I found some kind of reddish-brown bugs on two of my chickens this morning. They were pretty thick around the vent area. I didn't do a very thorough check , but didn't see them anywhere else. I dusted the whole flock with Sevin, but is there anything else I can do? I can't spend very much money on it. If it's best to continue with the Sevin, how often should I use it until they're gone? Any help would be very much appreciated.