Biology and Chemical Reactions

All biology, or biological development, is dependent on chemical reactions. The rate at which chemical reactions occur is dependent on the amount of heat received at the reaction site. The more heat received, the faster the rate of the reactions; and, in the case of biological organisms, the faster their development within limits.

Temperature Thresholds

The growth and development of insects are dependent on temperature. As the temperature varies during the growing season (and especially from season to season), pest pressure and the need for pest control measures will vary. The cooler the temperatures, the slower the rate of growth and development of insects. As the temperature increases, development time decreases until the temperature becomes high enough to have a negative effect. These limits are defined as the temperature thresholds. The lower developmental threshold (T_L) for a species is the minimum temperature at which development can begin. The upper developmental threshold (T_U) is the temperature at which the rate of development ceases to increase and begins to decrease. Each insect species has its own particular development rate. A general curve illustrating how T_L and T_U are derived is shown in Figure 1.

Degree-Days (Heat Accumulation)

Degree-days (^oD) is a measurement unit that combines temperature and time. Figures 2A - 2D illustrate ^oD for a specific pest (cotton bollworm) at different temperatures. At the lowest temperature, the time to maturity required the most days. At the highest temperature, the time to maturity required the least days. In other words, temperature and time work together such that the time for development of the organism's life cycle, or any stage or portion of the life cycle, decreases as the temperature increases. The key point in Figure 2 is that the heat units required to complete development (maturity) were approximately the same (539 ^oD - 622 ^oD). This is termed as the physiological time for development of the organism. Although temperatures and days to maturity may vary, the organism's physiological time (combination of time and temperature) remains relatively constant.

Calculating degree days

One degree-day is accumulated when the temperature is one degree above the T_L for a 24-hour period. There are several methods used to calculate ^oD in the field. The simplest calculations are the "linear" methods. These types of calculations are based on the assumption that the rate of development is linear with temperature (Figure 1B). Field temperatures follow a cyclical pattern, each 24-hour period having a minimum temperature (T_min). The "averaging" method used to estimate ^oD first takes the average of the day's high and low temperatures, then subtracts from that figure the lower developmental threshold temperature for the specific pest or organism. The equation is ^oD = [(T_max - T_min) / 2] - T_L.

There are pre-calculated ^oD tables available that use more complex calculation methods (triangulation, sine curve and modeling). Different tables must be used for species having different thresholds. For example, in Table 1 there are three precalculated ^oD tables. For an organism with a T_L of 50^oF, one would use the middle table. If the day's T_min was 60^o and the T_max was 85^o, the ^oD value for that day would be 22.5. The new UC/IPM home page on the World Wide Web http://www.ipm.ucdavis.edu provides ^oD information for weather station sites located all over California. The weather network system set up specifically for the Lompoc Valley also calculates degree days. This information is available to Lompoc Valley growers.