Hello,
I am looking for some input from someone who is familiar with quaking (or trembling) aspen trees and any possible associated problems with them.

I transplanted 4 aspen trees from a fairly young stand last fall (2013). It is now a year later. They are all approximately 7-8 feet tall and about 1-3 inches in diameter. I planted them around my back yard. The trees seemed to be doing really well after the transplant, and they still seem to be fairly healthy today.

However, on 3 of the 4 trees, I have noticed sections of the tree trunk that look swollen. Every now and then, it appears that the swollen section had a hole come through the bark, and I can see a couple of places on one of the trees that look like this has happened before and the area is scarred over now.
I don't know if this information will help, but, after it rains, the tree bark dries off fairly quickly, but these affected areas seem to stay wet for much longer than the rest of the tree.

I am no expert on trees or problems with trees, but to me, it seems like an insect growing underneath the bark and then it exits the tree, leaving the hole to scar over. I did a little research, and thought that maybe it was a twig gall fly, but I couldn't find any pictures of them that looked exactly like what I am experiencing. From what I found, they seem to affect the twigs, small branches, and leaves. I didn't really find anything occurring on the tree trunk itself. Again, I am not knowledgeable about tree problems such as this, so this is just a researched guess and may be not even close to what is actually causing this swelling, just what I thought it may be.

I have attached a picture of one of the affected areas, I have several other pictures as well, which I will try to upload.

If you know anything about what I am describing that can help, it would be HUGELY appreciated as I do not know if this is a real concern to the trees or not, and I really would hate to lose these trees, or possibly have this problem spread into other plants in my yard.

Thank you.