How important is residual chemical?
The promise of the continuing effects of residual chemical after an initial application is highly overrated. Roaches are really, really adept at avoiding noxious chemicals (thus the spreading effect mentioned in the answer to the question above). In order to make it impossible to avoid the so-called residual chemical, the pest control technician would have to apply an initial massive quantity of hazardaous chemical (a gross violation of the law) and then run from your house. It could make you sick and the chances are very good you would still have roaches.
