RoseMagazine.com provides this discussion of how rose galls get started.
How Galls Get Started:
Bacterial galls are tumors caused by a critter called > Agrobacterium> .
Crown galls, those tumors which attack the root crown of the plant near the soil’s surface, are produced by > A. tumefaciens> .
Cane and stem galls are produced by > A. rubi> .
These bacteria live in the soil and can survive long periods of adverse conditions in soil debris or within the systemic fluids of the plant. When the plant is wounded, the bacteria enter the tissues and form a soft, spongy tumor. The bacteria proliferate on the outer surfaces of the tumor. As the gall hardens, some of these outer tissues are sloughed to the ground carrying with them more bacteria which then re-infect previously non-infected, wounded areas of the plant.
Crown Gall is caused by a bacterium, not an insect.