They like young trees a friend of mine has an uncle that would cut back his trees every year and the butterflies would lay eggs on the fresh young leaves. He had thousands of worms on his 10 trees. We kept them in cornmeal in the ice box until we used them. You could catch as much as 20 bream on 1 worm cut into 3 or 4 pieces they are tough little buggers. I caught plenty of catfish on them while fishing for bream also, they're good bait if you can find them but its hard with all the pesticides that are used these days. I've only seen worms on a few mature trees and figure it's just by chance they were on them because I never saw them again.
A Bad Day of Fishing is Better Than a Good Day of Work!