Question
QUESTION: I've experienced some very significant damage to two elm trees (Princeton variety with Dutch Elm Disease resistance) over the last 2-3 weeks.  The location is coastal mid-coast Maine.  The trees were purchased from a nursery in the Spring of 2010 when about 12 ft tall and had grown to 15-16 ft tall when the damage occurred.  Some of the smaller branches were severed on an angle and fell to the ground on their own.  Branches up to 3/4 inches in diameter were partially severed, apparently toppled by gravity or wind and were left hanging down vertically in the trees.  From reading similar instances/questions on this website, it seems the damage may be attributable to twig pruner insects. If true, it is certainly alarming that 3/4 inch branches can be severed.  Are there other obvious suspects for such damage?

There are 12-20 ft autumn blaze maple and red oak trees nearby that suffered little or no similar damage.  On the assumption that the culprits are twig pruner insects I have removed and burned, as recommended,  the damaged branches in an attempt to eradicate or at least reduce the next generation of pests.  The two elms look pathetic with about half their foliage removed and I fear they will be completely ruined or even not survive if similar damage reoccurs in the future.  Is there anything else I can do as a preventative measure?

This is the first year I have noticed similar problems with our trees.  Could the relatively mild winter we experienced in Maine for 2011-12 be a contributory factor?  Could the insects have been introduced in the nursery stock planted two years ago and not shown up until now?

Please help!  This is depressing and I am desperate.

ANSWER: Sounds like an insect called a twig girdler or pruner.

There are two insects that cut off twigs of hardwoods: the twig girdler and the twig pruner. Both of these insects belong to a group of beetles known as the long-horned wood borers.



The adult twig girdler cuts twigs off trees in late summer and fall. Eggs are laid in the section of the twigs that are cut off. These eggs hatch into whitish-colored, legless larvae. The larvae grow slowly during the winter and spring months as they feed and tunnel within the twig, then grow rapidly through the summer emerging as adults in August and September. The larvae usually take about one year to develop into an adult.


Adult twig pruners deposit eggs in slits in the bark near the tips of twigs and small branches in the spring. Upon hatching, the larvae begin feeding under the bark. As they grow and develop, the larvae feed down the center of the stem toward its base. In late summer, they sever the twig by making a spiral cut from the center of the twig outward to, but not through the bark. This thin layer of bark cannot hold the twig onto the branch and eventually the twig breaks off and falls to the ground with the larva still inside. The larva pupates within the twig and emerges as an adult the following spring or fall. It takes about a year for the twig girdler to develop from an egg into an adult.


About the only way to control the twig girdler and twig pruner is to collect the severed twigs containing the larvae and destroy them by burning, shredding, or removing the twigs from the area. Control of these insects with insecticides is not practical.


To determine if the twig was cut by the twig girdler or the twig pruner, examine the end where the cut was made. If the end of the twig looks like it was chewed from the outside in, and has an appearance much like the end of a tree that was cut down by a beaver, then the cut was made by the twig girdler. If the end of the twig looks like it was cut with a saw and there appears to be a spiral patterned cut radiating from the center of the twig outward, then the cut was made by the twig pruner.  

There is not really a good preventive except gathering the fallen limbs and destroying the insects in them This will drastically reduce the population next year and the damage.The good news these insects usually do not kill the tree and the population varies greatly from year to year so more than likely there will be little damage next year.          



---------- FOLLOW-UP ----------

QUESTION: My damage sounds more like that from a twig pruner than twig girdler.  In practical terms, this may be academic, as the preventative measures are the same for both.  The thing that still puzzles me is that you state that in the case of both the girdler and the pruner, that the twigs/branches are severed in the late summer or fall.  My damage occurred in early and mid June.  In several cases the damaged branches were six feet long and the break occurred where the branch joined the main trunk of the tree and was close to an inch in diameter.  Are the timing and size of branch broken consistent with a twig pruner?

Answer
The size of the limbs being cut does not sound like pruners/girdlers. They usually lay their eggs in the smaller branches and cut these off. They can cut branches up to 1-1 /2 in diameter. The time is general and this spring was very unusual in early leaf budding out etc.I would not just go by the timing but look at the damage and how the branch was cut.  I think it sounds like pruners.  http://extension.missouri.edu/p/g7276