The heat unit concept is an integral part of the COTMAN crop monitoring program, i.e. for predicting the time after physiological cutout (NAWF=5) to terminate insecticides and to defoliate. However, some controversy and skepticism has arisen about the about the accuracy of the accumulated heat units rule (i.e., 850 HU's after NAWF=5) to determine when to defoliate. This is because the accumulated heat units for timing defoliation, i.e., 850 HU's after NAWF=5 (for optimum yields) have varied tremendously from year to year. It has been suggested that the actual temperatures of the developing bolls in the canopy may not be closely represented by ambient temperatures (measured in a meteorological site). Field studies were conducted in 2004 and 2005 to determine the best and most practical method of measuring boll temperatures compared to ambient temperatures. Measurements were made using thermocouples inserted into the boll versus directed handheld infrared thermometers. In addition, measurements were also made of diurnal changes in boll temperature, the effect of boll size on internal temperature, and the effect of canopy position on boll temperature. In the second part of the study, boll temperatures was measured weekly at NAWF5 (the last effective harvestable boll population) using infrared thermometers and thermocouple psychrometers in order to compare heat unit calculation from boll or ambient temperature for the purpose of determining the timing of defoliation. The results should help to verify the reliability of the current method of timing defoliation or provide information to fine tune this method.