The rates of degradation of common chemical pesticides applied to stored grains are known to be strongly dependent on grain moisture content and temperature. A finite difference mathematical model of the processes of heat and moisture transfer and pesticide decay in aerated grain is presented and its validity is supported by experimental evidence. Aeration is shown to reduce the rate of degradation of pesticides applied to stored grain, and render the rate of decay relatively insensitive to initial grain conditions. In the temperate and sub-tropical wheat growing regions of Australia, aeration can reduce usage of the pesticide methacrifos (0-2-methoxy carbonylprop-1-enyl 0,0-dimethyl phosphorothioate) by factors of 7 and 4 respectively. The effects of initial grain moisture content, initial grain temperature, fan operation, air flow rates and geographic location are modelled. Aeration of pesticide treated grain may reduce uncertainty within the grain handling industry, thus enabling more efficient planning and resource utilization.