Barnes OrchardUnderstanding Degree-days and Using them in Pest Management Decision Making

Gabriella Zilahi-Balogh and Douglas G. Pfeiffer

This page was prepared mainly by the senior author as part of a project for ENT 4987, Arthropod Management in Fruit Crops.

Department of Entomology, Virginia Tech, Blacksburg, VA 24061-0319

The understanding of  insect development plays an increasingly important  role in insect pest management decision making. This is because timing is crucial to the implementation of pest management decisions.  For example, knowing or predicting that an insect pest is in the egg or larval stage is important for initiating sampling programs or timing sprays.  This is where the concept of degree-days comes in.  Degree-days is a way of measuring insect growth or development in response to daily temperatures.


Role of Temperature on Development or Growth

Growth or development of trees and insects are dependent on several environmental factors including temperature (heat), light and humidity.  Because insects are cold-blooded, temperature has the greatest effect on insect development rates.  Here is a graph of development rate versus temperature:

This figure shows that development rate increases as temperature increases.  In the temperature range from 10 to 30 degrees, development rate changes almost linearly with increasing temperature.  At very low temperature there is no development, and at very high temperature development is retarded.

Since temperature influences tree growth in a similar manner to insects, development of the various growth stages in the tree is dependent on temperature.  Each stage of tree development requires a certain amount of heat units before growth to the next stage will occur.  This measure of accumulated heat over time is known as physiological time.  To illustrate this point, if there is a hot spell around bloom on apples, pink to petal fall stage can occur within three to four days compared to cool, rainy weather where bloom can extend for several weeks.  The same amount of heat units are accumulated in both situations.  However, the heat units accumulate faster under warmer conditions.

For many years, growers have observed the arrival or development of a particular insect pest with flower bud or leaf development and have timed the applications of sprays in the spring.  This is an indirect use of physiological time.  Using these natural timers are most useful early in the growing season when tree growth stages are readily observable. However, making accurate predictions on insect life stages throughout the growing season are needed.  This is done by measuring degree-days of the pest insects in question.

How development thresholds and degree-days are determined

Entomologists have determined lower threshold temperatures and degree-day totals for the life stages of many  insects by studying their development in the field and the laboratory.  The lower developmental threshold for a species is the temperature below which development stops. The upper developmental threshold is the temperature at which the rate of growth or development begins to decrease. Phenology models are then developed and used to predict various events or lifestages of an insect.   For more information on phenology models please read: What exactly is a phenology model and how can it help me? and what validation means.

The biofix is the date when degree-days begin to be accumulated , usually associated with a biological event, such as first sustained trap catch of male codling moth, tufted apple bud moth or oriental fruit moth in pheromone traps. The term "sustained catch" often causes some confusion. If 1-2 moths are collected in traps, followed by a period of no captures before resumption of more or less continual captures, then those early males are ignored. The "sustained catch" is the beginning of the continual period of moth activity; catches in more than one trap on two consecutive trapping days. Obviously this can't be determined on the day of the capture, rather only after the overall pattern of flight is seen. But since this event is only the beginning of accumulating degree- days, and not the treatment signal, the grower has time to make this evaluation. In practical terms, there may not be an important difference between first catch and first sustained catch. If 1-2 males are captured followed by an interval of no catches caused by cool weather, then very few degree-days will accumulate in the days between first catch and first sustained catch. The critical number of degree-days to signal the timing of a spray may occur at almost the same date.

Development Thresholds for Selected Tree Fruit Pests

Each insect stage requires a certain number of degree-days before it will develop to the next stage.  Upper and lower development thresholds have been determined for many of the tree fruit pests through carefully controlled laboratory and field experiments.

The following table provides development thresholds for a variety of tree fruit insect pests.

Insect or Disease
Lower Development 

Threshold (oF)

Upper  Development

Threshold (oF)

Codling Moth 50.0 88.0
Apple Maggot * 43.5    -
Tufted Apple Bud Moth 45.0 95.0
Oriental Fruit Moth 45.0 90.0
San Jose Scale 50.0 90.0
Peach Tree Borer * 50.0 87.8
European Red Mite * 51.1    -
Pear Rust Mite * 42.8    -
Pear Psylla *  41.0   -

* Taken from UC IPM site

There is additional information on phenology models available for a variety of  organisms.  Each model contains: degree-day accumulations required for specific developmental stages of an organism, method of calculation, reference source, location of the study, developmental thresholds, and cutoff method. More than one model may be presented for an organism if citations occur in the literature.

Calculating Degree Days (DD)

 The simplest method used to estimate the number of degree-days for one day is called the averaging method:

[(maximum temp. + minimum temp.)/2]-developmental threshold = DD

For example, on May 14, 1998 in Albemarle County, VA, maximum daily temperature and minimum daily temperatures were 70 and 52° F, respectively. Using 50° F for the lower development threshold for codling moth, degree days accumulated were (70 + 52)/2 - 50 = 11 DD.  Therefore, on May 14 , 11 degree-days were accumulated for codling moth.

There are other methods for calculating degree-days.  (See degree-day calculation methods at the UC Statewide IPM site and Virginia Tech, Quantitative Ecology site).

Obtaining Temperature Data

It is easy to measure maximum and minimum daily temperatures using a max/min thermometer at the orchard.  The thermometer should be placed in location in the orchard and protected from sun and wind and temperature measured at the same time each day.

Daily temperature data  and  degree-day information is available from weather stations in tree fruit growing counties in Virginia and West Virginia if you do not measure temperature at your site.

Other Internet Sites

UC Statewide IPM Project
New Mexico Climate Center
NOAA  Degree-days
National IPM Network - Southern Region
Other Resources of Fruit Information


Copyright.  Gabriella Zilahi-Balogh, Department of Entomology, Virginia Tech, Blacksburg, VA, 24061-0319.
December, 1998.