Crovect is permethrin, which presumably is one of the chemicals you are trying to avoid. However, as mentioned on a previous thread, it can be diluted significantly and still be effective against blowfly, but less potentially harmful to your shearer. It needs to be diluted with an oil, as opposed to water, to prevent it just running off the fleece. I have found dilution (about 5x) with red diesel very effective. I tried for the first time last year and got no blowfly strike, even though I didn't shear until July. I've also put it on my mule this year and so far it has prevented her getting sweet itch, in spite of the midges coming thick and fast in the last week.
You could also use paraffin for dilution, or liquid paraffin (aka pig oil) - obtainable in 5l packs from agricultural merchants.