-- Vincent P. Jones and Jay F. Brunner
Models are not a replacement for sampling but can be used to predict the best time to sample. For example, the phenology model for the western tentiform leafminer can be used to determine the best time to collect samples to estimate the level of parasitism in each generation.
How degree-day models work
In warm-blooded animals, body temperatures rarely vary more than a few degrees so the rate of chemical reaction is fairly constant and their development can be easily predicted by calendar time. However, insects and mites have no built-in mechanism to regulate their body temperature. In general, their body temperature is affected by the surrounding temperature, and development can best be predicted from accumulated heat units rather than calendar time.
Using heat-unit accumulation to predict an insect's development rate works because a specific number of heat units is required for the insect to complete a certain physiological process. The heat-unit scale is often called a physiological time scale. Whether the heat units accumulate quickly or slowly is immaterial (within reasonable limits). A good analogy would be the filling of a gallon container. It does not matter how fast or how slowly you do it, it still takes a gallon to fill it.
The temperature limits on physiological reactions are called the upper and lower developmental thresholds. When the temperature rises above the upper threshold, development stops and if temperatures continue to rise, the insect dies. When the temperature drops below the lower threshold, development stops, but insects rarely die unless the water in their cells freezes.
How degree day models are developed
The number of degree days needed for a certain insect to develop can be calculated in a laboratory. Normally, 30 or more insects are reared at a constant temperature and the time needed for each insect to complete each stage-egg,larva,pupa and adult-is recorded. This is repeated at several different temperatures.
The rate of development at the various temperatures is then plotted and from the graph the lower and upper development thresholds and the degree days needed to complete a stage of development can be calculated.
Field information from several different sites and several different years is required to validate the models.
- A horizontal cutoff, where degree-day accumulations above the upper threshold do not count; or
- A vertical cutoff where, once the upper threshold is surpassed, no more degree-days are accumulated until the temperature drops below the threshold again.
The degree-days accumulated are represented by the area under the curve within the upper and lower thresholds, shown in black in the figure. Although the horizontal cut-off method seems less reasonable from a biological standpoint, it does give better predictions of insect development in some cases. Often, a table is available in which orchardists can look up the maximum and minimum temperatures for each day and find the number of degree-days accumulated during that day for a particular insect (see available tables). These daily degree-days are accumulated from biofix, or a specified calendar date, and are used to predict the timing of critical life history events.
A direct calculation method where temperatures are recorded at frequent intervals (e.g. hourly) can also be used to determine degree-days. Degree-days are calculated by adding the number of degrees between the thresholds, then dividing the total by the number of reading times in a 24-hour period. Several computerized temperature monitoring devices, known as biophenometers, are available for accumulating degree-days in this way. Some allow the grower to program the thresholds and display degree-day accumulations at the touch of a button, while others require temperature information to be first downloaded into a computer and then analyzed. Be aware, however, that a direct calculation method will not always give the same degree-day values as a sine-wave method. Differences are greatest in the spring and fall. You should know the degree-day calculation method used by the model you are following, and if you are using a different method of calculating degree-days you should know how to adjust values to predict the same life history events. For example, if a sine-wave model is used, the recommended timing of the first cover spray for codling moth control is 250 degree-days after biofix, or predicted 3% egg hatch. But if a biophenometer is used, the timing would be approximately 230 degree-days after biofix because the instrument tends to accumulate degree-days more slowly during the spring.
Using the calendar method, the first codling moth spray is applied 21 days after first bloom, a timing that is often several days before egg hatch has begun. Some years it could be as much as 18 days early, and the spray residue is effective for only 21 days.