I have never put anything on my chickens. I have found that during mosquito season I put fans in the coop that move the air over the roosts enough to make it hard for the mosquitos to fly and land on them, and my incidents of pox went down to near zero. They are much more likely to be bitten when they are still, on the roosts, and the mosquitos are more active at dusk and dawn and night when they are in roosting. I know of others around here that have spent the entire season dealing with pox. I had one case last year, and that was my first in 5 or 6 years, and was likely because the fans were off due to some modifications going on in my coop. I mount the fans in the eaves of my coop, orienting them pointing down and over towards my roosts, so no birds can get close to them and be injured. If you have windows or ventilation openings you could even mount fans there, as long as they move the air enough to disturb the flight of the mosquitos.