Twig Girdlers and Pruners

By Jeff Rugg

September 4, 2019 5 min read

Q: All of a sudden, a lot of small short branches broke off the oak tree in our front yard. The ground is covered with them. There are at least 50. There hasn't been a storm to damage them, and the squirrels don't seem to be in the tree. The oak is probably over 100 years old, and I don't want to lose it. What do you think is the cause?

A: Well, you eliminated two possible causes: storms and squirrels. That leaves three possibilities: two longhorn beetles and a fungus. Both the beetles are less than an inch long and rather skinny. They are called longhorn beetles because their antennae are longer than their bodies.

They are both common on all the normal forest trees: elm, hickory, honeylocust, linden, maple, oak, pecan, sweetgum and walnut. They are also found in smaller, ornamental trees such as dogwoods, redbuds and fruit trees.

Twig girdlers are about 3/4 of an inch long, brown and black in color and rarely seen. The adults hatch out of their pupas in mid-August to mid-October. The female chews through the bark all the way around the branch. She chews through the cambium layer so that the branch tip can't gather any water that could drown her young, and the branch can't produce resins and gums that would prevent the larva from infesting the branch. She then lays eggs under the bark on the end of the branch.

As the larvae begin hatching and eating the dying wood in the branch, the branch breaks off the tree and falls to the ground. The larvae will remain in the protection of the dead branch through the winter. In the spring and early summer, the larvae continue eating. In midsummer, they pupate, and the cycle starts over.

If you pick up and dispose of the fallen branches, you will remove a large number of next year's twig girdler beetles. The branches must be burned, run through a chipper or sent to the garbage. They cannot be composted or else the beetles will hatch out in the spring.

Twig pruners are only about a half-inch long. The adults hatch in the spring, and the female lays eggs on the ends of the tree branches. The larvae burrow into the stem and hollow it out. By the end of the summer, they are large enough to pupate, but before they do, they eat around the inside of the branch so that it will break and fall to the ground. The larva will continue eating for a few more weeks and then spend the winter in the pupa stage.

Again, you can pick up the fallen branches during the fall and winter and dispose of them so that the insects never hatch. Both of these insects only have one generation per year and don't cause enough damage to old mature forest trees to be too concerned. They can cause a lot of damage to small or young trees, and an insecticide may help in those cases.

Some dead branches, caused by each insect type, may not fall off the tree. You may notice short, dead branches hanging on the tree in the fall before they fall off. Some branches will fall off while the leaves are still green, and others will cling to the tree until the leaves are brown and dead. During late fall and winter, you can prune off any remaining dead twigs and dispose of them, too.

To determine which insect is causing the damage, look at the cut ends of the branch. The twig girdler cut is smooth around the outside where it was chewed, and ragged in the middle where it broke off. The twig pruner cut is smooth on the inside and ragged around the bark edge where it broke off. In the fall, if you slice open the twig and it is solid, it is a twig girdler; if it is hollowed out, it is a twig pruner.

The reason I don't think fungus is the problem is because those dead leaves and twigs stay on the tree. The last few inches of the leaves will die and often bend backward toward the branch, so it might look the same as the insect problem at first. Prune off the dead branch and scrape off the bark between the dead and living areas. It will be green on the living side and black on the dead side. The disease is called Botryosphaeria canker, and it can affect more than a hundred species of trees.

Email questions to Jeff Rugg at [email protected] To find out more about Jeff Rugg and read features by other Creators Syndicate writers and cartoonists, visit the Creators Syndicate website at www.creators.com.

Photo credit: Antranias at Pixabay

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