<<< Don't treat for flies before clipping. The shearers wont be impressed.>>>
I agree absolutely with Harmony on this point. Crovect sticks to the fleece for a good 6 weeks, three months to be on the safe side. Crovecting closer to shearing than that means that the shearer is exposed to the chemicals for each sheep he/she shears. Then when the fleece is scoured, either commercially of by craft workers, the effluent will contaminate the water supply with the drainage from the scouring. Finally there may still be some residue left on the fleece after scouring, which will mean any craft worker, or mill worker will be exposed to the chemicals for the whole time they are working with that fleece.
The 'rise' of a fleece is associated with temperature, as is the arrival of flies, so if your sheep get their rise in June, you are probably in a colder area so flies are unlikely to arrive before May. Watch your sheep very carefully every day for any hints of flies, fly interest or signs of strike. Treat with Crovect if there are flies, but check your shearer is happy to go ahead with removing the fleece, then condemn those fleeces as contaminated.