Coleopteran work crews are trimming the forest canopy. Their leavings are scattered all over the leaf litter, in a profusion that I’ve never seen. Next spring’s hickory trees will therefore cast less shade and we’ll see the beetles’ legacy in a patchwork of sunlight. Canopy openings will admit more sky into the understory, fattening citrine spring-light with blues and reds.
The arborists are Oncideres cingulata, hickory twig-girdlers. The beetles’ bodies are stubby, just a half inch long, and their colors match the chestnut-ash blotching of hickory twigs. In late summer and autumn adults feed on the delicate bark of hickory twigs, chewing the soft phloem tissues through which the tree transports its sugars. These twigs also serve as mating sites, all-you-can-eat-buffets doubling as dating clubs.
Twigs then serve as nurseries. Females lay eggs under the bark, sometimes peppering a single twig with a dozen or more piercings. Then, the mothers go to work with their sharp mandibles, gnawing a ring around the twig. They start at the bark then, lathe-like, they rotate until the twig breaks away, leaving a few torn wood strands at the center of a clean cut. It is these shed twigs that are strewn everywhere. Lately I’ve seen one hundred or more on a short woodland walk.
Eggs hatch in late autumn and the larvae set to work within the fallen twigs, using symbiotic fungi to turn inedible wood into a paste of yum. In spring, when the weather has warmed, the larvae expand their activities, riddling the wood with tunnels. Beetle runnelling? They stay under the bark, emerging only to expel sawdust. In summer, the full-grown larvae gnaw small chambers into the twig, then curl up and pupate. The adults that emerges fly to the treetops to start on fresh twigs. Why the majority of this species’ life cycle is spent on the ground, inside fallen twigs, rather than on unfelled twigs, is a mystery. Perhaps the girdling shuts down the flow of defensive chemicals from the trees’ branches. The ground is warmer, too, giving the larvae more work days. Or woodpeckers may be less likely to drill the youngsters from fallen twigs than from standing branches. Whatever the reason, the stem litter underfoot evinces the beetles’ success.