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Tests were conducted to verify the disinfestation of small rectangular hay bales (mixture of alfalfa/bromegrass) in a laboratory heat treatment unit. Cavities were made on top of the hay bales and Hessian fly- (Mayetiola destructor (Say)) infested wheat seedlings contained in polyester mesh bags were placed in them. For each bale tested, three mesh bags of Hessian fly puparia were used. A bale inserted with the mesh bags was thermally treated. The experimental tests were done in a completely random order. Ambient air (20 27oC) was circulated through three control mesh bags containing infested wheat seedlings. An estimated number of 1220 puparia were used for each disinfestation test and the control. The density of the bales ranged from 113 to 148 kg/m3, and the initial moisture content ranged from 8.5 to 9.9% wet basis. The average temperatures used in the heat treatment ranged from 73 to 76oC, and the average relative humidity ranged from 39 to 53%. The thermally treated insect bags and the controls were sent for emergence test. About 241 insects survived out of the approximate number of 1220 puparia used as control. This represented a pre-treatment viability of about 20%, which was unexpectedly low. The thermal treatment ensured a total mortality of the Hessian fly puparia. This study has shown that thermal treatment could be used to disinfest hay bales to meet quarantine and phytosanitary regulations.