Southern by choice
Herd Master
The permethrin is perfect for the task, but I have not been able to find or verify any milk withholding requirement for it. Perhaps @Goat Whisperer or @Southern by choice or maybe @babsbag would know for sure? Maybe someone else? I can't find it listed at FARAD http://www.farad.org/WDIlookup/
Since it is a man made substance designed to mimic a natural substance, I should think that a 5-7 day withdrawal (just to be safe) should be more than adequate.
http://www.livingwithbugs.com/permethrin_pyrethrum.html
The synthetic version (permethrin) is much safer in general and far less toxic than pyrethrum is a botanical insecticide which is natural made from the chrysanthemum plant. The synthetic version cannot be used on cats.