A phosphine-resistant (R) and a laboratory-susceptible (S) strain of Cryptolestes ferrugineus (Stephens) and malathion-resistant (R) and a laboratory-susceptible (S) strain of Sitophilus granarius (L.) and Oryzaephilus surinamensis (L.) were exposed as adults to a controlled atmosphere generated from mixing nitrogen and compressed air to give an oxygen content of 1%. These exposures all took place at 20 degrees C and 50% r.h. They lasted for 4, 6 and 8 d for C. ferrugineus and S. granarius, and for 2, 4 and 6 d for O. surinamensis, Simultaneously, similar exposures were carried out which included up to three periods at higher oxygen levels (3, 5 or 10%) during the course of the exposure to 1% oxygen. These periods with increased oxygen concentrations lasted for 8 (only C. ferrugineus) or 16 h (all three species). There was no significant difference (P > 0.05) in the response of the two strains of C. ferrugineus to the atmosphere so further analysis was carried out using pooled data of both strains. However, there was a significant difference (P < 0.05) between the strains of the other two species. This difference was not the same for both species as the R strain was more tolerant than the S strain of S. granarius whereas the reverse was true of O. surinamensis. Adult C. ferrugineus of either strain appeared the most tolerant to the 1% oxygen atmosphere, followed by the R strain of S. granarius, then its S strain and finally the S and then the R strain of O. surinamensis. Mortality of all species increased as the exposure period increased. Comparisons of mortalities between the continuous 1% exposures and interruptions with higher oxygen levels produced similar patterns of response for the three species, even when their tolerances to the atmosphere differed. There was increased survival as the level of oxygen during the interruption increased and this increase was accentuated with an increase in duration of the interruption. However, the significance of this increased survival decreases as the exposure period increases. The use of an exposure period, which produced over 99% mortality for the uninterrupted 1% oxygen exposure, meant that there was no increase in survival even after three 16-h interruptions of 5% oxygen during such an exposure. Crown Copyright (C) 1996 Published by Elsevier Science Ltd.
