Calculating Degree Days

For calculating degree days (DD), it is recognized that growth only occurs within a range of temperatures. The minimum temperature below which no growth occurs is called the minimum threshold. Rate of growth increases with higher temperatures up to a maximum developmental threshold. Thresholds are determined experimentally and are different for each species.

Degree days can be easily calculated by using the average daily temperature minus the minimum threshold temperature. Using the average temperature method we calculate DD for each day, determining the number of degrees above the threshold. DD for a single day are calculated as follows:

ORANGE WHEAT BLOSSOM MIDGE OR WHEAT MIDGE.
Developmental Threshold: Minimum = 40 F
Example: Daily minimum and maximum: 39 and 58

Degree days are totaled over a period of days to determine when an insect has reached a certain stage. The date to start accumulations will differ between species. Common starting times are when temperatures first exceed the minimum threshold.

Although DD are useful in predicting development of many insect species, these predictions are only estimates. The accuracy of a DD estimate is dependent on the temperatures used in calculating degree days. DD should be calculated with temperatures that represent the environment where insects are developing. Temperatures at one site give only a rough estimate of insect development at another site miles away.

The primary use of DD in IPM is to time scouting for pest species. DD can also be used to predict when an event will occur. By knowing the number of DD accumulated to date, we can estimate future DD by using average maximum and minimum temperatures, such as 5 or 30 year averages. These averages give only an approximation of when an event occurs, but they can be useful in planning our sampling and control activities. By using DD we can eliminate unnecessary scouting, we can avoid missing injurious pest populations, and we can make better management decisions. Thus these techniques help prevent economic losses and excess use of pesticides.

Based on data from Canada, the threshold temperature for wheat midge development is 40 F. Observations indicate the following DD accumulations for events in the midge population.

DD	EVENT
450	the midge breaks the larval cocoon and moves close to soil surface and forms the pupal cocoon.
1300	10% of the females will have emerged
1475	about 50% of the females will have emerged
1600	about 90% of the females will have emerged.

Based on data from Langdon and Minot, in 1995 midge numbers were the highest when heading occurred during peak female emergence (1400 DD). Using the 40 degrees as a threshold for wheat development (normally wheat development is monitored with 32 degrees) heading should occur at around 1000 - 1100 DD. Therefore, if about 150 - 200 DD accumulate prior to wheat seeding the peak midge emergence and wheat heading may be synchronized. Using that same logic, we would anticipate that wheat planted prior to 150 - 200 DD accumulations would be at relatively low risk for midge infestation. Wheat planted after the accumulation of 600 DD should be heading and flowering after significant midge activity and should be at low risk to infestation.

HRSW planted FROM 200 to 600 DD will be heading at the time wheat midge are emerging

HRSW planted AFTER 600 DD will head after peak emergence and should be at low risk to midge infestation (higher risk of frost, however)

We must remember that temperature drives midge development; however, soil moisture must be adequate to allow adult emergence from the soil, and warm temperatures, calm conditions, and adequate moisture during heading are also required by midge in order for economic injury to occur.


		Department of Entomology, 202 Hultz Hall North Dakota State University, Fargo North Dakota Extension Entomologist: Janet Knodel