Mystery cocoons on balsam poplar

2013 June 22

There are some little balsam poplar[1] saplings about 2-3 feet tall growing beside the road leading up the hill to our house. On July 26, 2012 I noticed that pretty much every leaf on them had one of these little white cocoons firmly stuck to its upper surface. The cocoons were about a quarter inch long, and bright white.

The structure of the cocoon seems to be a tight silk sheet running across a shallow crease on the leaf (they all seemed to be straddling one of the leaf veins), and underneath it looks like there is a spindle-shaped silk envelope that we can just see the tip of in this next closeup, and probably contains the actual pupa.

Do I know what this is? No, I do not. I kept the leaf and cocoon to see if anything would ever emerge from it, but nothing did. It might have gotten too dry, or it might have needed to go through a cold winter before it would complete its lifecycle, or there could have been some other problem. Thinking about it, it is even possible that it was an abandoned larval shelter, and maybe there wasn’t even anything in it anymore when I spotted it.

Well, the balsam poplars are going to be there again next year, so I guess I’ll just have to try again. In the meantime, anybody recognize it? The closest thing I’ve found so far was a picture of a larval shelter for a variety of leaf-miner in the book “Tracks and Signs of Insects and Other Invertebrates”, but that wasn’t too close.

[1] We have a lot of Balsam Poplar around here, as it is a northern species that does well in cool climates. In fact, we are at the extreme southern part of its native range! The sap has a nice smell. Since our road just underwent a major bit of reconstruction[2] about eight years ago, the vegetation on the sides is still growing back and these are some of the trees that are recolonizing the disturbed area.

[2] According to our neighbors, this was the first time any significant roadwork had been done on that stretch of road in the previous 50 years, and it needed it. The rebuilding happened after somebody (not us) sent a very short and to-the-point letter to the editor of the local paper saying that it was “The worst half-mile of paved road in Houghton County. Try driving on it and see for yourself”. The construction crew dug it all the way down to completely rebuild the road base, and in the process dug up all the rotting logs that were still there from when portions of it had been an old-style “corduroy road”, built by laying down logs side-by-side. The county had evidently paved over the logs 50 years ago, rather than removing them.

One Response
  1. Jenny W. permalink
    June 22, 2013

    interesting note about the road! did you take apart the silk to see if there was anything in it?

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