Insect and Mite Control on Woody Ornamentals and Herbaceous Perennials

Bulletin 504

Detection and Monitoring of Insects and Mites
on Woody Ornamentals and Herbaceous Perennials

Degree-Days: Their Calculation and Use in Management of Turf and Tree/Shrub Pests

Reason for Using Degree-Days

Degree-Days (DD) are a method of accounting for heat units. Power companies use cooling degree-days and heating degree-days to calculate how much energy a customer needs to cool or heat a house. Plants and animals which do not regulate internal temperatures (often called "cold blooded") vary in their physiological development, or metabolism according to what temperature they are subjected to. In short, these organisms develop rapidly at warm temperatures and slowly at cool temperatures. Therefore, we can treat plants and animals like a house, the more energy (heat) added the faster things happen. Conversely, the cooler (less energy) the organism the slower it develops. If this rate of development related to temperature can be determined, a prediction of insect and/or plant development or activity can be made.

Using DD as a predictor takes into account cool vs warm weather. Calendar scheduling of controls will usually be too early or too late unless the year is an "average year."

Temperature Thresholds

Fortunately, most plants and animals develop within a specific range of temperatures. If the organism drops below a certain temperature, called the lower threshold, no development occurs (see Figure 1). Above this lower threshold, the rate of development increases with temperature in an almost straight-line fashion. Most organisms also have an upper threshold temperature at which development begins to deteriorate because of heat shock. If the organism’s temperature rises too far above this threshold, it will die. In nature, most insects and plants find habitats which have temperatures above the lower threshold for sufficient time to complete a generation of development, but rarely exceed the upper threshold temperature.

Several field crops and ornamental plants are occasionally grown outside their original habitats. Corn plants shut down their development above 86°F and Balsam fir tends to stop development above 90°F. Unfortunately, most state crop reporting services are based on corn DD models which have the relatively low upper threshold of 86°F. Most insect pests and other trees and shrubs do not stop development until temperatures reach 100-110°F.

In reviewing DD thresholds for many insects and plants, several lower thresholds seem to be common. Most soil dwelling insects and some cool season plants (e.g., conifers, maples) seem to have lower thresholds of 40°F (5°C) or 45°F (7°C). Most above ground feeding insects (turfgrass surface feeders and most tree/shrub scales and caterpillars) seem to have a lower threshold of 50°F (10°C).

For all practical purposes, associating insect activity and plant phenology with 50°F degree-days (DD₅₀) is generally satisfactory.

Methods of DD Calculation

There are many methods for calculating DD. The easiest method is to use the average temperature method (see Figure 2). This method calculates the day's DD units by subtracting the average daily temperature from the threshold.

The following table illustrates this technique with several daily temperatures.

Table 1. DD50 units for days with varying maximum and minimum temperatures.
Example	Max T	Min T	Ave T	DD50
1	50	30	40	01
2	60	40	50	02
3	70	40	55	5
4	75	55	65	15
1/ If the average temperature is below the threshold a 0 is used never negative units. 2/ If the average temperature is equal to the threshold a 0 is used.

Many state reporting services use a sine wave calculation (see Figure 3) or a small time-unit calculation. The sine wave method assumes that the rise and fall of daily temperatures approximates a sine wave pattern. This method also allows for the accumulation of those units of heat energy in a day when the temperature was above the threshold. In example 2 above, the temperature was above 50°F for part of the day and the insects were developing. The average method estimated that no activity occurred while the sine method would have estimated about 0.3 DD₅₀ units for that day.

The small time-unit calculations, are continuous temperature monitors which read the temperature every few minutes and use that fraction of a day to calculate the DD.

In retrospect, use caution when looking at DD reports. You need to know what thresholds were used (especially if a low upper threshold is in effect) and the method of calculation (ave, sine, etc.).

Conversion of Degree Day Units

Changing from DD-Centigrade (°C) to DD-Fahrenheit (°F) is very easy using the following formulae:

Note that (+ 32°) is not used in this conversion.

Converting between Ave-DD and Sine-DD is a bit more complicated and should be done with care. Each geographic location usually has a constant which can be added or subtracted for this conversion. This constant is determined by calculating Ave-DD and Sine-DD using 30- year average temperatures.

If you need to convert between average and sine DDs, you should probably seek the assistance of a crop consultant or extension agent.

Determining Degree-Day Targets for Pests

There are four methods generally used to determine DDpest activity association: 1. growth chamber studies; 2. field data using regression analysis; 3. field data using lowest coefficient of variation (CV) analysis; and 4. experienced guess-trial and error using average yearly temperatures.

The first three methods generally require special equipment, complicated calculations and detailed data bases. The "experienced guess" technique makes use of past experience to form a rough target DD. This target is then modified yearly ("trial and error") as new information is gathered using actual DD calculations.

To use the experienced guess technique, obtain an annual weather summary from NOAA (National Oceanic and Atmospheric Administration, National Climatic Data Center, Federal Building, Ashville, NC 28801) and calculate the AveDD₅₀ using the normal maximum and minimum temperatures.

Chart the cumulative DD₅₀ for each day from April 1 through August. See the following example:

Charting Cumulative Degree-Days (DD) - Pest Activity, an example.
Date	DD50	Pest Activity
.	.
.	.
May 1	143
May 2	150			0
May 3	158			*
May 4	166		0	*
May 5	175		*	Lilac Borer
May 6	183		*	Adults
May 7	192		Holly Leaf Miner	*
May 8	201		Adults	*
May 9	210	0	*	*
May 10	220	*	*	*
May 11	230	*	*	*
May 12	240	*	*	*
May 13	250	Pine Needle Scale	2	*
May 14	261	Crawlers		*
May 15	272	*		*
May 16	284	*		*
May 17	296	*		*
May 18	308	*		*
May 19	320	*		2
May 20	332	2
May 21	341
May 22	353
.	.
.	.

Let's say that you remember that you usually see pine needle scale crawlers in the third week of May, holly leaf miner adults in the first and second week of May, and lilac borers in your pheromone traps in the first three weeks of May. Your target DDs would then be 250-332, 143-261, and 143 to 332, respectively.

The next season you accumulate actual DDs50 and you notice that the pine needle scale crawlers were active from 290 to 340 DDs. This indicates that the prediction should be shifted slightly to more DD₅₀ units.

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