The Ivermectin last long enough to rid them of the infestation. However, you do have to clean out the area, spray and make sure as the others have said the area they live. I sprayed the entire yard where they were with permethrin cleaned the yard, buildings sprayed it etc. It was a cleanning nightmare. But once they were gone I treat every three months with the ivermectin drops. I have not seen any more in over a yr. now. So I feel I am free of the critters unless some birds or chickens( from next door stray over) to bring more in. There is always some vector to bring in such. I really am vigilant.