A Hole New Mystery to Consider

On my last trip to the Niobrara Valley Preserve, I photographed the bark of wildfire-killed pine trees in warm late day light.  I liked both the patterns and the color and was just trying to make some visually-interesting images.  As I was taking the photos, I saw numerous small holes in the trees but didn’t think much about them.  Holes in dead trees are not really unusual, after all.  Upon looking at the photos later, however, I noticed something intriguing – many of the holes seemed to have a funnel shape, or beveled edge, at the surface of the tree.


Tiny holes in the bark of a standing dead ponderosa pine tree killed by a 2012 wildfire.  The two near the center of the image have a funnel-shaped or beveled edge to them.  The Nature Conservancy’s Niobrara Valley Preserve, Nebraska.

I couldn’t come up with a good explanation for that beveling, so I did what I usually do when I can’t identify something – I sent the photos to people with more expertise and begged for help.  Ted MacRae was kind enough to respond that except for the flared ends, the perfectly round holes might have been created by adult pine sawyers (Monochamus spp.).  James Trager liked my speculation that perhaps something like a woodpecker might have chipped away at the holes, looking for an invertebrate meal, and added nuthatches and chickadees as other options.  But none of us have a good answer.

After hearing back from Ted and James, I looked more closely at other photos from that evening and noticed something else that might or might not be a useful clue.  Not only had the edges of some of the holes been chipped (?) away, but there were similar marks elsewhere on the bark.  I’m not sure if those are related to what happened around the margins of the holes or not.  In the photo below, look at the “pitting” – especially in the top left quarter of the image.

More holes in trees.

More holes in trees, along with tiny chipped or pitted marks, both around the insect(?) holes and elsewhere.

Is some creature (bird?  insect?  microbe?)  chipping away at the tree?  If so, why?  And is that creature chipping away at the edges of holes made by beetles too?  And if so, WHY?  Or, are the beveled edges of the round holes a separate phenomenon from the pitted surface of the bark elsewhere?

I guess it’s good for my brain’s health to ponder mysteries like this, and it’s fun to think through all the possibilities.  On the other hand, it would also be fun to KNOW WHAT THE HECK IS HAPPENING HERE, so if you have good theories – or even better, actual answers – please let me know!

I didn’t measure the holes, so I hesitate to give estimates of their size, but they were really small.  Maybe 3-4 mm across?  I didn’t give that info to Ted (but I should have), and I’m wondering now whether the holes were actually too small for pine sawyer beetles.


22 thoughts on “A Hole New Mystery to Consider

  1. Yes, 3–4 mm diameter would be too small for pine sawyers but about the right size for other longhorned beetles that breed in dead pine such as Xylotrechus sagittatus, Astylopsis sexguttata, and a few others. Still, the beveling around the edges of the holes is intriguing and begs explanation.

  2. Maybe the holes are from naïve bees of some sort. I know some bees will nest in dead wood although the hoes would be plugged at this time of year if there was larvae inside, unless something has eaten them. Pete Berthelsen would know more about the native bees.

  3. Could it be that the holes were capped with something? And the woodpeckers were pecking that cap away so they could get at what’s inside? Or could it be that their beaks/tongues weren’t quite long enough to get the food, so they chipped a bit to reach in deeper?

  4. Chris,
    I cannot say for sure that the chiseled margins around the holes that you saw were created by woodpeckers, but I have seen holes with margins like these in goldenrod galls that have been pecked at by Downy Woodpeckers. If the woodpecker finds the escape tunnel of the goldenrod gall fly in the gall, it punctures the membrane that covers the tunnel and chisels the characteristic margin around the hole before extracting the fly larva. You can see images of the holes by doing a Google search with keywords “woodpecker goldenrod gall hole.” Another good resource is https://naturallycuriouswithmaryholland.wordpress.com/tag/goldenrod-gall-fly/

  5. I agree with Mike Henry! What a stunningly interesting image, of which the holy mystery enhances this viewer’s appreciation!

  6. I sent an email to Backyard Farmer. Last year they put up an “apartment” for native bees in their Backyard Farmer Garden. Hopefully, they will have some good information.

    While out with the dogs yesterday, I saw the same types of holes in an old Maple log. Some were round, some were beveled.

  7. Chris,
    I was looking around a little bit on my last visit and wondered if TNC has considered a timber salvage project at Niobrara? I realize the terrain may be a little steep and since it’s closing in on three years since the fire, wood quality is beginning to decline more rapidly. Just curious.

    • Hi Eric, We talked about it, but I think the staff at the Preserve decided that it would probably create more disturbance to the site than any benefits would be worth. The upsides would be the removal of dead wood to make mop up after future prescribed burns easier, and to prevent the coming hassles of trying to move around the slopes with fallen trees everywhere. On the other hand, those dead trees create pretty nice habitat for lots of species, including whatever species contributed to those little holes in the bark… The discussion was more complex than that, of course, but we did have it.

  8. Thanks to everyone for the very smart comments and questions. I wish I could just sit next to one of those trees for a few weeks to see what happens, but alas, it doesn’t seem to be in the cards for the near future. I can’t answer most of the questions you posed, but I love thinking about them! thanks again.

  9. Perhaps some hymemopteran gathering raw material for nest-making, along the lines of Polistes wasps. The edges of the beetle exit holes would make easy sites for scraping off material.

  10. Pingback: Photo of the Week – January 15, 2015 | The Prairie Ecologist

  11. Pine beetles (various spp.) cause the tiny holes as they “bore” through the bark. However, the upper left part of that image is infact from an avian predator (likely woodpecker spp.) of the beetles. So you are right in guessing both coleopteran and aves. One of the signs of a beetle-infected conifer is prevalent woodpecker holes.

  12. First thing I think I wd do is peel off some bark & see what’s going on between the bark & the wood.


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