing wood defects that weaken or kill the tree.
• Most borers are attracted to weakened trees, but a few exotic borers (such as the Asian long horn beetle or the emerald ash borer) appear to attack healthy trees as well.
  

• Dead tree, dead top of tree, dead limbs or branches, or branches that lack vigor.
• Exit holes, where the insect has left the tree, that range in size from a pinhead to 3/4".
• Sawdust at the entrance of exit holes, or on the ground beneath.

                                                    Crown damage on oak from two-lined chestnut borer

• Adult boring insects deposit their eggs beneath the bark by piercing or chewing through it.
• Once hatched, the larvae feed by tunneling into young shoots, branches, trunks, roots, or underneath the bark.

• They kill trees, both old and young.
• Loss of terminals, branches and trunks weakens and deforms the entire tree.
• In municipal settings, trees with borers become hazardous when their wood is weakened.

• Trees suffering from environmental stress such as prolonged drought or site that is not suited to the species.
• Storm-damaged trees: their weak condition and open wounds are inviting to insect borers.
• Transplanted trees, especially when not watered the first year or two.
• Young nursery plantings growing near infested areas.
• Urban species that commonly have borers include white birch, European mountain ash, crabapple, and black locust.

• Plant the right tree in the right location in the first place.
• Minimize environmental stress, especially on young trees (e.g., with summer watering).
• Get a positive identification on host and insect. There are many different species of insect borers, and each one has a different life cycle, therefore requiring different timing on treatments.
• Remove dead or damaged wood, and be sure pruning cuts are made properly.
• For details on treatments, consult the current recommendations of your University Extension service.

Johnson, Warren T., and Howard H. Lyon.  1991.  Insects that Feed on Trees and Shrubs. 2nd edition, revised.  Ithaca NY: Cornell University Press. For other information, advice and help on this topic, call offices of your State Urban Forestry Coordinator or University Extension service, or visit urban forestry web sites.