Water balance of Dermatophagoides pteronyssinus (Acari: Pyroglyphidae) maintained by brief daily spells of elevated air humidity

A facility was constructed to expose cultured Dermatophagoides pteronyssinus (Trouessart) to 24-h cycles of changing relative humidity; low humidity was interrupted every day by a period of high humidity. The temperature was kept at 16 degrees C, a common temperature on the ground floor of Dutch houses. Partially dehydrated mites were exposed to various humidity regimes in the absence of food and liquid water. Some mites gained weight when moist air was given for only 1.5 h every day. In the presence of food, egg production was recorded when moist air was given for at least 3 h daily, whereas the average relative humidity was quite low and less than the critical equilibrium humidity of D. pteronyssinus. Brief spells of elevated humidity allowed populations to survive much longer in a microclimate that was otherwise too dry, and may be decisive for survival during the winter months. We found that average relative humidity was misleading as an indicator of mile survival and growth conditions.