We had a 25 year+ old forsythia bush near our rhubarb patch. Recently, I noticed large amounts of sawdust on the rhubarb leaves and ground under the bush at one side. When I pushed the bush, it moved easily and revealed hollow trunk near the ground. We have removed all that was above the ground (the plant was still green and sprouting) but I wonder if any treatment is needed. Should we dig out what is left of the roots?
A borer type insect is likely the cause.
I would remove the plant and dispose of the material.
You can treat the soil with an insecticide.