What is Phenolgy?
From the UC IPM page
Phenology models predict time of events in an organism's development. Development of many organisms which cannot internally regulate their own temperature, is dependent on temperatures to which they are exposed in the environment. Plants and invertebrates, including insects and nematodes, require a certain amount of heat to develop from one point in their life-cycle to another, e.g., from eggs to adults. Because of yearly variations in weather, calendar dates are not a good basis for making management decisions. Measuring the amount of heat accumulated over time provides a physiological time scale that is biologically more accurate than calendar days.
Two parameters are used when referring to the effect of temperature on growth and development. The lower developmental threshold for a species is the temperature below which development stop. The upper developmental threshold is less well defined, but is often taken as the temperature at which the rate of growth or development begins to decrease. For many organisms the upper thresholds are not used because data are lacking to obtain such estimates. Both lower and upper thresholds are determined through carefully controlled research and are unique for a specific organism.
The amount of heat needed by an organism to develop is known as physiological time. 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 in units called degree-days. For instance: if a species has a lower developmental threshold of 52° F, and the temperature remains at 53°F (or 1° above the lower developmental threshold) for 24 hours, one degree-day is accumulated.
Each stage of an organism's development has its own total heat requirement. Development can be estimated by accumulating degree-days between temperature thresholds throughout the season. The accumulation of degree-days from a starting point can help predict when a developmental stage will be reached. Degree-day monitoring does not indicate whether control action is warranted, but rather when a pest will reach susceptible life stages. If pests are abundant, monitoring degree-days helps to eliminate the guesswork otherwise required to determine the time for a control action.