Is ding dong ditching illegal

Ding dong ditch: To ring a random person's doorbell and run. Hence "ding dong" ditch.

No it is not illegal until you do it profusely or repeatedly to the same party, then it is considered harassment. If you are lucky, you will only be forced to talk to the police officer and apologize the family. The resident can file a police report for trespassing on private property.

Of course, if it somehow results in some damage or injury you may find yourself in legal trouble, i.e., being sued.