Technical Abstract: Data from long-term Tribolium castaneum pheromone trapping programs in two flour mills was used to evaluate the impact of structural fumigations (n=23) on pest populations. The two mills differed in mean number of beetles captured and proportion of traps with captures of one or more beetles, but in one of the mills the mean number of beetles captured was reduced after implementing a more intensive IPM program. Mean number of beetles per trap and proportion of traps with captures increased by 53±8% and 25±5% from one monitoring period to the next, but decreased by 85±5% and 71±5% when fumigation occurred between periods, respectively. Mean number of beetles per trap and proportion of traps with captures immediately after fumigation were both positively correlated with number captured per trap and proportion of traps with captures in the monitoring period immediately prior to fumigation. Mean daily air temperature inside the mill fluctuated with the season, and, although always warmer than the outside temperature the relative difference varied with season. Relationship between inside and outside temperature could be explained well by an exponential equation with the parameters a=20.4, b=2.2, and c=-15.2 (r2=0.698, which is 94% of the maximum r2 obtainable). Although outside temperature differed between spring and fall fumigations, inside temperature and reduction in beetle captures was not significantly affected by season. A better understanding of pest populations and the impact of structural treatments within commercial food facilities is critical for improving the management of pest populations and for the adoption of methyl bromide alternatives.