Check your chickies behinds with a flashlight when they're roosting at night, fluff the feathers and look for bugs and bug eggs. Check every individual, some are more vulnerable than others but if you find bugs treat them all.
I use DE where they dust, in their nests, their litter, run, and all over their coop. I don't care if any gets in their food as I use Food Grade Diatomaceous Earth (DE). I have no pest problems with my birds.