As long as they have opportunities to dust bathe (plain old dirt is fine) they should keep bugs to a minimum by themselves. Parasitic pests are picked up from wild birds, rodents etc so exposure to those animals increases the likelihood your girls end up with an excess of bugs, but if they have good access to dust bathing it hopefully won't become an infestation, though sometimes weather conditions can cause an outbreak.
Offering them chopped/minced fresh raw garlic daily builds up the amount of Sulphur in their flesh which makes them less tasty to bugs. Don't mix it into their food or water as too much gives you garlicky tasting eggs. Let them self medicate. I couldn't believe how much mine loved it when I gave it to them.
Check around their bottoms every now and then to see if you can see lice eggs which will be in whitish clumps at the base of the feather shafts. For red mite run your hand (use a paper towel if you are squeamish) under the roosting bar/s. They like to hide there and streaks of blood will alert you to a problem.
This is an excellent site for help in identifying your parasites (and it's a wealth of knowledge too on many other chicken related things):
http://www.the-chicken-chick.com/2012/08/poultry-lice-and-mites-identification.html