Resistance of the grain mite Acarus siro L. (Acarina, Acaridae) to unfavourable physical conditions beyond the limits of its development

The resistance of the grain mite Acarus siro to unfavourable physical conditions was studied to determine how long some members of a population might survive so that development and increase could continue if conditions became favourable. Preliminary experiments showed that: (i) the eggs formed the most resistant stage, both to extreme temperatures and to desiccation; (ii) the most resistant eggs were those in an early stage of development, and that resistance declined with age; and (iii) environmental conditions prior to oviposition influenced resistance, resistance being greater in eggs from populations reared at near threshold conditions than in those reared at ecologically more favourable temperatures and humidities. Experiments with eggs exposed to lethal high and low temperatures and to both favourable and desiccating humidities show that above 30°C and below 0°C, relative humidity ceased to affect survival, the eggs being killed by heat or cold before the effects of desiccation became important. Eggs were fairly susceptible to high temperatures; none survived longer than 10 h at 35°C or 35 min at 40°C. They showed greater tolerance to low temperatures, surviving up to a maximum of 12–14 days at −10°C and 24–26 days at −5°C. At 0°C, the maximum survival period varied from 80 days at 20% RH to 180 days at 90% RH. For eggs exposed at temperatures within the developmental temperature range to relative humidities from 20 to 50%, survival appeared to be directly related to humidity and inversely to temperature, maximum survival periods ranging from 20 days (20% RH) to 45 days (50% RH) at 5°C, and from 1 day (20% RH) to 3 days (50% RH) at 30°C.