Who left all these twigs in the woods? 2015 is the year of girdled hickory.

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.

Girdled

I’ll put a girdle around the twig / In forty minutes ~ Oncideres puck

Bark scars: eggs below.

Bark scars: eggs are nestled below, inside the wood.

14 thoughts on “Who left all these twigs in the woods? 2015 is the year of girdled hickory.

  1. Stephen Truslow

    We’ve been going to a little island with a summer cabin on it in Ontario for many years and a similar thing happens to the oak trees. The small twigs show a definite girdling brake but thankfully there aren’t too many twigs on the ground every year. The bigger threat are beavers who love poplars and oaks. Also in the past we had gypsy moth infestations but they have run their course.

    Another problem we had this summer was a profusion of rodents who love to girdle the juniper bushes. We often have black rat snakes on the island but I didn’t see any this year which may explain why there have been so many rodents. The island is only one acre and is basically a big rock with a pine needle base for topsoil. When it rains, it just runs back into the lake. Organic material decomposes very slowly. It’s a fascinating environment.

    Reply
    1. David George Haskell Post author

      This sounds like a wonderful place. A tough one for trees, though: so many creatures looking for phloem snacks. The girdlers that I’ve written about in this post will also use other trees, including oak, persimmon, and elm.

      Reply
  2. john f. kiser

    We have girdlers in our white oaks every year here at Tate Mountain, North Georgia, and this year more twigs and branches than we have ever had. Some branches three feet long. But the twigs have a neater rounded end compared to your hickories. I have seen one hickory branch, not twig, that had the end like your illustration. We have been burning twigs in fireplace. Will this help reduce the girdler population? John Kiser (Miriam Keener’s Uncle)

    Reply
    1. David George Haskell Post author

      Interesting observation. There is also an oak girdler — Oncideres quercus — but I do not know whether it lives in our region. As for burning, it will knock the population back, but the overall effect I suspect will depend on how many twigs escape the fire. One female can lay hundreds of eggs.

      Reply
  3. Richard Hitt

    I’ve been finding large numbers of these hickory twigs in the Nashville area, as well. I was trying to explain to a group just a few days ago how sometimes a fallen twig is not just a fallen twig. About halfway through, I realized I didn’t quite remember all the details as well as I would like. Thanks for the refresher on the details.

    Reply
    1. David George Haskell Post author

      Yes, fallen twigs are entryways to great stories! I enjoyed refreshing my memory for this post, having forgotten the species name. Very interesting to hear that you have a lot of them this year around Nashville.

      Reply
  4. Laura Cotterman

    Yes, I have noticed this phenomenon around here (Piedmont of NC) for many years–hickories and oaks– and have found the little egg holes by peering closely at dropped twigs. My sense is that the twigs hang on for a while, via that middle bit that the adult has not gnawed through, and then they come down in late-summer/fall storms when there is enough wind. Did not know about the symbiotic fungi! Wonderful! Could it be that the fungi can only get into the wood once the twig is on the ground?

    Reply
  5. Todd Crabtree

    Many girdled hickory twigs were also observed this weekend at Radnor Lake State Park in Nashville. I heard part of the story while hiking there with a bryologist. The adults are not strikingly colored, as many other long-horned beetles (Cerambycidae) are. The colors are subdued, suggesting camouflage, so they might be delicious to beetle eaters.
    “Girdled twigs often remain on the tree until sufficient wind dislodges them. Large infestations can result in a high percentage of twigs being girdled. Though this may reduce the vigor and overall appearance of the tree, the overall effect on the tree’s health is not severe. However, the twigs are unsightly and do not fall all at once so clean-up is a drawn out process.”
    http://www.hfrr.ksu.edu/doc1736.ashx

    Reply
    1. David George Haskell Post author

      Good point about camouflage. They must lack chemical defenses? Although some Cerambycids (e.g., native ash borers) are bright and waspish, with no poison that I know of. Batesian mimicry, perhaps.

      “…clean-up is a drawn out process” I can believe it! Bring in the fungi.

      Reply
  6. Anonymous

    I really appreciate this post. I have often observed those in my woods and never knew what was creating them. Mine are all oak girdlers as there are no hickories.

    Reply

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