“She’s over there on that knoll. You don’t have to worry about her. She knows this woods better than we.”
“Look at that trail!”
“That’s strange; it wasn’t like that last week!”
My wife and I (and the dog) were on our regular Sunday afternoon walk in the woods, when we saw one of our trails covered with small branches. It was late summer, and there had been no storms. Where did all these branches come from?
We went for a closer inspection. They were all hickory branches with the leaves still on. Looking closer, we found that the end of each branch was cut very smoothly the whole way around about 1/16 to ⅛ inch deep, leaving a center core that had snapped off.
Further inspection revealed quite a few lines cut into the bark lengthwise on the branch. Continuing our walk, we came to another section of trail covered with hickory branches. We circled back to the first spot, gathering a few samples on our way home. At home, we used our microscope to see what, if anything, was in the slits. We found most of the slits contained one creamy white egg.
We were sure it was not cicadas. They cut slits in the branch, but not a ring around the branch like this. What did this?
I stopped by the local county extension agent’s office to ask him about it. He took one look at the branch in my hand and said, “That’s from a twig girdler. There are several kinds; some target blackberries, some hardwood trees.”
I told him about the eggs in the slits. He remarked, “Isn’t God wonderful how He makes all these little things know how to do what they need to do?”
He sent us some interesting information about twig girdlers. Their scientific name is Oncideres Cingulata. They are a type of long-horned beetle that is mottled gray with antennae longer than the body. A very similar beetle called the twig pruner is brown instead of gray and attacks similar species.
Twig girdlers target mostly hickory and pecan trees, but do go after other hardwoods with smooth-barked twigs (honey locust, poplar, persimmon, redbud, etc.)
The females make slits around the twigs, depositing one egg in each slit, often at a bud scar. One female can lay 50 to 200 eggs! They then chew around and around the stem until only a small part in the middle is fastened. This eventually breaks off and falls to the ground. The egg hatches in about three weeks. The grub over-winters in the twig. Next spring the grub grows quickly, feeding on the wood under the bark. The grub makes small holes in the bark every so often to eject pellets of frass as it makes its way toward the end of the twig.
When mature, the grub uses shredded fibers to close off the tunnel it has made and form a pupation chamber. This occurs during August or September. After twelve to fourteen days, its metamorphosis is complete. The adult beetle chews a circular hole in the bark and emerges to see the world around it for the first time. It then flies up to a host tree, feeds on twig bark, and the whole process starts again.
Normally twig girdlers do little harm to the host tree, but can be a pest to pecan farmers by reducing nut yields.
In August, we went back to the hickory tree to gather a few twigs. After almost a year, you could still easily see that the ends were girdled. A clue for a twig with a grub in it was one with small holes in the bark. Finding them, we would feel gently around the area for the tunnels just under the bark.
At home, we carefully opened some tunnels to find grubs. After getting pictures, we closed the tunnels and put them in a container, checking on them every so often.
About two weeks later, we found our first pupa and more grubs. They do not pupate at the same time. Some twigs a foot or so long had three or more grubs inside. After several weeks, we saw an adult beetle. Putting fresh twigs in with them, we continued to check on them. Later, it was easy to see where the bark was eaten off the fresh twigs, especially after a number of beetles had emerged.
Our Hickory Mystery was solved, thanks to our friendly extension agent. And we do agree with him that God is wonderful to make twig girdlers know how to do what they are supposed to do.