There many reasons why roaches are so resilient and these infestations can be challenging to treat, but one in particular is the roach egg case (called the ootheca) itself. This case is is carried around by the female until she drops it in a favorable location once the eggs are ready to hatch. A single German cockroach casing can contain upwards of 50 eggs! Most applications cannot penetrate these casings, which are made out of a hardened protein substance, So, even if you kill the live roaches, there are still hidden eggs ready to hatch. It usually takes a few treatment cycles to fully resolve a roach infestation.