It has been suggested that Primitive sheep are less prone, or immune, to fly strike. This is not so - a sheep is a sheep and needs care.
We use Crovect on our lambs usually from mid May. We don't treat adults until after they are shorn, and we have had no problems for many years now. Your query has made me question whether we need to use Crovect at all

on adult sheep. It does give peace of mind and we both detest fly strike so much that in itself is I suppose a reason to carry on using it. By the time I harvest the fleece for handcrafts, any Crovect has been denatured by the winter weather.
For Shetlands, their fleece is thick so strike can be hidden, for example under the crotch or around the penis, so we check ours (two wethers) frequently
The only time we have had a real problem with strike was many years ago when a neighbour didn't deal with strike in his flock, and the flies seemed to go for our various breeds through the fence. He is no longer allowed to keep animals so the land is rented by proper livestock keepers who care for their animals, and we have double fencing with hedging which tends to stop stuff coming through from nextdoor anyway.