Maybe it is easier to look at the frass on the bark. It is like the larvae are very difficult to rear, but that's not true. For that reason birch trees are totally cut down and this is not necessary!
When dealing with S. spheciformis the bark shows frass clearly from 1-max. 2 above the ground. Sometimes you have to drudge a bit into the ground near the stem. When you remove some parts of the bark with a knife you will see the whole corridor with the larva clearly.
By the way, cutting trees as illustrated above is needless.