The original area where the larva had pushed out frass in little piles is visible in the lower right-hand corner. The larva had then moved to the upper left-hand corner and worked its way to the center at first leaving a broken line of frass and finally pushing out a pile of frass when it reached the center of the leaf. A photo I took on Sept. 22 shows the larva just starting the new mine in the upper left-hand corner. In one day it had cleared the large area shown in this photo. I also came across photos of larva of Mompha terminella which also show a larva with a green line running through the center. I don't know if this is characteristic of this species or whether other larva also look the same. At any rate, this gives me something to keep searching for.

I also found this link which mentions that larva are found in August and September although this site is from England so I'm not sure how this would match up with the time frame in the US because it also mentions that adults are out in July and August.