Question
I have a hybrid Elm tree, about 15 years old, and Dutch Elm disease resistant. Each spring for the  past few years many of the branch tips drop off. The tips have 3 or more leaves on them. The number of tips increases each year. Could it be the twig pruner or girdle beetle I've been reading about? Or something else? Is there a systemic treatment available?

Answer
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 a really good way to apply insecticides since the insects do not really feed on the limbs just cut them off after laying eggs  so a spray would net do any good. The good thing the overall health of the tree is not damage unless the population get so large that the majority of the limbs are cut. The destroying of the fallen limbs will greatly reduce the population that may hatch out next year. It may take a couple of years to see no more cutting but it will work.