Mystery: Why is the Sawfly chewing wood???

Subject: Swarms of wood chewing bugs
Location: grand rapids, michigan
April 12, 2014 4:35 pm
At first glance I thought “carpenter ants,” but that’s only because we have a massive carpenter ant issue in our neighborhood. Upon taking a closer look, I realized that they were definitely not carpenter ants. They come in swarms of 20-50 and land on any exposed wood in the area. At first it seemed that they were just congregating, but I noticed that the totem pole I’d been carving was starting to look smoother… Like someone had come around and sanded it for me. Since Wednesday of last week I’d noticed that the entire pole was covered in these things, but was too busy with other things to look closer. Today I went out and got a good look, and it was pretty clear that their mandibles were working extra hard. They were gorging away. I’ve had no luck identifying them on my own, and more than anything I’m just really curious what they are. They don’t seem to come from any particular direction, one minute there’s none , and five minutes later there’s dozens of them. They aren’t the best flyers, and seem to land in the grass every few feet before launching off again.
Signature: dave

Sawfly Chews Wood!!!  But Why???
Sawfly Chews Wood!!! But Why???

Hi Dave,
We wanted to contact Eric Eaton to get his opinion on this perplexing behavior prior to assembling the posting.  Chewing wood is generally a behavior associated with paper wasps and hornets that use the wood pulp in the construction of a nest.  The numbers of wasps you observed coming to your totem pole at the same time also implies that this is some type of social wasp, but it resembles a Wood Wasp more than a social wasp.  Here is what Eric Eaton wrote back.

Dolerus Sawfly chews wood, but why???
Dolerus Sawfly chews wood, but why???

Eric Eaton provided an identification, but cannot explain behavior
Ok, I can identify the wasps, but cannot explain the behavior….
The wasps are sawflies in the genus Dolerus (pretty sure anyway, definitely sawflies).  I’ll ask around and see if anyone can explain them flocking to a wood carving.

Sawfly attracted to wood shavings.
Sawfly attracted to wood shavings.

Update:  Thanks to Eric Eaton’s identification, we were able to locate an image of a Dolerus Sawfly on BugGuide that does indeed resemble the Sawflies in your images, and it is also crawling on some exposed wood.  There is no explanation regarding what is going on in these BugGuide images, nor in this BugGuide image.  This Cirrus Image website provides some information, including “Their flight is slow and clumsy, resembling that of a common firefly. Larvae feed on various grasses.”  But alas, there is no information on wood chewing activities.

Thanks for the identification!  Its quite odd, they definitely match the description, especially the clumsy flight.  On that note they certainly don’t fly together, they arrive one at a time within a minute or two of each other.  I had to take down some tree limbs today and sure enough the cut ends of the limbs were crawling with these guys within a few minutes.  The odd thing is they don’t interact with each other at all.  In fact it really looks like they are just sitting there.  If you get really close however, you can see their mandibles are hard at work.  They don’t really leave any marks, it seems that they just clean off any small dangling bits of wood.  Yesterday I noticed one had its abdomen curled down in an awkward position as though it was a bee trying to sting the wood.  But I haven’t seen any others do that.  But its like they can smell fresh cut wood for miles, because you won’t see them all day, but cut a log and they’re everywhere.
Thanks again!  You’ve definitely satisfied my curiosity, even if their behavior leaves a new mystery to solve.

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