Fluctuating hydrating and dehydrating relative humidities effects on the life cycle of Dermatophagoides farinae (Acari : Pyroglyphidae)

Reducing relative humidity to <50% in homes is recommended as one means of reducing dust mite populations in the homes of those who suffer allergies t o house dust mites. Because of some activities in the home (e.g., bathing, cooking, opening windows), it may not be possible to keep relative humidity constantly <50%. We determined how the fluctuating daily regimes of hydrat ing (75%) and dehydrating (35%) relative humidities affected the developmen t of Dermatophagoides farinae Hughes. The life cycle was completed (egg to adults) when mites were given regimes of 24 h at 75% RH, 8 h at 75% and 16 h at 35% RH, 6 h at 75% and 18 h at 35% RH, and 4 h at 75% and 20 h at 35% RH. The time required to complete development was inversely related to the amount of moist air given daily. Development took 58.3 +/- 1.44, 64.7 +/- 1 .87, and 82.4 +/- 2.39 d for 8, 6, and 4 h of moist air daily, respectively . In comparison, the life cycle was completed in 41.1 +/- 0.50 d when devel opment occurred at a constant 75% RH. Egg incubation time was significantly longer for fluctuating ambient relative humidity compared to a continuous 75 or 35% RH. Of the emerging larvae 53.8, 72.7, 62.7, and 85.0% completed the life cycle when given 4, 6, 8, and 24 h 75% RH daily and 35% RH for the remainder of the day. This study revealed that D. farinae can complete dev elopment when given only short periods of moist air daily but the rate of d evelopment is much slower than development at a constant 75% RH. Therefore, reducing ambient relative humidity does reduce the rate of development of mite populations and the accumulation of dust mite allergen.