Effect of relative humidity on life cycle of the house dust mites, Dermatophagoides farinae and D. pteronyssinus

Adopting the individual rearing method, the effect of relative humidities of 86,76,61 and 36% RH on the life cycle of Dermatophagoides farinae and D. pteronyssinus was observed under a constant temperature at 25℃. At 76% RH, the eggs of D. farinae developed to adult in the shortest duration of 39.6±6.6 (29-60) days; 8.1±0.1 days for egg, 8.2±0.3 days for larva, 17.0±5.7 days for protonymph and 6.6±0.4 days for tritonymph. In D. farinae, under each humidity, two types of the protonymphal stage were observed. One had a shorter stage of 5-10 days and another had that longer than 15 days. The former was named as the regular protonymph and the latter as the prolonged protonymph. At 61% RH, a few prolonged tritonymphs, which required more than 20 days before molting to the adults, were also observed. The appearance of the prolonged nymphal stage seemed to be related with the nutritional condition as well as humidity. The number of eggs laid by a female per day was normally 1 or 2 in D. farinae. However, some females occasionally laid 5 or 6 eggs in a day. At 86% RH, the largest total number of eggs per female (80.6±8.2) was observed. At 76% RH, numated female of D. farinae showed longest longevity of 188.8±60.9 days ranging from 92 to 378 days. The longevity of the female was usually longer than that of the male. D. pteronyssinus showed the shortest developmental duration also at 76% RH. It took 37.1±2.5 days with a range from 30 to 54 days; 6.2±0.3 days for egg, 10.7±0.3 days for larva, 8.6±1.0 days for protonymph and 11.4±2.2 days for tritonymph. The prolonged proto-and tritonymphs were not observed in this species unlike in D. farinae. At 76% RH. the largest total number of eggs, 76.2+22.2,was laid by a female of D. pteronyssinus. At 36% RH, all of D. farinae and D. pteronyssinus were dead before the protonymphal stage.