The use of accumulated heat units or degree days (DD) is now widely accepted as a predictive tool in crop management and pest control. The developmental rate of organisms can be determined and projected more accurately using heat unit accumulations rather than using calendar days. Field data were collected over a 3 to 5 year period and used to develop and verify plant growth and development models for the Acala and Deltapine cotton cultivars, and phenology models for the pink bollworm and tobacco budworm. The models are based on a developmental threshold temperature of 60 F. Degree days were calculated from daily minimum and maximum air temperatures using the triangulation method. Degree days are an accumulated product of temperature and time among the developmental thresholds for each day throughout the life span of an organism.