I recently have discovered A white cotton like ball under a piece of the bark on my Larch here..
Mealy Bugs:
Mealy bugs will appear as little cotton balls sitting on the leaves and branches of your bonsai. The insects hide within the little “balls” and are protected by wax-like coverings. They can commonly be seen in patches or clumps. During re-potting, and any other time when you notice symptoms, you should also examine your plant for root mealy bugs.
Symptoms: Your tree's growth will be slowed or stunted and leaves will droop and turn yellow. Mealy bugs can also cause the development of black sooty mold.
Treatment: You can treat your bonsai with either a contact or systemic insecticide. If you have a recurring issue with mealy bugs, an initial contact solution will be effective, but you may wish to use a systemic treatment to prevent re-infestation.