Sounds like it might be Vine Weevil to me - do they look like this?
If there were as many as you describe, they will have had a serious effect on the root system.
There are specialist treatments available for them from all good garden stores.
Treat, keep just moist, in the shade outside and keep your fingers crossed.
Good Luck.... any chance of a picture of the bugs and the tree ?
Regards,
Ian.