Monthly Mystery #2: Oak Skeletonizers

Well, a month has passed, and so far the only suggestion I’ve received about those pine needle bananas is that they might go well with lemon juice and cocktail sauce.  This month’s mystery begins with a photo taken earlier the same day (July 26, 2008).  Noah and I had arrived at the gate to Yosemite National Park and were waiting in line to be let in.  We got out of the car to have a look around, and soon noticed that many of the black oak (Quercus kelloggii) leaves looked like this:

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This general style of leaf feeding–consuming some tissue while leaving veins intact–is known as skeletonizing, and many beetles, caterpillars, and sawfly larvae do some version of this.  But neither of us had seen this distinctive zigzagging pattern before, and we had no clue what insect might be responsible.  I’ve wondered about this for four years, and at least one other person has noticed it too.  This fall, a photo of the same thing was posted to BugGuide.net, also on California black oak, and also at Yosemite.  A few weeks later, when Julia and I arrived in northern California, we spent a good amount of time inspecting black oak leaves, and found many interesting leaf mines, but none of these zigzags.  As soon as we got to Yosemite, though (well, a campground in the National Forest just outside it), sure enough…

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…it seemed virtually every leaf had these zigzags.  We searched for clues as to who might be responsible, but of course the feeding had all happened months earlier.  My only new insight was that the zigzags always seemed to start (or end?) at the leaf edge.  We were only able to find one instance where this wasn’t the case:

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A few weeks after that (in early November), we were in Madera Canyon in southeastern Arizona, and the oak leaves there had a mystery skeletonizer of their own.

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In some areas it was hard to find a single leaf that had not been carefully munched around the whole perimeter.  One leaf that had a couple of preexisting holes in it even had this skeletonizing around the margins of the holes:

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Once again, we were looking at old, brown feeding damage and were unlikely to discover the culprit.  However, I did find a little leaf-rolling weevil (Attelabidae: Himatolabus pubescens) doing something similar:

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Its style, however, was to eat in little patches, not quite at the leaf edge.

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This oak didn’t have any leaves that were skeletonized around the perimeter.  This was the most extensive fresh-looking feeding area I found in association with the weevil:

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There were also older patches on the same tree that may or may not have been related:

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Later that day, I found a broad-nosed weevil (Curculionidae: Pandeleteius) munching away on an oak leaf, but it too had a different pattern of skeletonizing:

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Oddly, the area where this weevil was feeding was already brown.  Maybe this species prefers to feed on dead leaf tissue?  Such a preference is not unheard of; tooth-nosed snout weevils (Attelabidae: Rhynchitinae) in the genus Eugnamptus are, as far as is known, leafminers exclusively in dead or dying leaves.

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So, my current thinking is that these different patterns on oak leaves are all the work of beetles, and maybe all weevils, but I would love to know for sure what species are responsible.

About Charley Eiseman

I am a freelance naturalist, endlessly fascinated by the interconnections of all the living and nonliving things around me. I am the lead author of Tracks & Sign of Insects and Other Invertebrates (Stackpole Books, 2010), and continue to collect photographs and information on this subject. These days I am especially drawn to galls, leaf mines, and other plant-insect interactions.
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2 Responses to Monthly Mystery #2: Oak Skeletonizers

  1. In the zig-zag and perimeter examples, the fact that the feeding band never changes width suggests feeding by an adult insect also – if it was a caterpillar or beetle larva one would expect the feeding band to become wider as the larva grew. This seems further support for your weevil hypothesis. Very interesting!

  2. Pingback: Arizona Bugs, part 2 | BugTracks

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