by Vincent P. Jones and Jay F. Brunner, originally published 1993
Phenology is the study of relationships between the weather and biological processes such as insect development. Phenology models, also known as degree-day models, can help predict the best timing of pest management activities such as pesticide applications. These models are based on the fact that an insect’s growth is closely linked to the temperature where it is found. Phenology models do not operate on a calendar-day basis but on a heat unit (degree-day) scale. They are usually begun at some easily detected event, such as first flight of codling moth, and are used to predict events that are important, but not easily sampled, such as egg hatch or peak moth flight.
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
Metabolic rates of animals are influenced by temperature. There is generally a temperature at which a chemical reaction goes fastest. At temperatures above the optimum, the enzyme that causes the reaction is disrupted and the reaction slows. If the temperature remains high too long, the enzyme can be permanently disrupted and the animal will die. At temperatures below the optimum, the reaction rate slows down and finally stops.
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.
Most degree-day models use a sine-wave curve to approximate the daily temperature cycle from night to day. The upper threshold can have at least two forms:
- A horizontal cutoff, where degree-day accumulations above the upper threshold do not count
- 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.
Degree-days are used to measure the number of accumulated heat units. A degree-day is the heat experienced by the insect when the temperature is one degree above the lower threshold for 24 hours. The starting point for degree-day accumulation can be some easily observed event, such as first moth capture in a pheromone trap, often referred to as a biological fix point or “biofix.” Alternatively, it can be a calendar date, such as March 1, before which there is generally no degree-day accumulation. Using a biofix generally gives a better prediction of future life history events, such as egg hatch, because of a better synchronization between the insect’s development and degree-day accumulations.
Degree-day models for the following tree fruit pests are available in Washington: codling moth, western cherry fruit fly, San Jose scale, western tentiform leafminer, apple maggot, pandemis leafroller, obliquebanded leafroller, Lacanobia subjuncta, campylomma, fireblight, apple scab, and storage scald. Other models availabel soon include oriental fruit moth, white apple leafhopper and peach twig borer. Probably the most successful degree-day model is the one for codling moth. It is used in most fruit growing regions of the United States to time sprays. The codling moth model is used to predict egg hatch based on the number of degree-days accumulated since biofix, which in this case is the first consistent flight of codling moth. The main objective of cover sprays is to kill the newly hatched codling moth larva before it bores into the fruit. The model allows us to get maximum longevity of the cover sprays. It is critical to apply them as close to the predicted time as possible. If the spray is applied too early the residue will have weathered before the eggs hatch and it must be reapplied that much sooner. When sprays are applied late, more larvae already will have entered the fruit where they are difficult to kill.
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.