Physical limits for complete development of the grain mite, Acarus siro L. (Acarina, Acaridae), in relation to its world distribution.
The following is virtually the author's summary. The investigation described de-fines the limits of temperature and humidity beyond which the grain mite Acarus siro L. [cf. RA.E, A 53 259] cannot complete its life-cycle. Small groups of adults or other stages were transferred from laboratory cultures to micro-cells, in which the mites were exposed to temperatures ranging from 0 to 35°C. [32 to 95°F.] and to a graded series of relative humidities from 60 per cent. upwards. Adult survival, oviposition and hatching were recorded. A few individuals were successfully reared at about 100 per cent. relative humidity and 15°C. [59°F.], but there was a high rate of juvenile mortality, apparently caused by moulds. The lower humidity limit varied with temperature. At 5°C. [41°F.], development was completed only at relative humidities above 65 per cent. At 10-20°C. [50-68°F.], a small proportion of individuals completed the developmental cycle at 62.5 per cent. relative humidity, but at 60 per cent. relative humidity adults survived for only short periods, few eggs were laid, and none hatched. At higher temperatures, the low humidity limit became higher; it was between 65 and 67.5 per cent. at 25°C. [77°F.] and between 70 and 72.5 per cent. at 30°C. [86°F.]. The upper temperature limit was between 31°C. [87.8°F.], at which a few individuals completed their development at 90 per cent. relative humidity and above, and 32°C. [89.6°F.], at which none did so. The lower temperature limit was below 5°C. but above 0°C., at which temperature no eggs hatched and other immature stages failed to develop although the adults were long-lived. The laboratory results help to explain the distribution and status of A. siro as a pest of stored wheat and other grains. It is only in countries with cool, moist climates that it becomes a major pest of stored grain. In tropical and subtropical regions, its development is prevented or restricted by unfavourably high temperatures or low relative humidities. In more temperate zones, its powers of increase and abundance are governed chiefly by the atmospheric humidity and related moisture content of the stored grain.