What is the green caterpillar boring holes in the barn wood. About 1/2 inch
long, only 1/16th around. Looks like they are coming from the ground, up
the cement foundation and going back into existing pin holes in the siding.
I imagine to lay eggs for the winter larvae, to start the cycle all over.

http://www.whatsthiscaterpillar.co.uk/america/index.htm
http://www.whatsthatbug.com/