I don't overuse it, however, it wasn't for mites. After this last rain, I was seeing quite a few Stick-tight fleas, and wanted to make sure the birds were treated, and outside around the coop was treated. I normally put a bit of Vasoline, or A&D ointment on combs, wattles, and legs every 2-3 days, and that tends to keep them free of them, but this past week I've noticed an increase, even though I was putting the Vasoline on them. I cleaned everything in the coop with Dawn Blue dish liquid, mixed about like for washing dishes, and used a 2 gallon sprayer to apply. I rinse the soap out of feed, water dishes, and nest boxes, but I don't rinse the roosts, and wood each time, just let it dry. This is usually very effective at keeping almost everything well under control, especially when done on a regular basis. It's not as effective on fleas, ticks, and stick-tight fleas, which is not usually a problem, but here in Florida we do get years that conditions are right to produce bumper crops of them.
In the meantime, since I posted, I did more research, and found what I consider to be a credible source of information.
Cornell University completed a 10 year study in 1993 on the effects of Carbaryl, which is the active ingredient in Sevin dust, the inactive ingredient is DE powder, so the biggest threat is inhalation, and a few other harmful effects known to be associated with the DE powder. I believe it was done back when they sold Sevin 10, and now you can only get Sevin 5. Upon reading, you will find that they tested mostly on lab rats, and pigs at a higher dosage, (high levels for 50 straight days) and higher exposure than any of us would experience in 3 lifetimes, unless we worked in one of the manufacturing plants, and were continuously exposed to it in a 100% pure form for a long time, so I'm not overly concerned about toxicity.
Like Ron said, it has a very slight mutagenic effect on it's own, but the mutagenic effect is greatly increased with the addition of nitrite, a food preservative, creating Nitrosocarbaryl. If I fed bacon to my chickens, I'd be a lot more concerned.
Since it appears that about 75% would leave their systems through normal elimination within a 3 day period, if my chickens might have ingested some (no, I didn't feed any to them) I think a week would be sufficient withdrawal time, and from now on I will wait a week before eating, or collecting eggs to hatch after dusting with Sevin dust.
http://pmep.cce.cornell.edu/profiles/extoxnet/carbaryl-dicrotophos/carbaryl-ext.html