Size is about 2mm compressed, and 2.5mm outstretched, when it's walking.

I saw what I think was the same maggots earlier this summer when they were smaller and white. They have pretty much decimated my loquat tree. I just spent a good deal of time today investigating, using a chisel. The maggots are found almost exclusively at the boundary between living and dead cambium. I found some beetle larvae also in my investigation, but they seemed to be a secondary infestation, taking advantage of the wounded plant. Unfortunately, I think I smashed all the beetle grubs, so I can't submit them for identification.

I'm inclined to try to culture these grubs to see what they turn into and would appreciate some advice on how to do so. I felled the tree, cutting it down just below the infestation (about 8 inches above the ground). I will chisel out pieces of cambium and refrigerate and freeze them for food.

While I was investigating, occasionally, a larva would fall off a chip I was removing. The individual in this first photo is an example. It crawled down the bark until it found a crevice and then started working its way into the crevice.

At first I thought this was some sort of fly larva, but as the second picture shows, each section appears to have 2 pairs of prolegs. Is that typical of flies? I didn't think so, but I realize I know very little about fly larvae.