Carpenter bees are commonly mistaken for bumble bees. There are two differences. Carpenter bees are generally larger. Carpenter bees have an abdomen which is shiny, metallic and black in color. Their abdomen has no body hair.
Bumblebees have yellow and black body hair on all body parts.
Old nests are used year after year. If the original nest is occupied, other female bees will drill new ones. A single nest one year will become two or three the next. Problems rapidly escalate and soon you may have hundreds of holes. When you have numerous nests, you will have numerous larva .