The Degree Day Unit is an inexact unit of measurement generally used by the pest control industry and lawn and landscape services regarding the amount of days, normally in the spring, of temperatures above 50 degrees Fahrenheit.
For many years insects and weeds were simply assumed to emerge in the early spring based on only average temperatures of the different areas. If unseasonable temperatures occurred especially extended winter conditions and/or snow the emergence of either would vary considerably. Each weed or insect, is based on studies of how many warm days they need to emerge so each month a total of 50 degree days are totaled and compared to how many are needed for each.
Temperature controls the developmental rate of many organisms. Plants and invertebrate animals, including insects and nematodes, require a certain amount of heat to develop from one point in their life cycles to another. This measure of accumulated heat is known as physiological time. Theoretically, physiological time provides a common reference for the development of organisms. The amount of heat required to complete a given organism's development does not vary—the combination of temperature (between thresholds) and time will always be the same. Physiological time is often expressed and approximated in units called degree-days (°D).