Ew! For some reason, I find these spindle galls a little icky-looking. They remind me of thick worms or maggots, but they are just the 'homes' that mites have created for their offspring to develop in. It makes the leaves of my black cherry look deformed, but it doesn't mean the tree is diseased or sick at all. It's just another microworld of our world, and nothing needs to be done about it.