TERRO Drain Insect Killer, 1-Pack, Foam Treatment for Indoor Infestations | Kills Cockroaches, Drain, House, & Fruit Flies, Extension Tube Application, Drain Fly Killer, Works on All Pipes

I recommend getting a large mixing bowl and fill it with two cups of white vinegar and a few tablespoons of baking soda and pour it down the drain.Then spray the drain and overflow with TERRO spray foam until it foams out of the drain and overflows. Let it sit for a couple of hours.wait a few days.You may need a second application depending on how much infestation there is.