How Deer Repellent Works
Repellents are designed to change behavior patterns by providing negative reinforcement. Depending on the active ingredient, the repellent will either be an unpleasant taste or smell to the deer, will mildly irritate their nasal passages or mouths, or will mimic a predator using predator urine. These three things will ideally cause the deer to leave the area and forage or feed elsewhere.
There are many deer repellent recipes or methods of using household items to make homemade deer repellent, like human hair or bars of soap hung on trees, but are generally unsuccessful. Mothballs are commonly touted as an animal pest repellent, but please note that using mothballs or any pest control product in a way it is not designed or directed for is illegal.