It is shown that the ecological hardiness of the webbing clothes moth, Tineola bisselliella, is due in part to the fact that a majority (85 to 100 percent) of middle-aged larvae do not perish under unfavourable condition (lack of suitable food, anti-feedant or weak biocidal properties of food substrate, lowered oxygen content). They convert instead to a physiologically inactive state that is a kind of torpidness of facultative diapause. These larvae can stay in closed silk cocoons without food and movement for 2-3 months. This inactive state is reversed when limiting conditions return to normal values. Thus the greater part of larvae in moth population can outlive prolonged extreme but sub-lethal conditions in a state of physiological rest. Such a fact must be considered during moth control. It is essential that all controlling conditions reach their lethal value.