Fumigation with carbonyl sulfide: a model for the interaction of concentration, time and temperature

The new fumigant carbonyl sulfide offers an alternative to both methyl brom ide and phosphine as a grain fumigant. Separate mathematical models for lev els of kill, based on quantitative toxicological studies were developed for adults and eggs of the rice weevil Sitophilus oryzae (L.). These models su ggest that fumigation exposure times for carbonyl sulfide will be a comprom ise between those of methyl bromide (typically 24 h) and phosphine (7-10d) to achieve a very high kill of all developmental stages. S. oryzae eggs wer e more difficult to kill with carbonyl sulfide fumigation than the adults. At 30 degreesC, a 25 gm(-3) fumigation killed 99.9% of adults in less than 1d, but took 4d to kill the same percentage of eggs. Models were generated to describe the mortality of adults at 10, 15, 20, 25 and 30 degreesC. From these models it is predicted that fumigation with carbonyl sulfide for 1-2 d at 30 g m(-3) will kill 99.9% of adults. Furthermore the models illustra te that fumigations with concentrations below 10 g m(-3) are unlikely to ki ll all adult S. oryzae. Significant variation was observed in the response of eggs to the fumigant over the temperature range of 10 to 30 degreesC. Mo dels were generated to describe the mortality of eggs at 10, 15, 20, 25 and 30'C. As the temperature was reduced below 25 degreesC, the time taken to achieve an effective fumigation increased. Extrapolating from the models, a 25 g m(-3) fumigation to control 99.9% of S. oryzae eggs will take 95h (4d ) at 30 degreesC, 77h (3.2d) at 25 degreesC, 120 h (5 d) at 20 degreesC, 17 4 h (7.5 d) at 15 degreesC and about 290 h (11 d) at 10 degreesC. The role of temperature in the time taken to kill eggs with carbonyl sulfide cannot be ignored. In order to achieve the desired level of kill of all developmen tal stages, the fumigation rates need to be set according to the most diffi cult life stage to kill, in this instance, the egg stage. (C) 2001 Elsevier Science Ltd. All rights reserved.