you have to act quickly. You can't begin to imagine how quickly this can become a big problem unless you have experienced it. Get the sheep in and take a really good look over them. Make sure you have the fly strike pour on stuff and treat them all whilst they are in. Personally I don't give too much regard to the dose, more interested in making sure I have a good strip down the spine and around the back-end and tail. Don't put it off. If you have seen flies laying their eggs they will be hatched and munching skin before you can blink!