Whether you are trying to control a certain insect with a pesticide, or trying to find an insect or plant in a certain stage of development you need to know when you should be out there spraying or looking. You can put a reminder on a calendar based on when the right time was in past years. But you know some years things happen a lot earlier or a lot later than other years. That is because certain conditions are required for growth and development.

Development can only occur within a certain temperature range, moisture regime and day length, etc. Some of these requirements like temperature and moisture vary from one year to the next, so the time of moth flight or the flowering of a plant varies from year to year. This relationship between weather and development is called phenology.

With insects, the most influential factor is usually temperature. Being cold-blooded, their development is directly related to temperature. The time it takes for an insect to develop from one stage to another depends on temperature. The insects develops faster when the temperature is optimal and slower when outside the optimum range.

Scientists have used the accumulation of daily maximum and minimum temperatures above a minimum threshold temperature (degree days or DD) to predict the stage of development of some insects. An insect can be expected to be in a certain stage of development when a certain number of degree days have accumulated regardless of the calendar date.

For an example, lets say a certain insect only develops when the temperature is above 45 degrees F. Today's average temperature is 46. You have accumulated 1 degree day for today. If the next day the average temperature is below 45 say 43 you do not accumulate any degree days because the average temperature was below the base or threshold temperature of 45.

A degree day model for spruce budworm, developed by Timothy J. Lysky, was described in a paper titled "Stochastic Model of Eastern Spruce Budworm Phenology on White Spruce and Balsam Fir." Degree day predictions developed in one location may not be accurate if applied to another location. This model was developed in Canada but did contain a number of plots just north of Minnesota and hopefully is close enough for our use.

This model uses 46.4 F as the threshold temperature and a starting date for accumulation of degree days of March 1. Based on this, the peak occurrence of the various life stages of spruce budworm can be expected at the following accumulated degree days:

Spruce budworm

accumulated degree days

Life stageWhite spruceBalsam fir
2nd instar92DD89DD
3rd instar103 DD108 DD
4th instar138 DD154 DD
5th instar186 DD204 DD
6th instar256 DD295 DD
Pupae370 DD413 DD
Moth437 DD467 DD

"All well and good" you say, "but how do I know what the accumulated degree days are for my locale?" Glad you asked! The Climatology Working Group has agreed to post this information on the Internet for us. Accumulated degree days for spruce budworm at 40 locations around the state can be found on the Internet at the Climatology Working Group web site at So for example, if you want to collect some moths from white spruce you should be out there by the time the accumulative degrees days reach 437, or if a control recommendation is to spray a pesticide when the budworm is in peak 4th instar you should be spraying at 138 DD on white spruce or 154 DD on balsam. You will want to use this information along with other information such as the development of the host, etc. before actually spraying. Give it a try. See if it seems to be accurate for your area.

We were not able to locate a similar model for the forest tent caterpillar. However, degree days for FTC using a threshold temperature of 32F and a starting date of January 1st is also posted on this web page. Using this model, the peak of the FTC hatch should occur at 400 DD based on our past experiences.