Mothballs are not very good deterrents for mice when used outside of the home. They are a little more than 50 percent effective using this method. When used in the outdoors, mothballs can be blown away by strong winds. Furthermore, they will readily dissolve in the rain.
Mothballs can be very toxic to pets and small children if ingested. They can cause severe sickness and sometimes could even be fatal. Mothballs work best indoors in well-ventilated areas; however, they are still not the best deterrent for mice. The best way to prevent rodent intrusion is to carefully search around the exterior of the house to find and permanently seal any open points of entry.