[Gwen]

Read the source long ago so no clue where it was, but it stated that it wasn't the water part of washing the clothing that reduces the effectiveness of the product but rather the beating clothing takes in the washer and dryer (which makes sense when you know, after reading this thread, that permethrin isn't water soluble). I would then hypothesize that hand washing and hanging dry would make a single treatment last longer. ETA I have pusses and no problems for them as long as I spray and dry outside (and keep them inside).