SUSCEPTIBILITY OF DEVELOPMENTAL STAGES OF SITOPHILUS GRANARIUS (L) (COLEOPTERA CYRCULIONIDAE) TO MODERATELY LOW TEMPERATURES

The mortality of fourth stage larvae and pre-pupae of Sitophilus granarius reared at 25°C caused by exposure to 15°C increased with increasing exposure to exceed 80 per cent in 7 wk. By this time most of the surviving individuals had reached the adult stage. Weevil cultures of age known to within a day were prepared at 25°C and transferred at various ages to 15°C for 8 wk. The developmental stages most susceptible to exposure to 15°C were the very young eggs and the pre-pupae and pupae. Mortality exceeded 80 per cent in cultures first exposed at 1 day old and exceeded 60 per cent for all those first exposed at 2 days old or at 23 to 26 days old. Most of the larval stages were little affected by exposure to 15°C which is just above the developmental threshold temperature of this species. Some of the adults that emerged from these cultures after they were returned to 25°C were kept for 7 wk. In the groups that had been exposed to 15°C as pre-pupae, the proportion of adults still alive at the end of 7 wk at 25°C was low and the fecundity of females was poor. The lowest mortality and highest fecundity were recorded for the groups exposed when 7-14 days old, i.e. as young larvae. © 1968.