Gallina--you might want to switch out the nestboxes. The oils in cedar are harmful to the airways of chickens, and would be especially bad if in an enclosed area (such as a nest box). Mixed in on the floor of a coop is probably not so bad, but if you can smell it, get it out. DE will help with mites (not totally prevent), but you could use it on the floor of nestbox. Most here use straw for the nestbox, as the hens like to "arrange" it. Pine shavings are great for litter, but lots of people use straw also.