Autumn is a season with a distinct feel and smell. The combination of generally comfortable temperatures, the low angle sunlight, and the odors of the newly decomposing summer accumulation of vegetation create a unique atmosphere that is uniquely stimulating. The activity of the natural world is intensified as organisms react to the changing season—reproductively and energetically. And it is easier to observe many organisms not usually seen since the vegetation is dying back. One such organism could very well serve as an inspiration for student research and inquiry.
From mid September to early November, take a walk along the wooded edge of almost any field in the eastern two thirds of KS and you might notice small twigs lying on the ground under the overhanging tree limbs. You might also find these twings along the ever increasing suburban walking trails where these trails pass beneath overhanging trees. These twigs may be from any number of trees from elm, hackberry, and honey locusts in the west to hickories, pecans and persimmons in the east. These are definitely twigs, usually no more than 15 mm in diameter at their thickest and about as long as your arm. As twigs there is not much to call your attention to them except for a couple of unusual observations. You’ll notice that they still have their leaves attached. In fact they are the brown, dead leaves or the dried green leaves of branches separated from the tree while still green—not the leaves of autumn. As you pick up one of these twigs you will notice that the butt-end appears to have been cut from the tree. This is the sign of twig girdlers.
Twig girdlers (Oncideres cingulata) are long-horned beetles (Cerambycidae) with an interesting life cycle. They range throughout much of the eastern U.S. In September adult females lays eggs towards the end of twigs of various hardwoods. After laying 3 or more eggs on an outer twig the female then girdles the twig by cutting about halfway through the twig with her mandibles. Later, fall winds in autumn help to separate the twigs from the tree—thus why they are found beneath trees this time of the year. The eggs are in the twig. They hatch in late fall or winter (at least in the southern states). The larval stage is spent entirely within the twig. The twig is food source and protective shelter. The larva pupates in late summer. The adults emerge shortly after and the cycle renews.
I’ve introduced numerous students to twig girdlers during autumn walks and fieldwork for biology classes. I’ve found them wherever I have taught—usually on campus or very near. In the past I’ve just focused on the interesting life history but recently it occurred to me that this organism might be a very fruitful and accessible organism for student-centered research.
The unique life history of this organism offers a rich source of questions for student study. They twigs are easy to find in the fall. In a very short time one student or a group of students can collect a large sample of girdled twigs from different tree species over a wide range of habitats. It’s fairly simple to determine the number of eggs on each twigs and it’s also easy to take measurements of the dimensions of the twigs. I expect that raising the eggs to larva to adults should be relatively easy, also. Quick, accessible questions come to mind. Is the type of tree selected as a host correlated with the size of twig selected? Eggs Laid? Pupae success? Is there a correlation between size of twig and offspring produced? What about parasitoids and parasites? What predators to the larva contend with?
If you are intrigued about twig girdlers you can learn more about them at: http://www.ag.auburn.edu/enpl/bulletins/twiggirdler/twiggirdler.htm
If you find some twigs near campus take your students for a stroll and see if you can’t get a good round of questions coming from your students about this interesting phenomenon. If you or your students find this organism interesting enough for further research I’d be interested to know. Good luck.
