Dusting is a superficial treatment for pests, so it shouldn't contaminate the inside of the eggs, just make sure you wash the shells.
Number of treatments will depend on what pest you're dealing with.
Lice are white or cream in color and live
on the bird, they feed on the feather shafts and loose skin cells. A series of two or three dustings about a week apart should work for them.
Mites on the other hand are small dark brown or reddish bugs that actually feed on the blood of the chickens they can live
off the bird sometimes for weeks, so if you have mites, you need to clean your whole coop and roosts and either use a dust or another product all over the coop and on the birds to get rid of them. Believe me they are a serious problem, they can weaken or kill a bird.
So I guess the first step for you is to go out at night with a flashlight and take a look at the rear end feathers of your birds to see what pest you're dealing with, then you can decide what to do from there.
Good Luck! Let us know what you find
