A few weeks ago I was sent the photos below, with these comments:
I found it on the bark of manzanita (Arctostaphylos) in Napa County, California on March 31. . . It resembles feeding sign of a rabbit because the scrapes go down to the cambium, but is in horizontal rows and goes too high up on the branches. It has no clear incisor marks either. I know some insects tunnel through the bark of trees and shrubs, but this looked like scrapes from a chisel or nail. The scrapes were about 1/8th inch wide, parallel, horizontal, varying lengths, and sometimes left short strips of bark hanging off the ends.
My response was: “If this is an insect sign, the closest thing I can think of is nest material collecting by paper wasps, hornets, etc., but in my experience they always strip parallel to the grain of the wood and use weathered wood rather than fresh bark. Also I have never seen so much stripping so close together. I’ve seen nest material gathering by squirrels, mice, etc. that looked superficially similar, but as you pointed out there don’t seem to be any incisor marks.” Anybody out there have any further insights?
BugTracks 
Well, if this was in an area where children pass by, I wouldn’t discount the possibility that it was, in fact, done with a nail or other small scraper. Kids do that sort of thing sometimes (at least, I did).
Always good to keep that possibility in mind, but I would be surprised if a kid vandalizing a shrub produced this pattern of precise, parallel, horizontal lines.
Yeah, that looks pretty darn focused for a kid!
I couldn’t tell but in the first picture where the tree had a groove in it, did the lines ‘skip’, like they would if a person was scratching the bark?
I was leaning drunk wasp at first, but that’s not really an option. 🙂
I don’t think so, but you’re right that the groove seems to have a strong influence on the start/end points of the lines.
A great mystery!
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