Crovect has to be applied every six weeks from just before the first flies until 6 weeks before the last ones. Here in southern Scotland that means for the lambs mid May and for the adults once they are shorn (usually June), until the end of August so cover extends to mid-October. To help prevent adult sheep from getting struck it is usual to crutch them ie clear the wool from around the tail, the bit which usually carries dung, round about late May or as soon as you see flies about.
Crovect doesn't stop flies but it does prevent eggs from hatching and kills any maggots which come into contact with it. Apply carefully according to the instructions so that no maggots develop on bits of wool which missed the sprayer. We put a little spot on the head too and sometimes on each hoof, as standing in dung can mean feet get struck too in a bad year.