Yes deer eat dahlias; it may not be their choice for main course, but they got at least one of mine two weeks ago- topped it- but I believe it's still growing.

Oh, no, Kim! Like nibbled off at ground level? Are the stems left lying there? Are the leaves eaten and the stems left? Or have they totally disappeared? I don't have much beebalm, but I have lots of things that eat my plants. Cutworms will eat plants by cutting the stems off and then eating what has fallen to the ground or occassionally by climbing the stems and eating holes in the leaves. They are gray, tan, or brown worms that will be in the dirt within a few inches of the base of the plants buried in the first couple of inches down. If you dig them up they curl up. I just squish them and then make a collar out of a plastic cup or other tube shaped plastic (a ring cut from a soda or milk bottle, a yogurt container with the bottom cut out, etc.) and place that around the plant, buried a quarter or half inch. You might have rabbits or groundhogs (woodchucks) since your yard backs up to a scrubby area and a park. You can send one of your sons over the banking to see if he can find any holes. I don't know how to get rid of them in the city other than with woodchuck bombs that you get at the hardware store or leaving your dog outside with a long enough leash so that he can protect the yard. (We use a 22 to get rid of the woodchucks - 3 just this weekend, and my broccoli are now just quarter inch tall stubs.) You could also check with your neighbors to see if they have had any problems with their garden or seen any critters. If there are still roots, there is hope. I'd look for cutworms and then put a collar around what's left - they may resprout, since beebalm is pretty vigorous.

very clean edge, it looks like they were cut with scissors, but no one has access to my back yard other than me and my dogs - if it were the dogs, I'd expect there to be some evidence or an overturned pot. I guess my question really is, what kinds of animals eat avocado leaves? I can only think that it was raccoons or rats that did this, thoughts?

Caterpillars and more. The cuttings will root in water if you have one or several "joints" below the water's surface. Once roots are a half-inch long, pot them up.