Here's a sign of a lazy pest controller. When dusting a roof void for insect control they stand in the opening of the manhole and just fire in the dust rsndomly, usually at the roof or some timber framing. Doing a thorough job requires actually getting in the roof void and dusting the whole area evenly. If you're ever in a roof void and see this white powdery dust, wear a respirator, gloves and overalls for your safety.
The dust was shot into the roof in one location and has coated the roof iron, not the intended location for insecticidal dust!