What are they?
What are the exterior signs of the presence of borers?
- 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
What kind of interior damage do they cause?
Why are borers a serious threat?
- 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.
What trees are susceptible?
- They kill trees, both old and young.
- Loss of terminals, branches and trunks weakens and deforms the
- In municipal settings, trees with borers become hazardous when
their wood is weakened.
How should this problem be handled?
- 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.
Where can I get more information?
- 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
- 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.