The GP Cub wing tends to cavatate and flex if the plane is flown too fast, then the wing explodes. Happened to one of mine when the throttle broke on my OS .70FS. It takes bass wood for a reason. Some planes designs will allow balsa or bass but this Cub likes Bass wood.
I cut my own stringers {scratch bulder} and when I can't get Bass Wood I will buy some clear Poplar and cut my stringers from that. I found out it depends on what state you are in what type of wood is common. I have a hard time finding bass wood and my son in law in Mi can get it all day long.