A comparison of the capacity for increase at a low temperature of foreign and Australian populations of Sitophilus oryzae (L.) and S. granarius (L.)

Abstract

The capacity for increase (rc) of three populations of S. oryzae and three populations of S. granarius from Great Britain and Canada was determined over 15 and 24 weeks respectively at 15°C in wheat of 14% moisture content. The fertility of these populations at 27°C, their body weight and chill-coma threshold was also measured. With both S. oryzae and S. granarius, values of rc for the foreign populations fell within the range previously observed when several Australian populations were reared under the same conditions. The cohort generation times (Tc) of the foreign S. oryzae populations were significantly shorter than those of the Australian populations because of the shorter immature development periods (D) of the former. Two of the foreign S. granarius populations had lower values for D than all but one of their Australian counterparts but their values of Tc were within the range previously observed for Australian weevils. When the Australian and foreign populations were considered together, the value of rc was more strongly correlated with fertility at 27°, an optimal temperature, than with body weight in both S. oryzae and S. granarius. With S. oryzae, rc was correlated with the chill-coma threshold of cold-acclimated weevils only. There were no correlations between rc and chill-coma thresholds in S. granarius. It was concluded that the observed differences in rc were related to differences in the vigour of the populations rather than to physiological differences in cold-tolerance. The likelihood of grain weevils becoming cold-tolerant as a result of grain aeration is discussed and the practical significance of differences in rc considered.