QuestionQUESTION: I have a large (80ft+) black walnut tree in the back yard. Lately, large branches are breaking off, and falling. The tree is laden with foliage and green pods. However, the branches that fell from rather high up, appear to be very brittle. I also noticed the bark on the fallen branch had a greenish residue all over it. Can you tell me what is happening? Should I keep paying someone to clear away the fallen branches, or should I consider having the tree taken down?
ANSWER: Sounds like it maybe an insect called a twig girdler.
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.
These insects are especially troublesome if the trees are located near or adjacent to woodlands containing pecan, hickory, and other hardwood trees such as oak, maple, walnut, and ash.
The adult twig girdler cuts twigs off pecan 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 pecan 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.
The green on the twigs sounds like a lichen and if it is not causing any problem. I would continue picking up the limbs and removing them from the property.
---------- FOLLOW-UP ----------
QUESTION: The falling twigs I've been picking up have just been an annoyance, but the branches falling now are as big as 3-4 inches thick. I'm afraid someone is going to get hurt. Will these insects eventually kill the tree? If so, I may as well have it taken down now, before it falls on my garage or my neighbor's house.
AnswerI would fertilize the tree with 10-10-10 fertilizer at the rate of 1 lb per inch of trunk diameter scattered around the tree and watered in good. Do this now and again in the Fall. This will encourage new strong growth. The insect will not kill the tree. Removing the fallen limbs will remove most of th insect population so next year the insects should not be a problem. But it does sound like you may also have a disease problem in the root system and the fertilizer will help grow new roots and strengthen the others. Try the fertilizer and if next spring the amount of foliage has not increased and it is still dropping large limbs then you may want to consider removing the tree. But I think you will see a difference in new growth.
