The capacity for increase at a low temperature of several Australian populations of Sitophilus oryzae (L.)

The capacity for increase (I c ) of one laboratory and seven field populations of young adult S. oryzae from different sites in Australia was determined over a thirty‐week period at 15°C, a supposedly marginal temperature, in wheat of 14% moisture content. The average value of I c was 0.0838 ± 0.0017 and the populations comprised three significantly different groups. Variations in the net rate of increase per generation (R 0 ) which averaged 34.7±1.71, had a major effect on the value of r c whereas variation in the cohort generation time (T c ), which averaged 41.6±.23 weeks had only a minor effect. The survivorship of adults of the populations did not differ significantly and 93% of females were alive after thirty weeks. Estimates of r c based on a fifteen‐week period and on an amended value of Tc differed little from those over thirty weeks. The capacity for increase at 15°C of a given population was found to be correlated with its fertility at 27°C and with its body‐weight rather than with its cold‐tolerance, as evidenced by chill‐coma temperature, or with its previous temperature‐history. The temperature experienced by the immature stages of weevils had a profound effect on r c in that weevils reared at 15 and 27°C had respective values of 0.0350 and 0.0707 when subsequently cultured at 15°C. Because all observed values of r c were higher than expected, possible errors in method were considered. An alternative estimate of R 0 agreed closely with that observed and indicated that only 17% of the immature stages survived at 15°C. Metabolic heating due to the higher than expected population density shortened the duration of the immature stages by 7% but did not affect their survival.