Q. I have a friend that lives in the basement level of our apartment building. She often gets these gross, large black cockroaches (she calls them waterbugs) in her apartment and sometimes sees them in the hallway. I’ve never seen black cockroaches on my second floor, but I have seen a few of the little German cockroaches in my apartment. Should I expect to get black cockroaches in my apartment eventually?

A. No, I don’t think you need to worry about black cockroaches invading your apartment. Believe it or not, cockroaches do tend to stratify themselves within a building with different species occupying different sites and even different levels. The black cockroaches that you have on the basement level are oriental cockroaches. They are commonly found only on the lower levels, in cellars, crawlspaces, basements, garages, or near floor drains or water pipes. This roach likes cool dark, damp, poorly ventilated areas. In warm weather, the oriental cockroach is often found outside around building foundations and enters through basement doors, crawlspace vents, window wells, and other openings. Oriental cockroaches are rarely found above the basement level of a building.

Moving up, you often find the large American cockroach occupying the ground floor or lower level of a building. American cockroaches are uncommon in residences but can be very common in commercial buildings like bakeries, greenhouses, and restaurants. They are found on the ground floor in boiler rooms, vending machine rooms, storage rooms, garbage rooms, sewers, and near steam pipes. The American cockroach sometimes visits the oriental cockroach on the basement level but it rarely travels above the first floor.

The familiar German cockroach, our most common cockroach pest, occupies the upper floors of a building. The German cockroach is usually found above basement level and is most common in the kitchen or bathrooms of residences, behind cabinets or appliances. It lives in warm, dry, protected areas but with direct access to food and water. Sometimes the brownbanded cockroach can be found in the same sites. This roach is found in drier areas on upper levels. Because it doesn’t require as much moisture as the German cockroach, it is more likely to be found away from kitchens and bathrooms, invading bedrooms and living rooms instead.

This ability to separate out and occupy different niches is just one more thing that makes cockroaches so successful. They don’t have to compete with other cockroach species for habitat and food; they each have their own preferred sites. Of course, there are always exceptions to these multi-level roach rules. For instance, the German cockroach can often be found anywhere in a building if conditions are right.