Have you tried Permethrin?
I've used Sawyer brand to wash my clothes in for year or so but this past spring I was tired of spending the cash for it. I went to the local farm store and found a quart for $12. I dilute it 1oz. per 5 gallons water. I use a bucket to soak sox,shirts,pants,hat in let them dry on the clothes line then, use the rinse cycle in the washing machine. Finally dry in the dryer.